Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. THE FATE OF FERTILISER PHOSPHORUS IN WHAREKO H E PODZOLS A thesis presented in part ial fulfi lment of the requirements for the degree of Doctor of Phi losophy in Soi l Science at Massey University Jennifer Kay Edwards 1 997 ABSTRACT During the 1980's and early 1990's, the then Ministry of Agriculture and Fisheries (MAF) Soil Fertility Service used the mass balance Computer Fertiliser Advisory Service (CFAS) model to make phosphorus (P) fertiliser recommendations where P requirements were calculated to replace losses from the cycling P pool via the soil and animals. In the late 1980's, concerns were raised that higher P application rates than those calculated by the CFAS model were necessary to maintain Olsen P levels on Wharekohe podzols. The soil loss factor (SLF) was identified as the model parameter which most likely led to the inability of the CFAS model to predict P requirements on these podzols. The new Outlook model also uses a mass balance approach incorporating a soil P loss parameter to calculate pasture P requirements. In this study the apparent limitation of the CFAS model to predict the maintenance P requirements of the Wharekohe soils, and the appropriateness of the soil loss parameter used in the New Outlook model, was investigated by (a) determining the fate of applied fertiliser P, (b) examining the possible mechanisms for any soil P retention or loss, (c) quantifying the SLF and (d) modeling the fate of applied fertiliser P. A chronosequence study found that pasture development resulted in an increase in total soil P to the top of the E horizon with increased P movement down the profile with increasing pasture age. The Wharekohe silt loam appears to have a maximum P storage capacity which is reached by 8 years in the 0-3 cm depth (approx. 166 kg applied P/ha) and by 11 years in the 0-7.5 cm depth (approx. 350 kg applied P/ha). The maximum total P storage capacity can mostly be attributed to a maximum inorganic P (Pi) storage capacity. Sodium hydroxide (NaOH) extractable iron and aluminium-Pi was found to be limited in the Wharekohe soil, due to its low sesquioxide content, in comparison to other New Zealand soils. Once the P storage capacity at each depth is reached there is little further accumulation of applied P and much of the P applied in subsequent application is lost from the topsoil in runoff waters. Up to 65% of the applied P could not be accounted for by animal loss or accumulation in the top 7.5 cm of older sites (>30 years). A glass house leaching study using intact soil cores confirmed that substantial quantities of applied P can be transported in subsurface water movement through Wharekohe podzols. Forty times more P moved through the Wharekohe soil cores than through cores of the yellow brown earth, Aponga clay (:::;45.6 jJg/ml in contrast to :::;1.07 jJg P/ml). In a field study using suction cups, concentrations of up to 18.65 jJg P/ml were obtained in soil water sampled under fertilised Wharekohe silt loam plots in comparison to <2 jJg P/ml under unfertilised controls. Movement of dissolved P occurred mostly as DIP after the application II of fertiliser P in the glasshouse and field studies. No difference in P movement could be detected in relation to development history in the glasshouse leaching study or in the field study, although the ability of the Wharekohe silt loam to retain added fertiliser P was found to decline with pasture development in a laboratory based P retention study. Soil loss factors calculated for the Wharekohe podzols from small plot field trials varied enormously (0.04 in the first year to 1.68 over the two year trial period) as a consequence of the large variation in the rate of P required to maintain a steady Olsen P level at each site. Consequently, it was not possible to determine if the SLF of 0.4 used for podzols in the CFAS model was appropriate. The component of the SLF due to non-labile P accumulation, calculated form the chronosequence data, decreased with pasture age. As P applied surplus to animal production requirements and P accumulation is lost from the root zone in runoff, the SLF should be reduced with increasing pasture age or else P runoff losses will increase. Relationships between pasture age and available Pi, organic P, strongly sorbed/precipitated and residual P, and total P accumulation in the top 7.5 cm of a Wharekohe silt loam were successfully modelled. The annual total soil P accumulation was described in a model which was then incorporated into the Phosphorus in Runoff in High Loss Soils (PRIHLS) model developed to predict potential runoff P losses. Runoff P losses predicted by PRIHLS from the Wharekohe silt loam are nearly 3 times higher from older pasture (>30 years) where the Outlook model is used to calculate P requirements (36 kg P/ha lost in runoff from a calculated P requirement of 44 kg P/ha) compared to the CFAS model (13 kg P/ha lost in runoff from a calculated P requirement of 21 kg P/ha), due to the higher soil loss parameter assigned to the Wharekohe soils in the Outlook model. Such high runoff P losses represent a cost to New Zealand both economically, and environmentally through increased P inputs to water ways leading to possible eutrophication. When runoff P losses have been quantified, through further research, they could be used in the PRIHLS model to predict P requirements and would enable more informed decisions to be made about balanced P fertiliser use on Wharekohe podzols. III ACKNOWLEDGMENTS I am very grateful to the following people for their help during the course of my PhD: Dr. Paul Gregg for his supervision, stimulating comments, enthusiasm and practical advice. Dr. Alec Mackay for his supervision, and endless enthusiasm and encouragement. Mr. Mike Richardson for his supervision, practical advice, support and encouragement. AgResearch for providing my scholarship and funding for this project. The Staff in the Department of Soil Science, Massey University, and AgResearch Grasslands, Palmerston North and Kaikohe, for their support and enthusiasm for this project. The AgResearch Ruakura Statistics team for their help with statistical analysis. In particular I wish to thank Dr. John Waller for his hours of support, enthusiasm, humour and especially patience in ensuring my statistical knowledge was greatly improved. Mr. Bob Fletcher for statistical and modelling advice and support. Mr. Peter Woods for his advice and support. Mr. Lance Currie for technical advice. Mr. Jamie Edwards, Mr. David Haynes, Mr. Peter Boyce, Mr. Bob Toes, Mrs. Glenys Wallace and Mr. Ross Wallace for providing technical assistance. AgResearch Soil Fertility Service staff for their assistance with soil analysis. The HortResearch Library Staff in Palmerston North for their patience in helping me find obscure references, library reminder notices and humour. The AgResearch computing staff for keeping me on-line. The HortResearch computing staff for their support and guidance. Mrs. Annette Richardson for her support and encouragement. Other users of the Soil Science Department Research Laboratories for their glassware and patience whenever I descended from Northland. My friends for their support and encouragement, especially Bob and Pru Wellington, Gary Mackay, Mike Bretherton and Lynne Vautier and Alec Mackay for providing me with food and shelter during my visits to Palmerston North. Maureen and Keith Edwards for their love and encouragement during my studies. My parents, Gwen and Jack Rowe, for their love and support, and ensuring that I received a sound education in my formative years without which study towards a PhD would not have been possible. And most importantly, my husband, Craig, for his support, love and patience. IV TABLE OF CONTENTS ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i i i TABLE OF CONTENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i v L IST OF F IGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x L IST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x i i i CHAPTER 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 CHAPTER 2 REVIEW OF THE LITERATURE 2. 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 .2 THE PHOSPHORUS CYCLE UNDER GRAZED PASTURE . . . . . . . . . . . . . . . . . . . . . . . . 5 2 .3 SOIL PHOSPHORUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2 .4 FATE OF FERTIL ISER P IN NEW ZEALAND SOILS UNDER PERMANENT PASTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4. 1 Total Soi l P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 .4 .2 Inorganic Soi l P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 .4 .3 . Organic Soi l P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0 2.4 .4 Losses of Appl ied P from the Pasture Root Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 2.4 .5 Other Chemical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 2 .5 FATE OF FERTIL ISER P IN PODZOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 2 .5 . 1 The Podzol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 2.5. 1 . 1 Podzol Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 2 .5 . 1 .2 Characteristics of Podzols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 2 .5 . 1 . 3 Podzol C lassification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 2 .5 .2 Phosphorus Chemistry of Podzols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 2 .5 .2 . 1 Phosphorus in Undeveloped Podzols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 2 .5 .2 .2 Podzol Pasture P Chronosequence Stud ies . . . . . . . . . . . . . . . . . . . . . . . 1 8 2 .5 .2 .3 Losses of P from New Zealand Podzols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 2 .6 MODELLING P FERTIL ISER REQU IREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2 .6 . 1 CFAS Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.6 . 1 . 1 Sensitivity of Calculated P Requirements to Incorrect Estimation of Model Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2 .6. 1 .2 Modifications to the CFAS Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.6. 1 . 3 Difficulties with the CFAS model on Wharekohe soi ls . . . . . . . . 29 v 2 .6 .2 P Requirement Models Developed Since the CFAS Model . . . . . . . . . . . . . 32 2 .7 SUMMARY OF L ITERATURE REViEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 CHAPTER 3 FATE O F APPLIED P 3. 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3 .2 OBJECTiVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3 .3 MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3 .3 . 1 S ite Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3 .3 .2 . Fert i l iser and Lime H istory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3 .3 .3 Soi l Sampl ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3 .3 .4 Soi l Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3. 3 .4 . 1 Total P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3 .3.4.2 O lsen P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3 .3 .5 Pasture Sampl ing and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3 .3 . 5. 1 Dry Matter Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3. 3 .5 .2 Botan ical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3 .3 .5 .3 N and P Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3 .3 .6 Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.4 RESULTS AND D ISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.4. 1 Effect of Pasture Development on the Fate of Appl ied P . . . . . . . . . . . . . . . . . 47 3 .4 . 1 . 1 Total P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3 .4. 1 .2 Losses of Appl ied P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 3 .4 .2 Effect of Pasture Development on Olsen P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.4. 3 Effect of Pasture Development on Pasture Production and Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 3.4 . 3. 1 Dry Matter Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 3.4. 3 .2 Botan ical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3 .4 .3 .3 Pasture N and P Concentration and Uptake . . . . . . . . . . . . . . . . . . . . . . . 66 3 .5 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 CHAPTER 4 CHANGE IN P FRACTIONS IN A PASTU RE DEVELO P M ENT C H RONOSEQU ENCE ON A WHAREKOHE PODZOL 4. 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.2 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1 4 .3 MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 4.3 . 1 Soi ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 VI 4.3 .2 Soil Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4 .3 .3 Soi l Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . 73 4 .3 .3 . 1 P Fractionation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4 .3 .3 .2 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 4 .3 .3 .3 Cation Exchange Capacity and Exchangeable Cations . . . . . . 76 4 .3 .3 .4 Total Calcium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 4 .3 .3 Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4 .4 RESULTS AND D ISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4 .4. 1 Effect of Pasture Age on the Accumulation of Appl ied P into Soi l Fractions in a Wharekohe si l t loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4 .4. 1 . 1 I norganic P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1 4.4. 1 .2 Organic P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1 4.4 . 1 . 3 Po/Pi Rat io . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.4 . 1 .4 Changes in P Fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 4.4 . 1 . 5 Spring 1 993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 4 .4 .2 Contribution of P Fractions to the SLF for the Wharekohe si lt loam . . 96 4.4 .3 Movement of P Through the Profi le of a Wharekohe s i l t loam . . . . . . . . . 98 4 .4 .4 The Influence of P Fert i l iser Form and L iming H istory on P Fractions in a Wharekohe silt loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.4 .5 Effect of Parent Material on P Fractions in Wharekohe Podzols 1 03 4.4.6 Effect of the Degree of Weathering of S i lt Sediments on P Fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 06 4 .5 SUMMARY AND CONCLUS IONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 08 4 .5 . 1 Effect of Pasture Age on the Accumulation of Appl ied P into Soi l Fractions in a Wharekohe si l t loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 08 4 .5 .2 Contribution of P Fractions to the SLF for a Wharekohe si l t loam . 1 1 0 4. 5 .3 Movement of P Through the Profi le of a Wharekohe s i l t loam . . . . . . . 1 1 0 4 .5 .4 I nfluence of P Fert i l iser Form and H istoric L ime Appl ication on P Fractions in Wharekohe podzols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 1 4 .5 .5 Effect of Parent Material on P Fractions in Wharekohe Podzols . . , 1 1 1 4 .5 .6 Effect of Degree of Weathering of S i lt Sediments on P Fractions . 1 1 1 CHAPTER 5 P MOVEM ENT IN SUBSU RFACE RUNOFF 5. 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 3 5 .2 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 5 5 .3 MATERIALS AND METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 5 5 .3 . 1 G lasshouse Trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 5 5 .3 . 1 . 1 Col lection and Preparation of Soi l Cores . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 5 vi i 5 .3 . 1 .2 Leaching Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 7 5 .3 . 1 . 3 Chemical Analysis of Leachate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 8 5 .3 . 1 .4 Statist ical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 9 5 .3 .2 Field Tria l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 9 5 .3 .2 . 1 F ield Sites and Ferti l iser Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 9 5 .3 .2 .2 Soi l Solution Col lection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 1 1 9 5 .3 .2 .3 Chemical Analysis of Soi l Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 20 5 .3 .2 .4 Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . '1 20 5 .4 RESULTS AND D ISCUSS ION . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 20 5 .4. 1 Glasshouse Trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 20 5 .4. 1 . 1 Amounts of D IP and DOP Lost by Leaching . . . . . . . . . . . . . . . . . . . . . 1 20 5 .4. 1 .2 Effect of Pasture Age on P Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 24 5 .4. 1 . 3 Effect of Parent Material on P Leach ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 24 - 5 .4. 1 .4 Effect of Time on P Leach ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 25 5 .4. 1 . 5 Water Movement Through Intact Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 28 5 .4 .2 Fie ld Trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 31 5 .4 .2 . 1 Movement of Appl ied P . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 31 5 .4 .2 .2 Effect of Pasture Age on P Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 38 5 .5 GENERAL DISCUSS ION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 39 5 .5 . 1 Comparison of the P Concentration in Leachate from the Intact Soi l Core and Soi l Solution Col lected in the Suction Cups . . . . . . . . . . . 1 39 5 .5 .2 Effect of Pasture Age and P Saturat ion on P Losses . . . . . . . . . . . . . . . . . . . . . 1 40 5 .5 .3 Effect of Waterlogging on P Loss . . . . . .. . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 42 5 .5 .4 Loss of P in Subsurface and Surface Runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 43 5 .5 .5 Loss of Particulate and Dissolved P . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 44 5 .5.6 Predicting Runoff P Losses from Soi l P Tests . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 1 44 5 .5 .7 Min imising P Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 45 5.6 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 1 49 CHAPTER 6 P RETENTION IN WHAREKOH E SOILS 6. 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 51 6 .2 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 53 6 .3 MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 53 6.4 RESULTS AND D ISCUSS ION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 54 6.4. 1 Effect of Pasture Age on P Retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 1 54 6 .4. 1 . 1 Possible Explanations for Differences in P Retention Between Developed S ites . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 57 6.4. 1 .2 Impl ications of Decreasing P Retention with Pasture Age on Model l ing P Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 64 ---------- vi i i 6 .4 .2 Effect of Soi l Weathering and Parent Material on P Retention . . . . . . 1 69 6 .5 CONCLUS IONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 72 CHAPTER 7 QUANTIFYI NG THE SLF FOR WHAREKO H E SOILS 7 . 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 74 7 .2 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 76 7 .3 MATERIALS AND METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 77 7 .3 . 1 Small-P lot F ield Trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 77 7 .3 . 1 . 1 Trial S ites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 77 7 .3 . 1 .2 Trial Design and Establ ishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 78 7 .3. 1 . 3 Pasture Sampling and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 82 7 . 3 . 1 .4 Soi l Sampl ing and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 83 7 .3 . 1 . 5 Calculation of the SLF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 84 7 .3 .2 Chronosequence Trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 85 7 .4 RESULTS AND D ISCUSS ION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 86 7 .4 . 1 Effect of Appl ied P on Pasture Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 86 7 .4. 1 . 1 Effect of Appl ied MCP on Pasture Growth . . . . . . . . . . . . . . . . . . . . . . . . 1 86 7 .4 . 1 .2 Effect of Fert i l iser Solubi l ity on Pasture Growth . . . . . . . . . . . . . . . . 1 92 7 .4 .2 Effect of Appl ied P on Avai lable P Soi l Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 94 7 .4 .2 . 1 Effect of MCP Application Rate on Maintaining O lsen P Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 94 7 .4 .2 .2 Effect of P Fert i l iser Solubi l ity on Maintain ing Avai lable Soi l P Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 7 .4. 3 Quantifying the SLF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 7 .4 .3 . 1 Effect of Soi l Ferti l ity Status on the SLF . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 7 .4 .3 .2 Effect of Parent Material on the SLF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 7 .4 .3 .3 Effect of Degree of Soi l Weathering on the SLF . . . . . . . . . . . . . . . 209 7 .4 .3 .4 Effect of P Ferti l iser Solubi l ity on the SLF . . . . . . . . . . . . . . . . . . . . . . . . 2 1 0 7 .4 .3 .5 Sensitiv ity the SLF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 0 7 .4 .3 .6 Effect of Pasture Age on the SLF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 4 7 .4 .3 .7 Impl ications of Soi l Loss Estimation on P Fert i l iser Requi rements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 8 7 .5 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 9 ix CHAPTER 8 MODELLING THE FATE OF P IN A WHAREKO H E SILT LOAM 8. 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 8 .2 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 8 .3 THE P CYCLE OF A GRAZED PASTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 8 .4 MODEL DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 8.4. 1 Soi l P Accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 8 .4 .2 Runoff P Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 8 .5 PREDICTING P ACCUMULATION IN EACH SOIL P POOL . . . . . . . . . . . . . . . . . . . . 228 8.6 PREDICTING TOTAL P ACCUMULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 8 .7 PREDICTING RUNOFF P LOSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 8 .7 . 1 Val idation of the PR IHLS Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 8 .7 .2 Effect of Fert i l iser and Stocking Rates on Runoff P Losses . . . . . . . . . . . 235 8 .8 USING THE PRIHLS MODEL TO PREDICT P REQUIREMENTS . . . . . . . . . . . 238 8 .9 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 CHAPTER 9 SUM MARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 GLOSSARY OF TERMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 APPEN DICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 x LIST OF FIGU RES Figure 2 . 1 Figure 3. 1 Figure 3.2 Figure 3. 3 Figure 3 .4 Figure 3. 5 Figure 3 .5 Figure 4. 1 Figure 4.2 Figure 4.3 F igure 4.4 Figure 4. 5 Figure 4.6 Figure 4 .7 Figure 4 .8 Figure 5. 1 steel Figure 5.2 Phosphorus cycle under grazed , fert i l ised pasture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Effect of pasture age on soil total P concentration (SED's varied, Table in Appendix 3. 1 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Comparison of total P measured in original 1 990 sites (0) and total P measured in additional sites in 1 993 ( . ) . . . . . . . . . . . . . . . . . . . . . . . ,. . . . . . . . . . . . . . . . . . 55 Fate of appl ied P showing losses from each depth with time from pasture development, a) 0-7 . 5 cm, b) 0 cm - top of E Horizon, c) soi l profi le to 30 cm below E Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Effect of pasture age on Olsen P (MAF Quicktest) for each depth and a calculated value for the 0-7 .5 cm depth . (Vertical bars=S. E .D . s for developed sites sampled to 0-3 cm, 3-7 .5 cm and 7 .5 cm to the E horizon) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Effect of pasture age on dry matter yield in 1 991 and 1 992 . . . . . . . . . . . . . 64 Effect of pasture age on P uptake in 1 991 and 1 992 . . . . . . . . . . . . . . . . . . . . . . . 64 Flow chart of sequential P extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Effect of pasture age on inorganic and organic P concentration in each depth of a pasture chronosequence on a Wharekohe si l t loam . . . . . . . 78 Effect of pasture age on organic P content (%) at each depth . . . . . . . . . 84 Comparison of inorganic and organic P fractions measured in each site at each depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Effect of pasture age on each P fraction at each depth . . . . . . . . . . . . . . . . . . . . 89 Effect of pasture age on pH at each depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 01 Effect of parent material on inorganic and organic P fractions in Wharekohe podzols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 04 Effect of degree of weathering of si l t sediments on inorganic and organic P fractions in the soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 07 Intact soil cores, with petrolatum (vasel ine) seal between galvanised casing and soi l , and the dual leachate col lection system . . . . . . . . . . . . . . . 1 1 6 Sum of d issolved inorganic P leached through soi l cores after four 1 1 .4 mm rainfa l l events on days 3, 5, 1 0 and 1 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 1 xi Figure 5 .3 D IP and DOP leached through cores after three 1 1 .4 mm rainfa l l events, a) 3 , b) 1 3 and c) 94 days after P fert i l iser appl ication . . . 1 22 Figure 5 .4 Effect of t ime on the amount of DIP leached from fert i l ised cores during each 1 1 .4 mm rainfal l event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 26 F igure 5 .5 Comparison of D IP leached from fert i l ised and unferti l i sed cores with t ime from P appl ication for the cores col lected from each site . . . . . . . 1 27 Figure 5 .6 Rainfal l recorded over period of field trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 32 Figure 5 .7 Dissolved inorgan ic P measured in water samples col lected from 2-7 .5 cm depth, a) 1 1 years under developed pasture, and b) 33/35 years under developed pasture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 33 Figure 5 .8 Dissolved inorganic P measured in water samples col lected from 7 .5- 1 3 cm depth, a) 1 1 years under developed pasture, and b) 33/35 years under developed pasture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 34 F igure 5 .9 Dissolved organic P measured in water samples col lected from 2-7 . 5 cm depth, a ) 1 1 years under developed pasture, and b ) 33/35 years under developed pasture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 36 Figure 5. 1 0 Dissolved organic P measured in water samples col lected from 7 .5-1 3 cm depth, a) 1 1 years under developed pasture, and b) 33/35 years under developed pasture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 37 Figure 5 . 1 1 Effect of t ime on D IP concentration in the 2-7 .5 cm with in the fert i l ised plots and 1 0 m away (controls) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 38 Figure 6. 1 Effect of increasing solution P concentration and shaking time on the retention of added P from 0 .01 M CaCb by Wharekohe si lt loam developed for 0, 1 1 and 35 years and Wharekohe sandy loam during a) 1 6 hour, b) 40 hour and c) 1 36 hour shaking periods . . . . . . . . . . . . . . . . . . . 1 55 F igure 6.2 Effect of pasture age on P storage by Wharekohe s i lt loam at various solution P concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 56 F igure 6 .3 Influence of shaking time on the storage of added P by a) Wharekohe si l t loam, 0 years, b) Wharekohe s i lt loam, 1 1 years, c) Wharekohe si l t loam, 35 years, d) Wharekohe sandy loam and e) Aponga clay . . . . 1 58 F igure 6 .4 Effect of cation species on the storage of added P by a) Wharekohe si lt loam, 0 years, b) Wharekohe si lt loam, 1 1 years, c) Wharekohe si lt loam, 35 years, d ) Wharekohe sandy loam and e) Aponga clay . . . . 1 60 F igure 6 .5 Effect of added P, pasture age and soi l type on the final pH of the shaking solution in a) CaCb shaken for 1 6 hours, b) CaCb shaken for 40 hours, c) CaCb shaken for 1 36 hours, and d) NaCI shaken for 40 hours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 63 xi i Figure 6.6 Effect of increasing solution P concentration on the storage of added P from 0 .01 M CaCb by Wharekohe si l t loam, Wharekohe sandy loam and Aponga clay during a) 1 6 hour, b) 40 hour and c) 1 36 hour shaking periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 70 Figure 7 . 1 Layout of smal l-plot field tria ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 80 F igure 7 .2 Effect of appl ied MCP on annual pasture yield for years 1 and 2 for a) Wharekohe s i l t loam NFt, b) Wharekohe s i l t loam Ft, c ) Wharekohe sandy loam and d) Aponga clay. (Vertical bars=S. E . D . ) . . . . . . . . . . . . . . . 1 87 Figure 7 .3 Effect of P ferti l iser solubi l ity on annual pasture yield for years 1 and 2 for Wharekohe si l t loam Ft, adjusted for covariates (OP not included) . (Vertical bars=S .E .D . ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 92 Figure 7 .4 Effect of appl ied MCP on Olsen P levels over t ime for a) Wharekohe si lt loam NFt, b) Wharekohe s i lt loam Ft, c) Wharekohe sandy loam and d) Aponga clay. Arrows ind icate P fert i l iser app l ication t imes . . 1 95 F igure 7 .5 Effect of P fert i l iser solubi l ity on Olsen P levels over t ime where a) MCP and b) SPR were appl ied on the Wharekohe s i lt loam Ft site. Arrows indicate P fert i l iser appl ication t imes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 F igure 7.6 Effect of appl ied SPR on Resin P levels over t ime on the Wharekohe s i lt loam Ft site . Arrows indicate P fert i l iser appl ication t imes . . . . . . . 205 Figure 8. 1 Fate of fert i l iser P in a grazed Wharekohe podzol . . . . . . . . . . . . . . . . . . . . . . . . . 225 Figure 8 .2 Effect of pasture age on predicted and measured a) Avai lable P i , b ) Organ ic P , c) Calcium Pi and d) Strongly sorbed and precipitated and residual P accumulation in the top 7 .5 cm of a Wharekohe si l t loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 F igure 8 .3 Effect of pasture age on pred icted and measured total P accumulation in the top 7 .5 cm of a Wharekohe silt loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 Figure 8.4 Predicted and measured P runoff losses for add itional sites sampled in 1 993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 F igure 8 .5 Effect of P appl ication rate and stocking type on annual soi l P accumulation and runoff P loss from the top 7 .5 cm of a Wharekohe s i lt loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 xi i i LIST OF TABLES Table 2 . 1 Changes in soi l Olsen P levels with P fert i l iser addition and calculated maintenance and current P requirements on three properties on Wharekohe soi ls in North land . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Table 3 . 1 Descriptions of Wharekohe si l t loam sites selected in 1 990 . . . . . . . . . . . . 39 Table 3 .2 P appl ication history of pasture chronosequence sites selected in 1 990 (additional sites selected in 1 993 in brackets) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Table 3 .3 L ime appl ication history of selected pasture chronosequence sites sampled in 1 990 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Table 3 .4 Total P content (kg P/ha) of Wharekohe s i lt loam samples . . . . . . . . . . . . . . 49 Table 3 .5 Accumulation of appl ied P at each depth to the top of the E horizon . 50 Table 3 .6 Accumulation of P appl ied from spring 1 990 - 1 993 in the top 7 .5 cm of original sites (add itional sites in brackets) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Table 3 .7 Accumulation of appl ied P in top 7 .5 cm of soi l sampled from the additional sites in 1 993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Table 3 .8 Losses of appl ied P from different aged pastures on the Wharekohe si l t loam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Table 4. 1 I norganic and organic P content (kg P/ha) of Wharekohe si lt loam samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Table 4.2 Estimated alkal i and acid P i accumulation in the original 8 and 32 year sites for a 3 year period compared to estimated accumulation for a 3 year period in MAF 'National Series' soi ls , TSP and RPR appl ied at twice maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Table 4. 3 Effect of parent material on the P fractions measured in Wharekohe podzols (% in brackets) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 04 Table 4.4 Effect of the degree of soi l weathering on the P fractions measured in the moderately leached yel low brown earth, Aponga clay, and the podzol , Wharekohe si lt loam, derived from simi lar si l ty parent material (% in brackets) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 06 Table 5. 1 Pattern of methylene blue stained water infiltration through intact soi l cores at the end of the leaching trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 29 Table 7 . 1 Soi l properties and slope at each sma"-plot field trial site, samples col lected in May 1 991 (pH and an ion storage capacity) and in late July 1 991 (Olsen P) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 78 Table 7 .2 Basal ferti l iser appl ied to each plot at each appl ication time . . . . . . . . 1 82 - ------ XIV Table 7 .3 Rate of appl ied P as MCP required to maintain a constant O lsen P test at each site for each t ime period (standard deviations in brackets) . 1 97 Table 7 .4 Effect of fert i l iser P solubi l ity on the rate of appl ied P required to maintain a constant avai lable P soi l test for each period (standard deviations in brackets) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Table 7 .5 Soi l Loss Factors determined for each sampl ing period at each site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Table 7 .6 Sensit ivity of the SLF to an incorrect estimation of the rate of P to maintain a steady O lsen P level and P uptake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1 Table 7 .7 Effect of pasture age on the SLFsPA for use in the CFAS model . . . . 21 5 CHAPTER 1 INTRODUCTION 1 Agriculture is the most important industry financially in the Northland region of New Zealand. The estimated annual total gross farm revenue (before expenses) averaged over the 5 year period from 1 992/93 to 1 996197 in Northland is $334 mi l l ion (Ministry of Agriculture and Fisheries, 1 996). Changes in farm revenue have far more effect on the economy of Northland than any other factor. Northland has never reached its ful l potential for agricultural production. A major reason for this is the low natural ferti l ity of the bulk of the pastoral soi ls in the region. Realising the potential production of these soi ls requires that the returns from agricultural products must exceed the cost of production ( including ferti l isers) for an extended period of time. In recent years, increasing costs combined with lower prices for agricultural products have led to a reduction in ferti l iser application rates. As a consequence approximately 65% of the Northland farms soi l tested by the then MAF Soil Fertil ity Service (SFS) in 1 990 had nutrient levels which were considered below optimum for the maintenance of pasture at 90% of maximum yield (D. Edmeades, pers. comm. ) . Most Northland soi ls have low natural fertil ity due to the strong weathering and leaching processes created by the mild, humid climate. The least ferti le soils are the podzol soi l group derived from sedimentary parent material which is low in phosphorus (P) . These soils cover 300 000 ha of the 1 .2 mi l l ion ha in Northland (Molloy, 1 988). As the podzols are predominantly found on gentle undulating topography, large areas have been bought into agricultural production and are frequently used for intensive pastoral farming such as dairy farms. The most widespread of the Northland podzols are the Wharekohe soi ls which cover approximately 60 000 ha (Molloy, 1 988). They are derived from mudstone (Wharekohe si lt loam) and sandstone (Wharekohe sandy loam) parent materials. However, as weathering is advanced, there is l ittle influence of the original parent material on soi l properties except for topsoi l texture. Wharekohe soi ls require large inputs of phosphate, sulphur, potassium, l ime and trace elements for establ ishing pasture successfully (Jackman, 1 961 ; Lambert, 1 961 ; During, 1 984). The appl ication of ferti l iser P in excess 2 of 1 00 kg P/ha ( 1 1 25 kg superphosphate/ha) is recommended during the first year of pasture establ ishment (Bal l inger, 1 953; Lambert, 1 961 , During, 1 984). Phosphorus accumulates in the soi l after development into permanent pasture as a result of the continued addition of P ferti l iser to the pasture cycle (Walker et aI . , 1 959; Jackman, 1 964a; Perrott and Sarathchandra, 1 987; Nguyen et aI . , 1 989). This cumulative build up of P can contribute to pasture growth. Eventually only maintenance appl ications of P are required to balance any losses from the pastoral system in order to maintain pasture growth at a required relative yield (Karlovsky, 1 966, Cornforth and Sinclair, 1 982, During, 1 984). Until this maintenance situation is reached, high capital dressings of P ferti l iser are appl ied to the Wharekohe soi ls for up to ten years to realise the production capacity of these soils (During, 1 967). Phosphorus ferti l iser is a major cost to the farmer accounting for approximately 20% of variable cash farm expenses (MAF, 1 993). Hence, in recent years much effort has been focussed on developing models to predict P fert i l iser requirements for pastoral farming. In 1 978 Sinclair and Cornforth, drawing on earl ier mass balance models used to calculate the P required for pasture maintenance in a particular pastoral system (Karlovsky, 1 966; 1 975a&b; 1 981 ) developed the Computer Fertil iser Advisory Service (CFAS) model used by the SFS of the then MAF and more recently AgResearch. The CFAS P model calculates the amount of P required as fertil iser to replace the P lost from the pastoral system to the soi l and through animals, in products and in transfer of excreta to unproductive and concentrated areas (Cornforth and Sinclai r, 1 982) . Parameters required in the calculation of the amount of P required to replace these losses in a pastoral system are stocking rate (SR), pasture uti l isation (PU) , the potential carrying capacity (CC), amount of P lost per stock unit (ALF, dependent on topography and stock type) and the amount of P lost from the pasture cycle to the soi l (SLF, expressed as a fraction of the total P uptake by pasture maintained at 90% of maximum yield) (Cornforth and Sinclair, 1 982). The calculated maintenance P rate is then corrected to account for the amount of avai lable soil P (measured by the Olsen P soi l test) through the use of a modifying factor. In order to overcome the difficulty in 3 estimating PU and the large errors associated with an incorrect estimation of CC, a modified version of the CFAS model was developed (Sinclai r and Cornforth, 1 984). Despite their low P retention, the Wharekohe soils were al located a high soil loss factor in the CFAS model . Al l other Northland soi ls of sedimentary origin require the use of the medium soi l loss factor in calculating P requirements. The modified CFAS model calculates that 20 kg P/ha is required to maintain 1 5 stock units and an Olsen P level of 20 on the flat to rol l ing Wharekohe soi ls. However, in 1 989 it came to the attention of scientific and advisory staff at the DSIR, Kaikohe, and MAF, Northland and Ruakura, that higher P appl ication rates than those calculated by the CFAS model were necessary to maintain required Olsen P levels on many of the farms in the district on Wharekohe soi ls. The model was able to predict, with some certainty, the P requirements of a volcanic soi l (also classified as high P loss) on one of the farms on which the P recommendation was insufficient to sustain Olsen P on the Wharekohe soi l . Therefore, the underestimation of the ALF can be ruled out. It was concluded that the SLF assigned to the Wharekohe soi ls in the CFAS model had been underestimated. Phosphorus can be lost from the pasture cycle via the soi l through the accumulation of plant unavailable P or from the soil as particulate and dissolved P in surface and subsurface runoff waters. The Wharekohe soi ls have a very low capacity to retain added P with Anion Storage Capacities often close to 0%. Hence, losses of P from the soil may form a significant component of the CFAS SLF. Substantial losses of appl ied P in runoff water represent a cost to New Zealand both economically and environmentally through increased P inputs of water ways leading to their possible eutrophication. Since the commencement of this PhD, the CFAS model has been replaced by the SFS. The new model, Outlook, has been developed, without the unquantifiable parameters PU and CC and with the abil ity to evaluate the economics of different P ferti l iser strategies using a water soluble P source, and used by the SFS since 1 994. Outlook also uses a soil loss parameter to estimate losses of P from the cycl ing P pool through plant unavai lable P accumulation and runoff P losses. Hence, an investigation of soi l P 4 loss in Wharekohe podzols is relevant to the use of the Outlook for predicting P requirements on these soi ls also. The objective of this research study was to investigate the apparent l imitation of the soi l loss factor used in the CFAS model to predict maintenance P requirements of the Wharekohe soils, and the appropriateness of the soi l loss parameter used in the new Outlook model , further by: a) determining the fate of appl ied ferti l iser P (Chapters 3 and 4), b) examining possible mechanisms for any soi l P retention or loss (Chapters 5 and 6), c) quantifying the amount of P lost from the pasture P cycle via the soil (SLF) in Wharekohe soi ls (Chapter 7), D) model l ing the fate of applied fertil iser P (Chapter 8) . The information derived from this PhD study wi l l be used to improve the economics and reduce the environmental impact of P fertil iser use on Wharekohe soils. 2. 1 INTRODUCTION CHAPTER 2 REVIEW OF TH E LITERATU RE 5 This chapter presents a review of the l iterature relevant to assessing the abil ity of the CFAS model to predict the maintenance P requirements of pastures growing on the Wharekohe soils. Firstly, a brief outl ine of the P cycle in grazed pasture systems and the forms and amounts of P found in the soi l is provided. Then a review of the chronosequence studies examining the fate of fertil iser P in New Zealand soi ls under permanent pasture is fol lowed by a discussion of the properties of podzols and the fate of applied P in podzolic soi ls. Final ly, modell ing of pasture maintenance P requirements, with particular reference to the CFAS model and the difficulties encountered in using the CFAS model on Wharekohe soils, is discussed. 2.2 THE PHOSPHORUS CYCLE U N DER G RAZED PASTU RE Under grazed pasture, P cycles through both above and below ground pools, summarised in figure 2. 1 . The chemical form in which P is found changes as it is transferred between these pools. Phosphorus is added to the cycle through the weathering of native parent material and the addition of P containing fert i l iser. Phosphorus can be lost from the cycle via animals through the transfer of P in excreta to unproductive sites and concentrated areas within paddocks, and by removal of P in animal products. Losses of P via the soi l occur through the accumulation of P in the soil by net organic immobilisation and net precipitation/sorption, and from the soil as particulate and dissolved P in surface and subsurface runoff waters. The loss of significant quantities of P from the soil as dissolved P in subsurface runoff waters is rare in New Zealand soils. As soi l losses have been identified as the most l ikely reason for the CFAS model under predicting the P requirements of Wharekohe soils, much of this l iterature review wi l l concentrate on the fate of appl ied P in the below ground component of the P cycle. Fertil iser P 1 Non-Labile Inorganic P • Plant P \ I Litter P i • Soil P Labile Inorganic P • Runoff P Loss Ani mal P Loss t Animal P I Faeces pi • • Organic P Figure 2. 1 Phosphorus cycle under grazed, fert i l i sed pasture. 6 2.3 SOIL PHOSPHORUS 7 The forms and amounts of P found in the soi l have been well reviewed by many authors (Larsen, 1 967; Russel l , 1 973; Ryden et a I . , 1 973; Dalal , 1 977; Parfitt, 1 978; Sample et aI . , 1 980; Tate, 1 984; Rowarth, 1 987) . A brief description of the forms and chemistry of P found in the soi l fol lows. Phosphorus is relatively immobi le in the soi l , it is not added by the atmosphere and so the P status of a virgin soil is dependent on the primary P content of the parent material, most commonly apatite with some iron and aluminium phosphates in acid soils (Norrish and Rosser, 1 983). With time, the primary P is weathered through the action of cl imate and vegetation to give secondary inorganic mineral P, and inorganic P in solution which is in turn converted to solid and solution organic P (Syers and Walker, 1 969a&b; Wi l l iams and Walker, 1 969a&b; Adams and Walker, 1 975). Inorganic P (Pi) in the soi l solution provides P for plant uptake. There is no evidence that Po is directly avai lable for plant uptake under field conditions. Soil organic P (Po) derived from plant residues, animal excreta, soil fauna and micro-organisms becomes avai lable for plant uptake following mineral isation into inorganic P (Cosgrove, 1 977). Mineral isation of organic P can contribute large quantities of P to the avai lable pool in New Zealand during spring (Saunders and Metson, 1 971 ). P which can easi ly contribute to the solution P and plant uptake is termed labile. The type of inorganic P polymers found in solution are governed by protonation and complex formation, and the amount is governed by precipitation and sorption reactions (Larsen, 1 967) . A very small proportion of the total soil P is found in solution at any time, approximately 0. 1 to 1 �g P/ml in soi ls which have not been recently ferti l ised (Larsen, 1 967) . Inorganic P reacts with free Ca and Mg (alkaline soi ls), and Fe, AI , and Mn (acid soi ls) to form insoluble precipitates (Reviewed by Sample et aI . , 1 980; Brady, 1 984). Inorganic P is also adsorbed onto variable charged surfaces of iron oxides, aluminium hydroxides and clay minerals through reactions with short-range order and crystal l ine hydrous 8 oxides and short-range order aluminosil icates (Saunders, 1 965; Syers et aI . , 1 971 ) . Eventually the adsorbed P can become absorbed as the P slowly diffuses into the soi l particles on which i t had been adsorbed (Barrow, 1 983). The identification of up to 50% of organic P compounds remains unknown. Known organic P compounds include a large proportion of inositol phosphates, smaller amounts of phosphol ipids and nucleic acids, traces of sugar phosphates, phosphoproteins, glycerophosphates and phosphonates (Anderson, 1 967; Omotoso and Wild, 1 970; Tate, 1 984). The amount of organic P in the soil is control led by the processes of immobi l isation �nd mineralisation (Dalal, 1 977). Organic P can be stabil ised against mineral isation by reactions with soi l surfaces as with inorganic P (Wi l l iams and Saunders, 1 956; Will iams et aI . , 1 958; Jackman, 1 964b). Phosphorus fractionation schemes aimed at quantifying the amount of P in various labile and non-labi le soi l pools are reviewed in chapter 4. 2.4 FATE OF FERTI LISER PHOSPHORUS IN N EW ZEALAN D SOILS U N DER PERMANENT PASTURE One way of examining the fate of appl ied ferti l iser P in the soi l is by determining changes in the quantity and type of P accumulation in the soil in relation to the appl ication of fert i l iser P over time from pasture development, that is chronosequence studies. Many of the early New Zealand chronosequence studies investigated the accumulation of nutrients into organic matter (OM) due to concerns that the accumulation of nutrients in OM made them unavai lable for plant uptake (Jackman, 1 951 ; Walker et a I . , 1 959; Jackman, 1 964a&b). As ferti l iser P prices escalated and shortages were perceived in the 1 980's, emphasis in chronosequence work shifted to examining P requirements for pasture maintenance and the effects of withholding or reducing P (Lynch and Davies 1 964; Grigg 1 966; Nguyen et aI . , 1 989). 9 This section reviews the fate of appl ied P in New Zealand soils, as determined from chronosequence studies, excluding podzols. The fate of applied P in podzols is reviewed in section 2 .5. 2.4. 1 Total Soil P Total soi l P accumulates under pasture with time where P application rates exceed losses from the pasture cycle (Ooak, 1 942; Jackman, 1 951 ; Walker et a I . , 1 959; Saunders 1 959a; Jackman, 1 964a; Lambert et aI . , 1 988; Nguyen et a I . , 1 989). The rate of soi l P accumulation wi l l depend on factors such as soi l type, the form of ferti l iser P, the P appl ication rate and animal losses (Saunders, 1 959a; Lambert et aI. , 1 988; Nguyen et a I . , 1 989; Perrott et aI . , 1 992a). 2.4.2 Inorganic Soil P As with total P, the application of P ferti l iser over time onto permanent pasture results in an increase in inorganic soi l P (Pi) (Jackman, 1 951 ; Quin and Rickard, 1 983; Perrott and Sarathchandra, 1 987; Nguyen et aI . , 1 989; Perrott et a I . , 1 992a), with more Pi accumulating at higher rates of P appl ication (Nguyen et aI . , 1 989; Perrott et aI . , 1 992a). Any changes in net annual plant available Pi will be dependent on the annual appl ication of fertil iser P in respect to pasture production and uti l isation (Grigg, 1 966; Nguyen et aI . , 1 989; Perrott et aI . , 1 992a) . Olsen P (NaHC03 extractable plant avai lable Pi) wil l increase where the P application rate is in excess of the pasture maintenance requirements to balance losses of P from the cycl ing P pool (Edmeades et aI . , 1 991 a). Many papers have detai led the mechanisms for Pi retention in New Zealand soils. The fol lowing is a brief summary of their findings. The application of fert i l iser P results in an increase in both P associated with Fe and AI (alkali extractable Pi) and Ca-Pi (acid extractable Pi) (Perrott et a I . , 1 989; Condron and Goh, 1 989; Perrott et a I . , 1 992a) . In acid weathered New Zealand soils, most of the Pi 1 0 accumulates as alkali extractable AI- and Fe-Pi , where soluble P ferti l iser is appl ied (Saunders, 1 959a; Steele, 1 976; Grigg and Crouchley, 1 980; Condron and Goh, 1 989; Perrott and Mansel l , 1 989; Perrott et aI . , 1 989; Floate and Enright, 1 991 ; Perrott et a I . , 1 992a; Rowarth et aI . , 1 992a). Most of this alkali P has been found to be associated with AI , rather than Fe, in most acid weathered New Zealand soi ls (Saunders, 1 959b; Saunders, 1 965; Syers et a I . , 1 971 ; Grigg and Crouchley, 1 980) as is the case overseas (Wil l iams et a I . , 1 958; Udo and Uzu, 1 972; Lopez-Hernandez and Burnham, 1 974; Borggaard et aI . , 1 990; Singh and Gilkes, 1 991 ; Gi lkes and Hughes, 1 994). The appl ication of phosphate rock (Grigg and Crouchley, 1 980; Perrott et aI . , 1 992a), large rates of soluble P fertil isers (Rickard and Quin, 1 981 ) and lime (Condron and Goh, 1 989) have been shown to increase the Ca bound Pi fraction in relation to the sorbed P in acid weathered New Zealand soi ls. Calcium Pi accumulates in the soil as a result of the accumulation of residual Ca-Pi from P ferti l isers and the conversion of soluble Pi to Ca- Pi at high pH where the cation exchange complex is saturated with Ca (Laverty and McLean, 1 961 ; Chang and Chu, 1 961 ; Sample et a I . , 1 980). 2.4.3 Organic Soil P Most of the P applied with pasture development accumulates initially as organic soi l P (Po) in New Zealand soils where OM levels are low and C :N ratios are generally high (Jackman, 1 951 ; Walker et aI . , 1 959; Jackman, 1 960). As low N levels are overcome by N fixation by pasture legumes, C/N ratios are lowered encouraging the immobi l isation of Po and a decrease in the C/Po ratios (Jackman, 1 951 ; Walker et aI . , 1 959; Jackman, 1 960). The rate of accumulation of Po, and hence the Po/Pt ratio, decreases over time as the Po content approaches equi l ibrium (steady state) (Jackman, 1 951 ; Walker et aI. , 1 959; Haynes and Wi l l iams, 1 992; Condron and Goh, 1 989; Perrott et aI . , 1 989). The more labile Po pool reaches equi l ibrium first (Perrott et a I . , 1 989). The time to Po equi l ibrium varies and depends on P supply, soi l type and pH. Phosphorus ferti l iser application has l ittle impact on Po accumulation provided existing soil P levels, from native P or historic ferti l iser P appl ication, are adequate to supply the rate of PM accumulation. Negligible to smal l increases in organic P, C and N were found 1 1 between top dressed and un-top dressed pastures on volcanic (Burgess and Davies, 1 951 ; Saunders, 1 959a(uncorrected for bulk density)) and moderately weathered sedimentary (Floate and Enright, 1 991 ; Perrott et aI . , 1 992b(uncorrected for bulk density)) soi ls and between low P input and high P input farmlets established on 50 year old unferti l ised pastures (Lambert et aI . , 1 988). On the South Island Lismore stony loam under irrigation, Po did not increase to the same extent without added P (Nguyen et aI . , 1 989), but cessation of P fert i l iser, resulted in continuing Po accumulation with an associated decrease in Pi under pasture (Nguyen et a I . , 1 989). Non al lophanic soi ls have lower steady state OM and Po contents and reach this steady state more quickly than al lophanic soils presumably because AI stabil ises OM and Po against mineral isation through sorption and precipitation in a similar way to Pi (Jackman and Black, 1 951 ; Jackman, 1 955b; Wi l l iams et a I . , 1 958; Jackman, 1 964a&b). In a study by Jackman ( 1 964a), Po contents appeared to be independent of C, N , and S contents to some extent, indicating the operation of a separate mechanism for the retention of Po other than accumulation in OM (Jackman, 1 964a). The soi l Po pool not only includes Po found in OM, but also includes Po complexes with low C: Po ratios which are not part of the bulk OM (Kowalenko, 1 978) and can be stabil ised against mineralisation through reactions with soi l sesquioxides. Increasing soi l pH has been shown to slow the accumulation of Po in favour of Pi accumulation on a Lismore silt loam under irrigation (Quin and Rickard, 1 981 ; Condron and Goh, 1 989). Lime, which increased pH from 5.8 to 6.6, would have increased mineralisation of Po by enhancing microbiological activity (Condron and Goh, 1 989). However, increasing pH would also have led to increased solubil ity of some organic P species which are less soluble when they occur as sparingly soluble iron and aluminium salts (at low pH) than when they are sparingly soluble calcium salts (at high pH). Also the desorption rate of some organic P species, reflected by decreasing NaOH extractable Po rather than NaHC03 extractable Po (Condron and Goh, 1 989) may have increased at the higher pH. The accumulation of Pi and Po in New Zealand soi ls with pasture development is reviewed in more detail in Chapter 4. 2.4.4 Losses of Applied P from the Pasture Root Zone 1 2 P is considered to be immobile within most NZ soi ls with appl ied P generally confined to the root zone (Jackman, 1 951 ; Saunders, 1 959a; Walker et aI . , 1 959; Jackman, 1 964a; Saggar et aI . , 1 992). However, there is also some indirect evidence for more substantial losses of P through the soil where P accumulated down the profi le in other soi ls (Doak, 1 942; Nguyen and Goh, 1 992). Phosphorus can be lost from the pasture root zone as particulate and dissolved P in surface and subsurface runoff waters. In addition to the movement of P in runoff waters, P can be physically moved down the soi l profi le via the transport of dung and higher ferti l ity top soi l to lower depths by earthworms (Mackay et aI . , 1 982) and the decomposition of pasture roots in situ. (Batten et aI . , 1 979). The accumulation of P down the profi le of New Zealand soi ls is reviewed in Chapters 3 and 4, whi le the movement of P in runoff waters is reviewed in Chapter 5. 2.4.5 Other Chemical Characteristics Carbon, N and S levels rise and C/N and CIS ratios are lowered in the top soi l over time under permanent pasture, with the application of fert i l iser and l ime (Walker et aI . , 1 959; Jackman, 1 964a; Perrott and Sarathchandra, 1 987). Where the soi l is ploughed prior to pasture establ ishment, an in itial fal l in both C and N contents, due to the mineral isation of organic matter, may occur (Walker et aI . , 1 959). Continuous P appl ication results in increasing CEC, as P precipitates with AI and Fe resulting in an increased number of H ions from the phosphate radical avai lable for cation exchange (Saunders, 1 959a). Increasing OM, and hence humus levels, also lead to increasing CEC. The pH and base content increase as Ca from the P ferti l iser and l ime replace the H ions which are then leached from the soil (Saunders, 1 959a; Haynes and Wil l iams, 1 992). However, acidification also occurs with the development of leguminous pastures, and where l ime is not appl ied, pH levels wil l fal l (Bolan et aI . , 1 991 ) . 2.5 FATE OF FERTI LISER P IN PODZOLS 1 3 Before reviewing the fate of appl ied P in podzols, an understanding of podzolisation and how this changes the chemistry of a soil is necessary. 2.5.1 The Podzol 2.5. 1. 1 Podzol Development Podzol isation refers to the process whereby sesquioxides and humus are transferred to an i l luvial horizon leaving an eluvial horizon high in residual quartz and secondary si l ica and low in sesquioxides (Taylor and Pohlen, 1 970). This process is sometimes, but not necessari ly, accompanied by clay i l luviation. Podzol isation occurs throughout the world under acid leaching conditions through the influence of mor humus and high rainfal l . In New Zealand, particular tree species which produce mor humus and are associated with podzolisation are kauri (Agathis australis), rimu (Dacrydium cupressinum), Hall's totara (Podocarpus cunninghamil), kaikawaka or pahautea (Ubocedrus plumosa) and hard beech (Nothofagus sp. ) (Atkinson, 1 980) . 2.5. 1.2 Characteristics of Podzols The podzols are characterised by low natural nutrient status, pH, CEC, total base content and Anion Storage Capacities (previously known as the P Retention test, Saunders, 1 965) in their A horizons, and high variable charge and Anion Storage Capacities in their B horizons due to the accumulation, at depth, of sesquioxides and organic matter (New Zealand Soil Bureau, 1 968; Blakemore, 1 980). The A horizon of podzol ic soi ls is more coarsely textured than the less weathered soils derived from similar parent material leading to greater storage of plant avai lable water and hence drought tolerance (Gradwell , 1 980; Jackson, 1 980). However, in their natural state, some podzol ic A horizons, including that of the Wharekohe si lt loam, are massive in structure with low porosity and water storage capacities (New Zealand Soil Bureau, 1 4 1 968; Gibbs, 1 980). The dense E and B horizons result in poor drainage and waterlogging of the A horizon during wet months (Jackson, 1 980; Mol loy, 1 988). 2.5. 1.3 Podzol Classification There are sti l l differing view points through the world over the classification of podzols. The term podzol is a Russian word which was adopted by the nineteenth century soil scientist v.v. Dokuchaev to describe soils with a grey or whitish horizon near their surface. Modern Russian classification includes three types of podzols, i ) eluvial podzols characterised by the absence of both clay and sesquioxide accumulation, ii) the eluvial clay bearing type characterised by an elevated clay content below the eluvial horizon due to soi l weathering in situ and i i i ) the eluvial-i l luvial type characterised by a simultaneous accumulation of clay and sesquioxides in the i l luvial horizon, the only Russian definition which fits the western concept of a podzol (Petersen, 1 984) . The western classification systems place greater emphasis on the i l luvial (B) horizon. In the USA most podzols are classified as Spodosols (Soil Survey Staff, 1 975). The chemical criteria for a soil being classified as a spodosol is that the spodic horizon (B horizon) contains pyrophosphate (pH 1 0) extractable Fe + AI at a level greater than 20% of the clay content, the spodic horizon looses 25% of its CEC after shaking with citrate­ dithionate and that pyrophosphate extractable Fe + AI divided by dithionite extractable Fe+AI is greater than 0.5. Where extractable iron is less than 0. 1 %, extractable carbon is substituted. The Wharekohe silt loam does not fit the USDA definition of a spodosol due to its low extractable Fe + AI : clay ratio as a result of the very high clay content of the B horizon. It is classified as an Ultisol, Typic Albaquult (Soil Survey Staff, 1 975). Some British and Canadian podzols under their respective classification systems are also excluded from the spodosol class due to their not meeting the required chemical criteria (Avery et aI . , 1 977; Wang and Rees, 1 980). In the latest New Zealand soil classification (Hewitt, 1 992), podzols are defined as "acid soi ls with low base saturation having an horizon of accumulation of aluminium occurring 1 5 as complexes with organic matter and/or as short-range-order minerals (typical ly with si l icon as al lophane/imogolite) . Iron (typically as ferrhydrite) may or may not be accumulated with aluminium. This horizon is usually associated with an overlying E horizon indicating translocation. The E horizon may be missing as a result of erosion, ploughing or bioturbation or it may be masked by organic matter. " Many areas contain ing Wharekohe soi ls have been disturbed by Kauri gum digging and pasture establ ishment so that parts of the E and B horizons are found in the A horizons and the E horizon is often missing. The Wharekohe soi ls would be classified as densipan podzols in this current New Zealand Soil Classification. 2.5. 1.4 The Wharekohe Podzol The Wharekohe podzols have developed in Northland New Zealand from sedimentary sandstones, si ltstones, claystones, greywacke or colluvium and alluvium derived from erosion of these rocks (Gibbs, 1 980). They are usually associated with podocarp forests dominated by kauri trees under which H ions from the organiC complexes in the leaves replace other cations in the soil promoting the downward movement of the cations, humus and clay particles (Bloomfield, 1 953 a&b). Wharekohe soi ls are often found in a complex with other less weathered and leached soils due to the historic pattern of vegetation distribution. They are characterised by a massive grey A horizon, often only 1 0 cm deep, overlying a dense, white si l ica E horizon or pan, underneath which l ies the clay and sesquioxide rich B horizon. The properties of Wharekohe soils which need to be overcome to achieve reasonable production using legume based pastures include very low pH (pH 4-4.5), very high C:N ratios, inadequate amounts of most essential plant nutrients including P, and very poor drainage (Mol loy, 1 988). The presence of the E horizon pan and the flat to rol l ing topography result in the A horizon becoming waterlogged in winter. The E horizon also restricts roots from obtaining soi l moisture from lower down the soil profi le during dry summers. 2.5.2 Phosphorus Chemistry of Podzols 2.5.2. 1 Phosphorus in Undeveloped Podzols 1 6 Podzols generally contain less P i n their undeveloped state than other soils. In an undeveloped Wharekohe podzol (New Zealand Soil Bureau, 1 968) under kauri forest, the total P content was three times greater in the surface humus layer than the 1 00 �g PIg of soil in the A horizon. Total P then increased down the profi le so that the B horizon had a higher P content of 1 50-1 70 �g PIg. Organic P decreased sl ightly with depth, whi le Pi increased accounting for the total P increase down the profi le. The A horizon of the Wharekohe silt loam had the lowest total P content of the 54 soils recorded in the New Zealand Soil Bureau Bul letin 26 (1 968) and the lowest ever recorded in a topsoil by Deventer (Netherlands) (Jackman, 1 960). The Northern podzol ised yel low brown earth (Waikare si lty clay loam), the other Northern podzol (One Tree Point loamy sand), and the South Island podzol (Okarito peaty loam) described in the New Zealand Soil Bureau Bul letin 26 (1 968), also contained low levels of total P and simi lar patterns of P down their profi les. In contrast, the undeveloped i l luvial-humic ferruginous and gley podzols investigated in a Russian study had different quantities and patterns of P content down the profi le (Pereversev and Koshleva, 1 991 ) . Amounts of total P in each horizon were up to 30 times higher than those measured in the Wharekohe soi l . Both organic and inorganic P increased with depth from the E horizon on the two i l luvial-humic ferruginous podzols studied. In contrast, in the gley podzol, a large decrease in organic P overlaying the smaller increase in Pi with depth, resulted in total P decreasing with depth. In the gley podzol , sesquioxide P compounds made up a larger component of total P in the A horizon, however with depth, residual calcium bound P became more important. In these acid Russian podzols on silty, sandy deposits, apatite and residual P were the largest P fractions. Their high total P and Ca-P contents indicate that these Russian podzols may not be as weathered as the Wharekohe podzol due possibly to differences in vegetation and climatic conditions. 1 7 Calcium, AI and Fe-Pi , and the more stable Po extractable by NaOH are usually the largest P fractions in less weathered undeveloped New Zealand topsoi ls (Walker and Syers, 1 976; Perrott et aI . , 1 989; Haynes and Wi l l iams, 1 992) . In contrast most of the P in highly weathered sedimentary topsoi ls is expected to be found in the occluded Pi and more stable Po forms, as weathering of yel low brown earths to podzols leads to a reduction in the amount of non-occluded Fe and AI-P, Ca-P and organic P (Walker and Syers, 1 976). In a review of the l iterature concerning P fractions found in soils in natural ecosystems overseas, most highly weathered spodosols, ultisols and oxisols were also found to have negl ig ible Ca-P. However, the ult isols and oxisols contained the highest proportion of Fe and AI-Pi due to their high sesquioxide content (Cross and Sch lesinger, 1 995). Most of the highly weathered soi ls in the survey contained relatively low levels of labi le P i . However, in the spodosols with the lowest total P content (close to that in Wharekohe soi ls) , the labi le P pool was larger than the NaOH extractable Fe and AI-Pi (Pare and Bern ier, 1 989; Trasar-Cepeda et aI . , 1 990). The more stable NaOH Po was the largest Po fraction in al l soi ls in the survey. The Anion Storage Capacity of New Zealand podzolic A horizons is low, in the range of 0-20% (Mol loy, 1 988). Wharekohe A horizons generally have very low P retentions closer to O. In contrast, the Bh and Bms horizons exhibit very high P retention in the vicin ity of 80-95% due to the accumulation of iron and aluminium at depth (New Zealand Soi l Bureau, 1 968; Mol loy 1 988). Such a difference in Anion Storage Capacity between horizons is not as great in other New Zealand soi ls (Saunders, 1 965). The retention of P by A horizons samples of Canadian podzols shaken in solution with a range of added P levels has been shown to be related to oxalate-extractable Fe and AI (Laverdiere et a I . , 1 981 ; Laverdiere and Karam, 1 984) . Tamm oxalate extractable AI had a higher correlation with Anion Storage Capacity than tamm oxalate extractable Fe for most soi ls in a study of major New Zealand soi l groups including podzols (Saunders, 1 965). Anion Storage Capacity in the study by Saunders ( 1 965) was closely related to the chemical composition of the parent material and the degree of weathering and leaching of the soi l profi le. It is not surprising that the highly weathered and leached Wharekohe podzols, which are very low in sesquioxides in their A horizons, have some of the lowest Anion Storage Capacities in New Zealand. 2.5.2.2 --.�-------- Podzol Pasture P Chronosequence Studies 1 8 As with other soils, pasture development on podzols, accompanied by ferti l iser and l ime appl ication, results in an increase in pH, exchangeable Ca and K, N , P and a reduction in the C:N ratio (Powell and Taylor, 1 980; O'Connor, 1 980; Lee et aI . , 1 983). Organic P accumulation with pasture development fol lowed a simi lar pattern in a Maimai gley podzol (Powell and Taylor, 1 980) to that in other soils reviewed in 2.4.3 (Jackman, 1 951 ; Jackman, 1 960; Walker et aI . , 1 959; Condron and Goh, 1 989). Pasture development resulted in the net accumulation of P solely as organic P in the first two years from development, however both inorganic and organic P increased in the third and final year of the study (Powell and Taylor, 1 980). In a comparison of superphosphate fert i l ised and unfert i l ised pasture on the Northern podzol Te Koporu sand, (Jackman, 1 955a), 25% of the appl ied P which had accumulated in the top 1 5 cm was in the organic form after 9 years. Appl ied P has been shown to have l ittle impact on the accumulation of Po under pasture provided existing soi l P levels are adequate (2.4.3). Inadequate native P combined with the very high C :N and N :Po ratios of the Te Koporu sand in an undeveloped state have most l ikely resulted in the accumulation of further Po where ferti l iser P was appl ied. Organic P accumulation in a less weathered South Island upland podzol of higher total P content was not enhanced by the addition of fert i l iser fol lowing pasture development presumably because native P levels were sufficient to supply the accumulation of organic matter (Floate and Enright, 1 991 ) . Podzolic soi ls low in AI are expected to reach a steady state Po content which is lower and more rapidly obtained than in other soils with higher AI contents which have a greater capacity to stabi l ise the Po compounds (section 2.4.3) . Lime stimulated the mineralisation of organic P in the Te Koporu podzol (Jackman, 1 955a) as was shown for other New Zealand soi ls (2.4 .3) . ------- 1 9 The appl ication of 1 0 and 20 kg P/ha/yr over 5 years has been shown to increase Olsen P levels in the South Island Maungatua upland podzol in a simi lar manner to 4 upland yel low brown earths of simi lar parent material (Floate and Enright, 1 991 ) . However, where P application was increased to 40 kg P/ha, the resulting increases in Olsen P at al l sites were lower for the podzolic soi l and the driest yel low brown earth. Leaching of P from the podzol ised yellow brown earth and a high P retention in the yellow brown earth may have contributed to lower NaHC03 extractable Pi at both sites. The abil ity of podzol ic soi ls to retain added P has been shown to reduce as the few avai lable P retention sites become occupied by appl ied P ferti l iser (Jorgensen and Borggaard, 1 992). 2.5.2.3 Losses of P from New Zealand Podzols Large losses of applied P from the top soi l have been estimated for New Zealand podzols under pasture. In a P fertil iser form comparison trial on a Te Kopuru podzol soi l in Northland, only about 1 0% of the fertil iser P applied at 34 kg P/ha/yr had been retained in the top 1 5 cm after 9 years of application (Jackman, 1 955a). The author assumed that the remaining P (apart from animal losses) had moved down the profi le to the iron pan. Where superphosphate was applied, the proportion of Po increased with depth. Accumulation of Po at depth does not necessari ly mean that P is moving as Po to depth, as Pi may be converted to Po in situ. This study of Jackman's did not differentiate these mechanisms. Losses of appl ied P from South Island podzols have also been reported. Floate and Enright ( 1 991 ) only recovered 1 8% of the 240 kg/ha P appl ied to a Maungatua upland podzol over 5 years in the top 7 .5 cm compared to up to 68% for the upland yellow brown earths in the same study. Sampl ing to 30 cm fai led to recover further P in the podzol but increased recovery in the other soils. Removal of P in clippings could only account for another 4% of the P not recovered at the podzol ised site and so either lateral P movement or movement of P beyond 30 cm in runoff waters was the most l ikely pathway for P loss. 20 Of the 220 kg P/ha appl ied over a 5 year period, 82% was recovered in the top 1 5 cm in an Addison peaty loam whereas only 33% was recovered in the top 1 5 cm of the areas of Addison silt loam on the same site (O'Connor, 1 980). O'Connor concluded that this was most l ikely due to movement of P from crests (silt loam) to hollows (peaty loam) rather than a greater affinity for P by the peat loam as both soi ls had low Anion Storage Capacities. In an earl ier study, McNaught and During ( 1 970) noted that the inabil ity of 84 kg P/ha/yr to prevent P deficiency in pasture on an Addison soil was most l ikely due to the very low P retention accompanied by high rainfal l inducing lateral movement of water and nutrients on this soil with a shallow pan. Phosphorus (acid extractable) fai led to accumulate in an Okarito si lt loam beyond 3 years despite continued application of approximately 40 kg P/ha/yr as superphosphate (Lee et a I . , 1 983). The Okarito silt loam has an impenetrable pan close to the surface, as in Wharekohe soi ls, which restricts water movement resulting in waterlogging for large parts of the year and lateral movement of runoff water. Such large losses of P from these New Zealand podzols are unl ikely to be accounted for by the physical movement of P alone, and hence losses of P dissolved in surface and subsurface runoff water are l ikely. Large losses of applied P have been recorded in the laboratory from repacked cores of the Northland podzols, Wharekohe si lt loam and Te Kopuru sand (up to 36%) (Hogg and Cooper, 1 964) and the South Island Addison gley podzol (42%) (McSweeney and Mul ler, 1 979). Considerable movement of P down repacked cores of an Austral ian gley podzol has also been recorded (Lefroy et a I . , 1 995). These laboratory studies indicate the potential for subsurface runoff losses of P from the podzols. The above review of P losses from podzols are not surprising given their low abil ity to retain added P and the heavy rainfall experienced by the podzolic soi ls of New Zealand. The potential for subsurface runoff P losses from the Wharekohe podzol has already been demonstrated and if they behave in the field as the other New Zealand podzols examined in this l iterature review, significant losses of applied P from the profi le could be experienced. Large losses of P from the soi l profi le have a cost not only economical ly, through inefficient ferti l iser use, but also to the environment, through increased P loading 21 of water ways inducing the potential for eutrophication. Methods for reducing runoff P losses from soi ls susceptible to large P losses in this way are reviewed in chapter 5. 2.6 MODELLING P FERTILISER REQUIREM ENTS The accumulation of P over time under permanent pasture with ferti l iser P appl ication can contribute to pasture growth so that eventually only maintenance applications of P are required to balance any losses from the pasture P cycle and maintain pasture growth at the required relative yield and stocking rate (Karlovsky, 1 966; Cornforth and Sinclair, 1 982) . Over the past 30 years numerous models for predicting the amount of P required to maintain pasture production have been developed (Karlovsky, 1 966; Bowden and Bennett, 1 975; Helyar and Godden, 1 977; Cornforth and Sinclair, 1 982; Metherell et aI . , 1 995). Most P ferti l iser requirement models are based on the Mitscherl ich equation. Y = 1 00 - Be�x where Y = relative yield (%) 1 00 = maximum yield B = 1 00 - production without ferti l iser C = response factor X = fert i l iser addition The Cornforth and Sinclai r ( 1 982) approach differs from other models incorporating the Mitscherlich equation, in that P losses are related to the required relative yield giving a P maintenance requirement curve rather than a pastoral response curve (Helyar and Godden, 1 977). Models used to investigate the cycl ing of P under grasslands and to predict P requirements for grazed pastures and some of the l imitations of these models have been 22 well reviewed by Rowarth, ( 1 987) . This literature review includes a brief description of the Computerised Ferti l iser Advisory Scheme (CFAS) model developed by Cornforth and Sinclair ( 1 982) and examines the sensitivity of the model to variation in parameter estimation on maintenance P requirements, in order to determine which aspectls of the model contribute to the apparent underestimation of P requirements on the Wharekohe podzols. 2.6. 1 CFAS Model The CFAS model has been developed from a balance sheet approach to the calculation of P maintenance requirements originally proposed by Karlovsky ( 1 966; 1 975a&b; 1 981 ; 1 982) . The amount of P required to maintain pasture production is calculated as the amount of P required to replace losses of P from the cycl ing P pool via soi l P accumulation and runoff P losses, and animal products and excretal transfer, where a 'steady state' exists. A steady state refers to the size of the cycl ing pool, reflected by Olsen P status, remaining constant from year to year. Cornforth and Sinclair ( 1 982) confined their model to pastures maintained at a 'steady state' so that there was no need to quantify flux rates between various compartments of the cycl ing P pool or to consider residual value of ferti l iser P which are important aspects of other models (Blair et a I . , 1 977; Bowden and Bennett, 1 975). However, in podzols and other soi ls of low Anion Storage Capacity, the retention of added P in the soi l decreases with increasing P fertil iser appl ication and pasture age (Weaver et a I . , 1 988; Borggaard et aI . , 1 990). If the net rate of accumulation of non-labile P in the soi l decreases with increasing P application and pasture age, the P cycle wi l l not be in a 'steady state', and the calculation of P requirements from the CFAS model would be inappropriate. The maintenance P equation involves the calculation of i) the total P lost from the cycl ing pool when the relative yield is 90% of the maximum and ii) the total P loss associated with the required level of production, giving an overal l equation of: 23 Maintenance P requirement = log1O{1 00/[1 00 - (8550 x SR)(CC x PUm x CC x (0.005 x CC + 0.275) x (PU x ALF x 0.0301 + SLF x 5.79). The parameters used in the calculation of the P maintenance requirement are: i) stocking rate (SR) - number of stock units ha-1 , where each stock unit is considered to consume 550 kg of dry matter annually. i i ) pasture uti l isation (PU) - % of pasture grown which is eaten by stock. i i i } potential carrying capacity (CC) - number of stock units ha-1 which can be carried where PU=90% and pasture production is maintained at 95% of Ymax. iv} animal loss factor (ALF) - amount of P lost per stock unit (kg/SU) , in animal products and by transfer of excreta to concentrated and unproductive areas, grazing pasture maintained at 90% of Ymax (dependent on topography and grazing intensity). v) soil loss factor (SLF) - amount of P lost from the pasture cycle to the soil, by the accumulation of organic and inorganic compounds which are unavai lable to plants, erosion, and leaching, expressed as a fraction of the total P uptake by pasture maintained at 90% of Ymax. (Cornforth and Sinclair, 1 982) Once the maintenance P requirement has been calculated using the model , it is adjusted to account for the amount of avai lable P in the soi l (measured by the Olsen P test) through the use of a modifying factor. The modifying factor was calculated using publ ished data for changing Olsen P with P appl ication. The output of the model is then the current P requirement necessary to sustain or gain pasture maintenance at 90% of Ymax. The modifying factor corresponding to a particular Olsen P wi l l depend on the relative yield of the pasture, in relation to current and potential stocking rates, and the P loss category of the soi l (Cornforth and Sinclair, 1 984) . Three P loss categories exist, low, medium and high. 2.6. 1. 1 24 Sensitivity of Calculated P Requirements to Incorrect Estimation of Model Parameters The impact of incorrect estimation of model parameters on the calculation of P maintenance requirements has been investigated by several authors (Parker, 1 982; Rowarth, 1 987; Scobie and St-Pierre, 1 987a). The examples examined by Parker ( 1 982) can be related to a dairy farm on a Wharekohe soil where the CC is considered to be 1 8 (Cornforth, 1 988) and the ALF is 0.9 (Cornforth and Sinclair, 1 984) . Stocking Rate Stocking rate can be difficult to estimate where areas of the farm differ in their development, soil type, topography and stock type. The incorrect estimation of stocking rate can have a large impact on P requirements particularly at higher stocking rates. Changing the SR from 1 5 to 1 6 can result in a 35% higher P requirement where CC= 18, ALF = 0.90 (Parker, 1 982) Pasture Utilisation Under hi l l country and extensive grazing conditions pasture uti l isation is difficult to assess (Sinclair and Cornforth, 1 984) . The incorrect estimation of PU has greater impact when the PU is low. For example, changing the PU form 85% to 80% results in a 1 0% increase in the maintenance P requirement, whereas changing the PU from 70% to 65% results in a 35% increase in the maintenance P requirement, where CC=1 8, ALF=0.9, SLF=OA and SR= 1 3. Pasture util isation can change markedly on the Wharekohe podzol when heavy rainfall leads to waterlogging, pugging and the loss of pasture which is trampled into the top soi l . Carrying Capacity In the example given by Parker ( 1 982) changing the CC from 1 8 to 20 resulted in a 1 2% decrease in the maintenance P requirement, where SR =1 3, ALF=0.9, SLF=OAO and PU=80%. He concluded that an incorrect estimate of CC was shown to have the smal lest impact on the P maintenance requirement of all the parameters. However where the SR is increased to 1 6, a change in CC from 1 8 to 20 results in a 26% decrease in the amount of P required for pasture maintenance. ----�- 25 Animal Loss Factor The ALF values used in the model were calculated from a single large grazing trial on hi l l country (Gi l l ingham, 1 980a&b; Gi l l ingham et aI . , 1 980a) and hence these values may have been influenced by any of the aspects of trial design, paddock size or grazing management peculiar to that trial (Rowarth, 1 987). Rowarth's data supported the use of a common ALF over a range of stocking rates for a given topography in the CFAS model. However, her estimates of ALF were lower than those used by the model and she attributed this to lower pasture P concentration, animals selectively grazing pasture with a higher P concentration and the topographically flatter nature of her trial area. The model assumes that the animal loss is l inearly related to the pasture P concentration. However, where the P concentration of pasture does not lead to a deficiency, the removal of P in animal products does not increase l inearly with pasture P concentration and animals excrete P in excess to their requirements (Bromfield, 1961 ; Barrow and Lambourne, 1962; Rowarth, 1 987). Consequently, where P concentration in pasture is higher than 0.35%, there wi l l be less P transferred per stocking unit than is presently accounted for in the model. Such a discrepancy has a negl igible impact on the maintenance P requirement. Even substituting an incorrect ALF in the model has l ittle effect, for example changing the ALF from 0.9 to 1 . 1 where the SLF=0.40, PU=80%, CC=1 8 and SR=16, only results in a 1 0% difference in the calculated P requirement. Soil Loss Factor Estimates of SLF values used in the model were calculated, from P rate trial data, where soi l loss was determined as the difference between ferti l iser P inputs and the amount of P removed in clippings where avai lable P (as measured by Olsen P) was in a steady state. However, due to the scarcity of wel l conducted and rel iable maintenance trials, much of the data was drawn from trials which were not really suitable for the calculation of SLF values (Cornforth and Sinclai r, 1 982). Little field data was avai lable to determine the SLF for Wharekohe podzols ( I . Cornforth, pers. \comm? There was also l ittle data avai lable to determine if differences in trial conduct can influence the calculation of SLF values (Rowarth, 1 987). Total rel iance was placed on the Olsen P soi l test for assessing steady state. The Olsen P test is known to be variable, both spatially and temporally (Edmeades et a I . , 1 988), and some doubt as to its reliabil ity for establishing a 'steady state' exists (Rowarth, 1 987) . 26 Soi l loss factors determined for the three soi l P loss categories were 0. 1 0 ( low soil P loss), 0.25 (medium soi l P loss), and 0.40 (high soil P loss). The soil P loss categories appeared to be closely related to P retention except for in soils with low P retention capacities under moderate to high rainfal l or irrigation where subsurface runoff P losses probably contributed to a higher SLF (Cornforth and Sinclair, 1 982) . Groupings were tentative and the authors of the model acknowledged the need for further research. Not only may a soil be incorrectly categorised, but a soi l placed in the low SLF category could feasibly have a lower SLF than 0. 1 , whi le a soil categorised as a h igh SLF soil may in fact have a higher SLF than 0.4. The northern podzols were initially placed in the high SLF category (Cornforth and Sinclair, 1 984), however this has been changed at times by the computer fertil iser advisory service to a medium soil loss as a result of consultant observations rather than any scientific investigations (P. Shannon pers. comm. ). Consultant observations have also indicated that a higher SLF may be more appropriate for the Wharekohe sandy loam compared to the Wharekohe silt loam (P.Shannon pers. comm.) . Parker ( 1 982) noted that using the wrong SLF in the model can result in large errors in the calculation of maintenance P requirements. For example, 36% more P is required when a low P loss soil is accidental ly classified as a medium loss soi l and 26% too much P calculated when a medium loss soil is classified as a high loss soi l , where CC=1 8, ALF=0.9, SR= 1 3 and PU=80%. Modifying Factor The modifying factors used to predict current fert i l iser requirements have been establ ished from trial data where the optimum Olsen P value to maintain pasture production at a particular RY and the Olsen P value at which no fertil iser P input is required to maintain production are used to establ ish a l inear relationship between the modifying factor and Olsen P for each RY. Problems with applying the modifying factor arise through anomal ies in trial data used to calculate the modifying factors and through the use of incorrect estimates of RY or Olsen P. Olsen P is of particular concern as variabi l ity of up to 40% can be encountered in its estimation under field conditions (Edmeades et aI . , 1 988), although variabi l ity wi l l be less under intensively sampled trial plot conditions. 27 The modifying factors used in the CFAS model calculate a current P requirement aimed at obtaining optimum labile P levels. If a farmer is below the optimal labi le P level and SR, then the P recommendation is aimed at gradually increasing the soil P to optimum levels over several years making ferti l iser costs more affordable for the farmer (P Shannon pers. comm. ) . However, this approach does not take into account that a more rapid increase in P appl ication rate and hence soil P levels may in fact be more economic as farm finance structures change. Scobie and St-Pierre ( 1 987b) showed that capital dressings to raise soi l P levels to optimum in one year are more economic than the approach of Cornforth and Sinclair ( 1 984) where a more gradual build up in soi l P is recommended through the use of the modifying factor. Capital P dressings have become more common over the last three years in Northland where soil P levels are frequently below optimum due to a decrease in ferti l iser use in recent years. Economic Impact of Incorrect Parameter Estimation Scobie and Pierre ( 1 987a) examined the cost of incorrectly determining P maintenance on gross profit margin and determined that an error of 20% would be needed in al l parameters to induce a significant economic loss. Economic loss was greatest where P was underestimated rather than overestimated, with a more than 20% error in all parameters resulting in a 1 0% loss in gross margin of course net loss more. However, the authors used high unit P costs (an average of $2.50 compared to $1 .66 (superphosphate) and $1 .34 (RPR) per kg P calculated in 1 991 ) and a very low kg butterfat pay out ($2.50 compared to the $5.53 averaged over the 5 years from 1 989 to 1 993) and hence economic losses would be far greater in the 1 990s where incorrect parameter estimation leads to an underestimation of required P . 2.6. 1.2 Modifications to the CFAS Model Since its inception, various modifications have been made to the CFAS model and also in the way it is used by consultants to overcome some of the practical problems associated with its use. The rate constant 0.005 used to estimate pasture P concentration at 90% Ymax from the CC was lowered to 0.003 to lower estimated pasture P concentrations, and hence estimated losses of P from the pasture cycle. Where a rate constant of 0.005 previously equated to a pasture concentration of 0.4% at ---- - 28 a CC of 25, substituting 0.003 equates to a pasture concentration of 0.35% for a CC of 25. As previously mentioned the SLF allocated to the Wharekohe podzols has been changed at times by the computer fertil iser advisory service from 0.4 (high P loss) to 0.25 (medium P loss). In order to overcome problems associated with the estimation of PU and CC, which can have a marked effect on the P requirements calculated by the orig inal CFAS version, Sinclair and Cornforth ( 1 984) developed a modified version. The ful l version has been modified by assuming that relative yield should not be allowed to fal l to such low levels that species composition and pasture qual ity decl ine. Hence, the fol lowing assumptions apply: RY should be maintained at 75% when SR is half CC, when SR=CC then RY should equal 95% and that for other stocking rates RY should be l inearly related to SR:CC. These assumptions result in a modification of the model where PU is substituted by (SR*8550)/(55CC+40SR) and hence: Maintenance P (kg/ha)= loglO[1 00/(45 - 40SRlCC) x CC x (0.003 x CC + 0.275) x [(SR x 8550 x ALF x 0.0301 )/(55CC + 40SR)+SLF x 5.79] The impact of incorrect CC estimation has been substantial ly reduced, for example a CC change from 1 8 to 20, as g iven in the earl ier example, resu lts in only a 1 % decrease in calcu lated P requirements compared to a 26% decrease when the fu l l version is used. The ful l version was sti l l advocated when PU and CC cou ld be accurately predicted. The CFAS model , although having the advantage of simplicity and ease of use in its modified form, suffers from the lack of economic, soil P status, pasture or animal production outcomes with changing ferti l iser application strategies (rates, timing of appl ication or form) . Consequently, the CFAS model has also been modified to overcome some of these shortcomings. The CFAS model has been extended to include the economic consequences of fertil iser decisions (Scobie and St-Pierre, 1 987a; Sinclair and Rodriguez Julia, 1 993), the residual 29 effects of fertil iser use (Scobie and St-Pierre, 1 987b; Metherel l , 1 988), animal response to pasture consumption and qual ity (St-Pierre and Scobie, 1 987a; Metherel l , 1 988), and the incorporation of environmental risk (St-Pierre and Scobie, 1 987b). Metherell 's were the only changes incorporated into the CFAS model by the SFS. Metherell ( 1 988) used the current soi l P status and fertil iser input to calculate the expected relative yield, which combined with other parameters from the CFAS model , was used to predict the size of the cycling P pool and ferti l iser recommendations for the fol lowing three years. The CFAS model was used by MAF consultants to predict P requirements for New Zealand pastoral systems until March 1 994. The present AgResearch SFS is no longer using the CFAS model for the fol lowing reasons: i) misuse of the maintenance model in the pasture development phase where a 'steady state' does not exist, i i ) lack of economic outcome, and i i i ) PU and CC parameters being difficult to quantify. 2.6. 1.3 Difficulties with the CFAS model on Wharekohe soils As stated in the Chapter 1 , in 1 989 it was real ised that higher P rates than predicted by the CFAS model were required to maintain appropriate Olsen P levels on many of the farms in Northland on Wharekohe soils. To document the existence of a problem with the CFAS model, soi l tests and ferti l iser requirements dating back to 1 984 from 3 farms on Wharekohe and volcanic soi ls were examined (Table 2. 1 ) . Included in the tables are the maintenance P requirements (calculated from the modified CFAS model (Sinclair and Cornforth, 1 984) and current P requirements (maintenance requirement multipl ied by the modifying factor based on the Olsen P soi l test) for each site. All of the farms examined in table 2. 1 are dairy farms. The Chamberlain and Inverarity properties have been under establ ished pasture for more than 30 years. The sites examined on Baxters' property were on newly established pasture (early 1 980s). The Olsen P values, due to variabil ity (40%) associated with their estimation, cannot be interpreted as infal l ible estimates of the true mean of the Olsen P for each site in each Table 2 . 1 Changes i n soil Olsen P levels with P fertil iser addition and calculated maintenance and current P requirements on three properties on Wharekohe soi ls in Northland. a) Baxter's property near Kerikeri Date Olsen P Applied P Maintenance P Current P (MAF Quicktest) Requirement Requirement (kg/ha) (kg/ha) (kg/ha) Site A: Wharekohe silt loam 1 984 spring 26 25 1 2.5 1 1 .5 1 985 autumn 1 8 24 1 2.5 1 9 1 985 spring 25 24 1 2.5 1 2.5 1 986 autumn 1 7 24 1 2.5 20 1 986 spring 1 2 Site B : Wharekohe silt loam 1 984 spring 35 25 1 2.5 3 1 985 autumn 27 24 1 2.5 1 0.5 1 985 spring 1 8 24 1 2.5 1 9 1 986 autumn 1 2 24 1 2.5 45 1 986 spring 1 5 Where: RY=88.3%; SR=1 5; CC=1 8; ALF=0.9; SLF=OAO b) Chamberlain's property near Taheke Date Olsen P Applied P Maintenance P Current P (MAF Quicktest) Requirement Requirement (kg/ha) (kg/ha) (kg/ha) Site A: Wharekohe sandy loam 1 988 autumn 22 56 28 34 1 989 autumn 1 4 31 26 49 1 990 autumn 1 1 Site B: Wharekohe sandy loam 1 988 autumn 1 1 56 28 58 1 989 autumn 1 3 60 26 51 1 990 autumn 1 8 Where: RY=90%; SR=16( 1 988/89), 1 5. 5( 1 989/90); CC=1 8; ALF=0.9; SLF=OAO 30 c) Inverarity's propery near Taheke Date Olsen P Applied P Maintenance P Current P (MAF Quicktest) Requirement Requirement (kg/ha) (kg/ha) (kg/ha) Site A: Wharekohe sandy loam (WK) 1 984 spring 21 52 26 34 1 985 52 26 34 1 986 52 26 34 1 987 95 26 34 1 988 autumn 8 Site B: Ruatangata friable clay (Rl) 1 984 spring 1 3 52 25 48 1 985 52 25 48 1 986 52 25 48 1 987 95 25 48 1 988 autumn 25 Where: RY=89.4 (WK), 86 (RT); SR=1 5.5; CC= 18 (WK), 20 (RT); ALF=0.9; SLF=O.4 (WK &RT) 3 1 year, especially as each soi l test result was from a l imited, but recommended, number of cores to give a bulk soil sample with no repl ication. However, the cumulative Olsen P test values measured over time can be used to establ ish possible trends in the changing avai labi l ity of P in the soi l . Olsen P levels have fal len, despite P applications in excess of maintenance requirements calculated by the CFAS model, on al l of the Wharekohe soi ls which were in itially above or near the optimum Olsen P value of 25. In contrast, Olsen P levels on the volcanic Ruatangata soil on Inveraritys' have increased, from low to optimum Olsen P, with the same amount of P as was appl ied to the Wharekohe sandy loam on that farm. Where the Olsen P increased from low to medium on the Wharekohe sandy loam site on Chamberlains' , inputs of P were greater than on the site where Olsen P values dropped on that farm. Both the Wharekohe soils and Ruatangata soils have the same SLF in the model of 0.40 (although at times 0.25 has been used for Wharekohe soi ls). The maintenance P requirements calculated by the CFAS model (Table 2. 1 ) appear to be inadequate to balance the P losses from the system on the Wharekohe soils. 32 It is unl ikely that either an incorrect CC or ALF assessment are contributing reasons for a drop in the Olsen P levels on the Wharekohe soi ls. The CC of 1 8 is most l ikely an underestimation. A value closer to 20 may be more real istic (Lambert et aI . , 1 979; Rumbal l and Boyd, 1 980). However, the use of the modified version of the CFAS model reduced the effects of an incorrect estimation of CC. The ALF was expected to have been similar at both the Wharekohe and volcanic sites on the Inverarity property due to simi lar topography. Differences in animal loss/ha cannot be discounted ful ly as stocking rates may have been different for the soi l types and smal l changes in SR can have a large impact on P requirements as described earlier. Each of the dairy farms described in table 2. 1 have large areas of volcanic soils. Given the difficulty in grazing Wharekohe soils in wetter months and the higher production usually obtained on volcanic soils (reflected by the higher CC values), SRs are more l ikely to have been overestimated on the Wharekohe soils on the Chamberlain and Inverarity properties as the SR was calculated as an average over the whole farm. The calculation of stocking rates for each soi l type was not possible on these properties. However, the overestimation of SRs would have led to an overestimation of calculated P requirements and so the surveys sti l l indicate that calculated P requirements were most l ikely insufficient to maintain Olsen P levels in most cases. Under estimation of the SLF appears to be the major reason for low predictions of P requirements on Wharekohe soi ls. The other parameters in the model are unl ikely to be influential as P fertil iser predictions for volcanic soi ls have not been of concern on at least one farm in Table 2. 1 . 2.6.2 P Requirement Models Developed Since the CFAS Model Other models which have been developed in New Zealand to predict pasture P requirements include the incorporation of the residual effect of applied P (McCal l and Thorrold, 1 991 ) , the use of reactive phosphate rock (RPR) (Sinclair and Johstone, 1 994; Sinclair et aI . , 1 993; Roberts et ai , 1 994; Metherel l , 1 994), and the economic impact of ferti l iser management strategies (Metherel l et a I . , 1 995). 33 The present AgResearch SFS is now marketing the 'Outlook' model developed by Metherel l et al . ( 1 995) to consultants who make ferti l iser P recommendations to farmers ( Introduced in 1 994) . The Outlook model uses the mass balance approach where P requirements are calculated to replace P lost from the cycling P pool via non-labile P accumulation and runoff losses, and animal losses in excretal transfer and product removal , simi lar to the CFAS model. The Outlook model also assumes that the cycl ing P pool is maintained in a steady state in the calculation of P maintenance requirements. The Outlook model accounts for P added to the cycling pool via soi l weathering whereas the CFAS model did not. The animal loss parameter has been assigned different values, divided into product losses and animal transfer rates, than those used in the CFAS model based on trials reviewed by Metherel l ( 1 994). The model assumes that soi l P loss wi l l be a constant proportion of the labile P pool, estimated from the Olsen P test by a curvi l inear relationship for each soi l group category. The CFAS model also assumed soil loss was a constant proportion of the labile P pool , but expressed the soil loss factor as a proportion of P pasture uptake. Outlook divides the soi l groups into 4 soi l loss categories; sedimentary (soil loss 0.04), volcanic (soil loss 0.05), pumice (0. 1 O) and high loss, the podzols (soi l loss 0. 1 0). The Outlook model is extremely sensitive to the al location of soil group categories with maintenance P requirements calculated for the podzol soi l group more than twice those calculated for the sedimentary and volcanic soi l groups (Metherel l et aI . , 1 995). Phosphorus requirements calculated by the Outlook model for northern podzols are more than twice those calculated from the CFAS model . The higher soi l losses assigned to the podzols by the Outlook model compared to the CFAS model may address the apparent underestimation of soil P loss found in the case studies in table 2. 1 and is investigated in chapter 7. 2.7 SU M MARY OF LITERATU RE REVIEW • In a grazed pasture system, P cycles through above and below ground pools. Phosphorus is added to the cycl ing pool from the weathering of minerals in the native parent material and P fertil iser appl ication, and lost from the cycl ing pool through transfer of excreta to unproductive areas and concentrated areas such as campsites, in animal ------ 34 products and through soil losses such as net immobil isation, net sorption/precipitation, erosion and runoff waters. • Total P and Pi accumulate in the soi l under pasture with time where P appl ication rates exceed losses from the pasture cycle. Inorganic P accumulates mostly as P associated with Fe and AI in the acid, weathered soils of New Zealand. Organic P accumulation dominates in the initial years fol lowing pasture development where OM levels are low and C :N ratios are high, as N fixation by pasture legumes leads to a lowering of C/N rations encouraging OM accumulation and the organic immobi l isation of P . The rate of Po accumulation slows as an equi l ibrium is approached with the most labile Po pools reaching equil ibrium first. P is considered to be immobi le in most New Zealand soi ls . • Podzolisation results in low sesquioxide contents and consequently low Anion Storage Capacities, and extremely low natural P contents in the A horizons of the more weathered podzols. The largest proportion of th is natural P is found in the labile P pool. Pasture development on podzols leads to P accumulation which initially accumulates as Po. Podzol ic soi ls low in AI are expected to reach an equi l ibrium Po content which is lower and more rapidly obtained than in other soi ls with higher AI content which have a greater capacity to stabi l ise Po compounds. The abil ity of podzols to retain added P is reduced with increased ferti l iser application as the few avai lable P retention sites become occupied with the added P. Large losses of P have been recorded from the A horizons of New Zealand podzols and the potential for subsurface P runoff losses has been demonstrated. • The accumulation of P over time under permanent pasture with ferti l iser P appl ication can contribute to pasture growth so that eventual ly only maintenance appl ications of P are required to balance any losses from the pasture P cycle and maintain pasture growth at the required relative yield and stocking rate. Various models have been developed to predict P requirements for pastoral systems in New Zealand. During the 1 980's and early 1 990's, the then MAF SFS used the Cornforth and Sinclair CFAS model, where P requirements are calculated to replace losses from the cycl ing P pool, to make ferti l iser P recommendations. Investigation of several farms, where the CFAS model was underestimating the amount of P required to maintain soil Olsen P values on Wharekohe 35 soi ls, has identified the CFAS SLF as the parameter most l ikely leading to the inabil ity of the CFAS model to predict P requirements on these podzols. The Wharekohe sandy loam may require a higher SLF than the Wharekohe si lt loam . • The CFAS model is no longer used by the present AgResearch SFS who are now marketing the new Outlook model to ferti l iser consultants. H igher soi l P losses have been assigned to the podzols in the Outlook model, resulting in calculated P requirements of twice those calculated by the CFAS model, which may address the apparent underestimation of soi l P loss determined for the Wharekohe soils in this review . • This review of the l iterature has confirmed the need to establish the extent and nature of soi l P losses from the Wharekohe podzols so that pasture P requirements can be modified and unnecessary costs, to the farmer economically and to the environment through increased potential for eutrophication, be minimised. 3.1 INTRODUCTION CHAPTER 3 FATE OF APPLIED P 36 When a phosphatic fert i l i ser is appl ied to pasture the water soluble component is d issolved by moisture from both the air and surrounding soi l . The dissolved fert i l iser P can either remain in solution, form sparingly soluble in it ial reaction products, become weakly sorbed, be taken up by plants or be lost from the P cycl ing pool . Phosphorus is lost from the P cycl ing pool via the soi l through both its accumulation within the soil profi le as non-labi le P and loss from the soi l profi le through water movement as surface soi l erosion and leaching either vert ical ly or lateral ly. These P losses constitute the soi l loss factor (SLF) used in the CFAS model for New Zealand cond itions. Phosphorus is general ly considered to be an immobi le anion in most soi ls and this is certainly the case in most New Zealand soi ls where a large port ion of the appl ied P is retained in the topsoi l (Jackman, 1 951 ; Walker et aI . , 1 959; Haynes and Wi l l iams, 1 992) . However, evidence exists for losses of appl ied P from podzol ic soi ls both in New Zea land (Jackman, 1 955a; O'Connor, 1 980; Lee et a I . , 1 983) and overseas (Khanna et aI . , 1 992; Lefroy et a I . , 1 995) suggesting that in these soi ls P is mobi le. In comparison to other mineral soi ls, the Wharekohe podzols of North land New Zealand are characterised by very low anion storage capacities, which may eventual ly l imit P accumulation and lead to the movement of appl ied P down and through the profi le and eventual ly from the soi l to the wider environment. Therefore, it is possib le that the magnitude and type of soil P losses from the pool of cycl ing P in Wharekohe podzols may change with time from in itial pasture development. Any losses of appl ied P from the soi l represent a cost both to the farmer and to New Zealand society through increased P load ing of water ways creati ng the potential for eutroph ication. ------ 37 Losses of P from the soil can be quantified by di rectly measuring P loss in erosion material and runoff water. However, such experiments can be time consuming, expensive and do not provide a historic perspective on P losses. Alternatively, where farm fert i l iser and stocking history are known, appl ied P which has been lost from the soi l can be calculated as: P lost from soi l = Appl ied P - (Animal P Loss + P accumulated in the soi l ) . The original DSIR Grasslands Kaikohe Research Station provided an ideal environment to assess the fate of appl ied P on Wharekohe podzols at various stages of pasture development, as pastures have been developed over a period spanning 24 years, and reasonably accurate fert i l iser and stocking records were avai lable. 3.2 OBJ ECTIVES This chapter considers the fol lowing specific objectives of the research project: 1 . The determination of the fate of appl ied fert i l iser P in a pasture development chronosequence on the Wharekohe si lt loam at the Kaikohe Research Station by measuring total soil P and, by calculation , P losses. (The accumulation of P into plant unavai lable soi l fractions in the chronosequence are presented and d iscussed separately in Chapter 4. ) 2. The determination of the effect of the stage of pasture development on plant avai lable P (as measured by Olsen P) . 3 . The determination of the effect of the stage of pasture development on pasture production and P uptake. 3.3 MATERIALS AN D M ETHODS 38 To meet the above objectives, two separate investigations were conducted. Soi ls for the first study were sampled in the spring of 1 990. To confirm and extend the initial findings, further samples were col lected in the spring of 1 993. 3.3. 1 Site Selection Wharekohe silt loams are found on the rol l ing areas of the Kaikohe Research station. There are two variants on the station, the Wharekohe si lt loam (with pan) and the Wharekohe si l t loam (without pan) , the pan being the dense pale eluvial (E ) horizon. In addition, the Wharekohe si l t loams exist in a complex with the podzol ised yel low-brown earth Hukerenui si l t loam. Most of the Kaikohe station had been dug over by Kauri gum diggers and hence the topsoi l , and in some places the E horizon, have been disturbed on all sites including the undeveloped sites. Where areas have been developed to pasture, the process of pasture development involved deep ploughing which has resulted in further inversion of the soi l 's upper horizons and often disturbance of the E and B horizons. Hence, in some areas, the E horizon has been broken up and bits of E and B horizon have been turned up into the surface soi l . Therefore, large un iform areas of Wharekohe si l t loam with the pan intact are d ifficult to find on the Kaikohe Station. Wharekohe si lt loam (with pan) was the soi l type chosen for this study because it is the more dominant variant and is considered to be the "typical" Wharekohe podzol in Northland. It is at the extreme end of the leaching scale and hence different sites are expected to behave more simi larly in respect to P chemistry than the less leached Wharekohe s i lt loam without pan. The latter is l ikely to be more variable and is easi ly confused with the less weathered Hukerenui and disturbed areas of Wharekohe s i lt loam with pan. In spring, 1 990, non-campsite areas of Wharekohe silt loam (with pan) on s lopes of between 4° and 1 4 0, covering at least 1 00 square metres were identified over a ------- - 39 range of pasture ages (Table 3 . 1 ) . The sites had s imi lar annual P fert i l iser histories as out l ined in 3 .2.2 . In order to confirm that the conclusions reached from the individual p its in 1 990 were applicable to a wider area, four further sites from each of the areas developed to pasture in 1 958, 1 960, 1 967 and 1 982 and the undeveloped area were identified as described above in spring 1 993. Sampling sites were spread over 2 to 3 paddocks for each pasture age and a simi lar area in the undeveloped region. Table 3 . 1 . Descriptions of Wharekohe si lt loam sites selected in 1 990. Year Pasture Age Slope Developed 1 990 (years) Undeveloped Undeveloped 5° 40' Undeveloped Undeveloped 1 1 ° 30' 1 982 8 6° 20' 1 967 23 1 4° 1 965 25 4° 20' 1 960 30 9° 1 958 32 6° 1 0' 3.3.2 Ferti l iser and Lime History Fertiliser History Average Depth to Average Depth of E top of the E Horizon horizon (em) (em) 1 2. 8 1 0 . 0 1 4. 0 7 . 1 1 7 . 5 1 1 . 0 1 0. 1 8 .9 1 3.0 1 5.4 1 4.4 8 .6 23.8 1 1 . 8 The total amount of P appl ied since in it ial pasture development at the sites selected in 1 990 and 1 993 are presented in Table 3 .2. Approximately 1 000 kg/ha of superphosphate was commonly appl ied in the first year of pasture development at a l l pasture age sites except the youngest area, developed in 1 982 , where 800 kg/ha superphosphate was appl ied in it ia l ly . 40 Table 3.2 P app l ication history of pasture chronosequence sites selected in 1 990 (additional sites selected in 1 993 in brackets) . Year Developed Pasture Age Total P Pasture Age Total P 1 990 Application 1 993 Application (years) to 1 990 (years) to 1 993 (kg P/ha) (kg P/ha) Undeveloped ( 1 ) Undeveloped 0 Undeveloped 0 Undeveloped (2) Undeveloped 0 - - 1 982 8 394 1 1 540 (526) 1 967 23 91 0 26 1 056 ( 1 037) 1 965 25 908 - - 1 960 30 947 33 1 095 ( 1 074) 1 958 32 1 01 0 35 1 1 56 ( 1 1 42) I n the years leading up to the 1 980's, the annual P appl ication rate varied between pasture age sites due to d ifferent trial requirements but averaged 24 kg P/ha, except on the area developed in 1 967 wh ich received an average of 33 kg P/ha in each of the 1 0 years to 1 978. In later years al l sites received approximately 30 kg P/ha/year, except the area developed in 1 960 which received 26 kg P/ha/year. Unti l 1 986 , P was appl ied as superphosphate normal ly on an annual basis. From the spring of 1 986 unti l 1 990, P was appl ied as "Longl ife" , a product contain ing 30% reactive phosphate rock and 70% superphosphate ( 1 1 % P) . 41 Al l the orig inal sites selected in 1 990 received 70 kg P/ha in 1 991 , 37 kg P/ha in 1 992 and 39 kg P/ha in 1 993. This was appl ied as superphosphate in 1 991 and 1 992, and RPR in 1 993. All P was appl ied as RPR from 1 991 to 1 993 to the rest of the Kaikohe Station including the additional sites selected in 1 993 at 5 1 -56 kg P/ha in 1 991 , 37 kg P/ha in 1 992 and 39 kg P/ha in 1 993. The most recent fert i l iser appl ications to each sampl ing time were March, 1 990 and January, 1 993. Lime History The total amount of l ime appl ied in the 7 years from in it ial pasture development and in the 1 0 years prior to sampl ing at the sites sampled in 1 990 are presented in Table 3 .3 . The amounts of l ime appl ied to each developed s ite during the in it ial pasture development stage were 2.24 tonne/ha ( 1 960 site) , 5 tonne/ha ( 1 967 site) and 6 tonne/ha ( 1 958 and 1 982 sites) . Lime was last appl ied to a l l sites in autumn 1 989 at 2 tonne/ha. No further l ime had been appl ied to any sites from 1 989 to the 1 993 sampl ing. Table 3 .3 Year Developed 1 982 1 965 1 967 1 960 1 958 Lime appl ication history of selected pasture chronosequence sites sampled in 1 990. Pasture Age 1 990 Total Lime Application Total Lime Application (Years) in 7 years From In it ial 1 98 1 - 1 990 Pasture Development (tonne/ha) (tonne/ha) 8 9. 1 1 1 . 1 23 unknown 5 .0 25 1 0.6 7 .2 30 4 .5 5 .0 32 1 5. 3 5 .0 42 Al l of the sites sampled in 1 993 from the 1 982 and 1 967 developed sites had simi lar l ime histories to the sites sampled in 1 990. However, l ime appl ication varied enormously over the areas developed to pasture in 1 958 and 1 960 in the first years fol lowing in it ial pasture development, due to the requirements of l ime appl ication rate trials. The sites developed to pasture in 1 960 and sampled in 1 993 received a higher average l ime rate than the 1 960 site sampled in 1 990, whi le the sites developed to pasture in 1 958 and sampled in 1 993 had received a lower average l ime rate than the 1 958 site sampled in 1 990. 3.3.3 Soil Sampl i ng Spring 1990 I n it ial ly one 1 0x 1 0 m site was selected for each of the pasture age sites (described in 3 .2 . 1 ) to determine if the low anion storage capacity of the Wharekohe soil l im its the accumulation of appl ied P leading to P movement down or from the soi l profi le. Four soi l pits were dug at each site, one pit on each side of a 1 0x 1 0 m area. A typical soi l profi le is shown in plate 1 . Soi l was sampled down a transect l ine of the profi le on one side of each pit where the E horizon was greater than 5 cm thick (marked by an arrow on each plate) . Approximately one ki logram of soi l was removed with a knife at each of 6 depths: 0-3 cm, 3-7 . 5 cm, 7 .5 cm to the top of the E horizon, E horizon, 0-1 0 cm below the E horizon and 20-30 cm below the E horizon, shown in P late 1 . Soi l bulk density was determined at each depth by removing 3 intact soi l cores ( 4 .8 cm diameter by 2 .3 cm) which were dried overn ight at 1 05°C and weighed. Spring 1993 In spring 1 993, 1 Ox 1 0 m areas were selected from within the 4 additional sites (described in 3 .2 . 1 ) for each pasture age and the original sites were resampled in order to investigate whether the orig ina l sites were representative of a wider area. 43 Plate 1 Profi le of an undeveloped Wharekohe s i lt loam. 44 20 soil cores (2 .5 cm diameter by 7 .5 cm depth) were removed from each site and bu lked. Soi l bu lk density was determined by removing 5 intact soi l cores (7 . 1 cm diameter by 7 .5 cm) which were dried overn ight at 1 05°C and weighed. Al l soi l samples col lected were air dried and sieved to <2 mm particle size to remove rocks and roots. Sub-samples for chemical analysis were removed by successive mixing and halving. The sub-samples used for total P were ground to < 1 50J.lm particle size in a ring grinder. 3.3.4 Soil Analysis 3.3.4. 1 Total P Total P was measured after d igestion in a tri-acid m ixture of concentrated HN03 : HCI : HCI04 (5 :5 :7 ) ( Bolan and Hedley, 1 987) . One gram of soi l was digested with 25 ml of tri-acid in a 250 ml Erlenmeyer flask at 260°C unti l white fumes appeared. The digest was made up to 50 ml in volume and a l lowed to settle before an al iquot was removed for inorganic P determination colorimetrica l ly by the phosphomolybdate method of Murphy and Ri ley ( 1 962) . 3.3.4.2 Olsen P Olsen P was extracted using the MAF Olsen P Quicktest method (modified from Olsen et aI . , 1 954) where soil was shaken with 0 .5 M NaHC03 (pH 8 .5) in a 1 :20 (v/v) ratio at 25°C for 30 minutes. The suspension was fi ltered (Advantec Toyo No.2) and the inorgan ic P determined colorimetrical ly by the phospho molybdate method of Murphy and Ri ley ( 1 962) with an autoanalyser. 3.3.5 Pasture Sampling and Analysis 45 Pasture samples were col lected regularly from beneath animal exclusive cages for two years from December 1 990-1 992 at the 1 990 soi l sampl ing sites. Fresh caged areas were pretrimmed to mower height at the start of each growth period. Dry matter yield and chemical analysis samples were col lected at i ntervals determined by pasture growth over a two year period. Pasture botanical composition samples were also col lected at seven of the growth periods. 3.3.5. 1 Dry Matter Yield Samples for dry matter yield were col lected with a mower set to approximately 3 .5 cm above ground level cutt ing strips 0 .46 m wide and 2 .25 m long. The samples were dried overn ight at 65°C before weighing. 3.3.5.2 Botanical Composition Pasture botan ical composition samples were also col lected with hand shears at mower height. Botanical composition was determined by hand sorting each pasture sample into its grass, clover, lotus, weed and dead matter components wh ich were then dried overn ight at 65°C and weighed. 3.3.5.3 N and P Concentration At the same time as yield measurements were made, samples for N and P analysis were col lected with hand shears at mower height, oven dried at 65°C and stored in airtight plastic bags unt i l gr inding to <2 mm. Herbage N and P concentration were determined after Kjeldahl d igestion (McKenzie and Wal lace, 1 954) by analysis with an autoanalyser (Twine and Wi l l iams, 1 97 1 ) . 3.3.6 Statistical Analysis Soil Data 46 Olsen P and total P results were subjected to analysis of variance using Genstat to determine differences between sites for each depth and between depths for each site. Stat ist ical analysis of total P was conducted on log transformed results using the unbalanced analysis of variance program REML in Genstat. Herbage Data The relationsh ip between each of dry matter yield, N and P concentration and uptake, and botanical composition with pasture age were determined using FLEXI (Upsdal l , 1 994) , a Bayesian smoothing program which produces curves representing each relationship and their 95% confidence intervals . . Trial Design Pasture was establ ished on the Kaikohe Research Station in large blocks at a time, start ing from the road frontage in the late 1 950's. Consequently the study of pasture age effects could not be approached as a conventional trial design. There is effectively no repl ication of pasture age. Hence, the analysis of variance i ndicates d ifferences between sites rather than pasture ages and it is only possible to say that soi l data from one site was sign ificantly different from another site. Although repl icates for the spring 1 993 soil sampl ing were sampled over a wider area from sites spaced further apart in d ifferent paddocks, this second sampl ing sti l l did not overcome the problem of a lack of randomisation of pasture age. 47 3.4 RESU LTS AND DISCUSSION 3.4. 1 Effect of Pasture Development on the Fate of Applied P 3.4. 1. 1 Total P Spring 1990 Total P concentration decreased significantly (P<0.05) with depth to the E horizon on al l sites (Figure 3 . 1 ) . Other New Zealand studies, where fert i l i ser P was applied over time, have also shown decreasing total P with depth down the profi le (Doak, 1 942; Jackman , 1 95 1 ; Jackman, 1 955a; Walker et a I . , 1 959, Saunders, 1 959a, Haynes and Wi l l iams, 1 992) . Total P concentration of the samples taken from the E and B horizons (0-1 0 cm and 20-30 cm below E) were lower than in the A horizon and showed l itt le variabi l ity between the three lower depths at each site. Despite problems with the interpretation of site data, some clear trends with pasture development are evident. Low natural total P content and its low variabi l ity in undeveloped Wharekohe top soi ls , combined with the number of older developed sites, mean that some confidence can be placed in any changes in P status reflecting pasture age and hence appl ied l ime and P. Total P concentration increased with pasture development at a l l sampl ing depths above the E horizon (F igure 3. 1 ) indicating that P moved down the profi le to the top of the E horizon . There was no significant difference in the total P concentrations for the 0-3 cm depth of the developed sites. I n the 3-7 .5 cm depth, total P concentration in the 8 year old site was sign ificantly lower (P<0.001 ) than in the four older developed sites. A s imi lar pattern occurred at the 7 .5 cm - E depth where the total P concentration in the 8 year old site was significantly lower (P<0.001 ) than al l the older developed sites except the oldest 32 year old site. 1 00 0 900 8 0 0 7 0 0 - Cl - 6 0 0 Cl ::::I - c. 5 0 0 iii - 400 0 I- 3 00 2 00 1 0 0 0 0 --+--0-3 em ---M-- E ho rizon 4 8 1 2 1 6 2 0 24 28 Y e a rs of D ev e l o p m e n t __ 3 -7 .5 em �7.5em - E h o rizon - -i( - E -1 0 em below . . • . · 20 -30 em below E 3 2 Figure 3. 1 Effect of pasture age on soi l total P concentration (S .E .D . s varied, Table in Appendix 3. 1 ) . 48 The lower total P concentration of the oldest site at the 7 .5 cm - E horizon depth was most l ikely due to the d i lution effect of the significantly higher depth (23 .8 cm) to the top of the E horizon at this site rather than pasture age. When the results were expressed as kg P/ha (Table 3 .4) , the oldest site contained the highest total P of any site at this depth. Some of the developed sites had significantly higher total P concentrations than the undeveloped sites for the two B horizon samples (0-1 0 and 20-30 cm below the E horizon) , although the magn itude of differences in total P concentration i n and below the E horizon between a l l the sites were smal l . The h igher P concentrations at some sites may have been due to the downward movement of appl ied P and its subsequent accumulation in the B horizon. Variation in the continuity and thickness of the E horizon was evident at pit sites and may have influenced the variation in the downward movement of appl ied P . 49 When the resu lts are expressed in kg/ha (Table 3 .4) a substantial port ion of appl ied P could be accounted for in the B horizon of some of the developed s ites, at least 20% of the P appl ied to the 23 year old site. However, the higher P contents of the B horizon of some developed sites may not have been due to downward movement of appl ied P and its subsequent accumulation in the B horizon as natural variation in native P content between the sites may also have accounted for the higher P concentrat ions recorded in the B horizon of some developed sites. Variation in native P content is most l ikely to be high in the B horizon of Wharekohe podzols. An anion storage capacity of 95% and total P concentrations, ranging from 1 50-1 70 ug PIg were recorded in the B horizon of an undeveloped Wharekohe si l t loam site (New Zealand Soi l Bureau, 1 968), yet in the present study much lower P levels were recorded (85 ug PIg measured in the 20-30 cm below E horizon depth of the 23 and 30 year old sites) . Table 3 .4 Total P content (kg P/ha) of Wharekohe s i lt loam samples. Pasture 0-3 em 3-7.5 em 7.5-E em E 0 -1 0 em 20-30 em Age horizon below E below E horizon horizon 0 20 23 28 33 1 1 2 1 8 1 67 1 28 92 55 24 23 23 22 1 267 93 92 95 1 1 5 25 1 87 220 1 40 92 26 49 30 1 74 1 91 1 67 48 87 98 32 1 85 234 247 44 20 1 4 Accumulation of appl ied P in the A horizon, from wh ich pasture nutrients are drawn, is presented in table 3 .5. Pasture roots draw most of their nutrients from the upper 7 .5 cm of soi l (the effective root zone) (Horne, 1 985; G i l l i ngham et a I . , 1 980b) with the 7 .5 cm - E horizon depth contributing nutrients to a lesser degree. The accumulation of P in the E and B horizons effectively represents a loss of P from the pasture cycle. 50 Table 3 .5 Accumulation of appl ied P at each depth to the top of the E horizon. Site P Applied P Acc. P Acc. P Acc. % applied P % appl ied P % applied P age (kg/ha) 0-3 cm 3-7 .5 7 .5 cm- accumulated accumulated accumulated (kg/ha) cm E* 0 - 7 .5 cm 7 .5 cm - E* 0 - E* (kg Iha) (kg Iha) 8 394 1 47 1 05 64 64 1 6 80 23 91 0 201 243 65 49 1 2 61 25 908 1 67 1 96 1 1 2 40 1 2 52 30 947 1 54 1 68 1 39 34 1 5 49 32 1 01 0 1 65 2 1 0 21 9 37 22 59 * E horizon. The substantial quantities of appl ied P accumulating below 7.5 cm to the top of the E horizon in the Wharekohe s i lt loam, up to 22%, are not unusual for a sedimentary soi l . Appl ied P has been shown to accumulate to depth in even greater quantities in other sedimentary soils, including a podzol , in New Zealand, but not in volcanic soi ls. In P fert i l i ser comparison trials of 4.5 years duration (Jackman, 1 955a) over a range of soi ls , the largest increases in P accumulation at depth were in the sed imentary soi ls compared to volcan ic soi ls . In contrast to the sedimentary soi ls, appl ied P was general ly restricted to the top sampl ing depth of central North Island volcanic soi ls, S cm in a yel low brown pumice soi l (Jackman, 1 95 1 ) , 1 0 cm in a yel low brown pumice soi l (Walker et aI . , 1 959) and 5 cm in a yel low brown loam with only m inor accumulation of appl ied P to 1 2. 5 cm (Saunders, 1 959a) . However, of the 35 kg P/ha/yr appl ied over a 35 year period to the surface of an i rrigated yel low grey earth at Winchmore, 283 kg P/ha (23%) and 1 29 kg P/ha ( 1 1 %) had accumulated in the 7 .5-1 5 cm and 1 5 -22 .5 cm depths respectively (Nguyen and Goh, 1 992) . In a yel low grey earth near Marton receiving 45 kg P/ha as superphosphate for 9 years, only 1 1 9 ug PIg and 62 ug PIg had accumulated in the 51 1 0-1 5 and 1 5-25 cm depths, respectively. In the West Coast Okarito podzol ( Lee et a I . , 1 983) , surface appl ied P at :::::30 kg P/ha/yr had moved downward to accumulate in the 5-1 0 cm depth on sites developed to pasture for 3, 5 and 1 3 years. However, only the 1 3 year old site was significantly different from the undeveloped site, 1 08 IJg/g more 0 .5 M H2S04 extractable P . The authors considered that the lack of differences was due to large variabi l ity in the P contents of the samples taken from each site. The greater percentage accumulation of appl ied P at depth in the above mentioned sedimentary soi ls does not necessari ly indicate greater movement of appl ied P through the profi le in comparison to the Wharekohe s i lt loam. The low Fe and AI contents and associated low anion storage capacity of the A horizon in the Wharekohe si lt loam and a maximum P capacity to accumulate P, may be restricting P accumulation in this horizon leading to large losses of appl ied P. Most of the appl ied P which accumulates as Pi in New Zealand soi ls has been shown to be associated with Fe and AI (Saunders, 1 959a; Steele, 1 976; Grigg and Crouchley, 1 980; Condron and Goh, 1 989; Perrott et a I . , 1 989; Perrott et a I . , 1 992a; Rowarth et a I . , 1 992a) . However, some Pi accumulates in the soil as Ca-P from relatively inert fert i l iser residues (Rowarth , 1 987; Condron and Goh, 1 989; Perrott et a I . , 1 992a). Hence RPR and insoluble PR residues from superphosphate were probably accumulating at each developed site in the present study. As RPR use continued beyond 1 990, the rate of net Ca-P accumulation is l ikely to have decreased from 1 991 as the dissolution of RPR in the soi l approaches RPR appl ication rates with cont inued appl ication (Edmeades et a I . , 1 991 b) . Fertil iser P residues were l i kely to be accumulating evenly over the older sites due to s imi lar site characteristics and the appl ication of simi lar quantities and forms of P. It is highly unl ikely that any other P fraction would have continued to accumulate evenly over al l of the developed sites. Hence, as smal l quantities of fert i l iser P residues are l i kely to continue accumulating in the soi l , the maximum P storage capacity referred to in this text appl ies to the capacity of the soil to sorb inorganic and organic P ontolinto soi l surfaces and form new insoluble P compounds in situ. The extent of ------ 52 accumulation of appl ied P into various inorganic and organic P fractions is investigated further in chapter 4 . The low P storage capacity of the Wharekohe si l t loam topsoi l is l im it ing the accumulation of appl ied P leading to P movement down the soi l profi le. Although it is difficult to determine the sign ificance of differences between s ites, total P accumulation at each depth appeared to be delayed unti l the depth immediately above had reached a maximum total P content. Hence, P may only move down the profi le of the Wharekohe silt loam in large quantities, to accumulate at depth, once the P content of the topsoil is near a maximum P storage capacity. In this study the maximum P storage capacity appears to be reached by 8 years under pasture in the 0-3 cm depth, and between 8 and 23 years for the 3-7 .5 cm depth . The consequence of such a low maximum P storage capacity would be large losses of appl ied P from the topsoi l of the older developed sites (explained further in 3 .4 . 1 .2) . Doak ( 1 942) also found that P (measured as HCI soluble P20S) moved s lowly down the profi le of a yel low grey earth over a 9 year period. Appl ied P in it ia l ly accumulated only in the top 5 cm of the soi l in Doak's study, with appl ied P only accumulating at 5-1 0 cm after 2 years, at 1 0-1 5 cm after 4 years, and at 1 5 to 25 cm after 5 years. In the present study, pasture ages with in 0 and 8 years or within 8 and 23 years were not avai lable to determine more precisely when P started to move into each depth in the Wharekohe s i lt loam. Spring 1993 Interestingly the concentrations of total P in the top 0-7 .5 cm of the additional 26 and 33 year old sites sampled in spring 1 993 (Figure 3 .2) were s imi lar to the 750 1-19lg measured in a Wharekohe s i lt loam by Perrott and Sarathchandra ( 1 987) in a survey of soi ls wh ich had been in pasture for at least 20 years. Comparison of the total P content (kg P/ha) of samples col lected in both spring 1 990 and 1 993 revealed that the 23 and 30 year old sites selected in 1 990 may not be 53 that representative of wider areas of s imi larly aged pastures on the Wharekohe si l t loam on the Kaikohe Research Station (Table 3.6) . The orig inal 23 year old s ite (developed in 1 967), and another site selected in the same paddock in 1 993, had far higher total P contents than the other 3 additional s ites despite the additional s ites receiving a l l their appl ied P in RPR form (RPR would be expected to increase total P in it ial ly through the accumulation of undissolved RPR residues) . Such high P contents in the original site indicate that a different ferti l iser history at the site may have occurred. The paddock has a large, steep area (30% of paddock) that fert i l iser trucks may have avoided and hence increased appl ication rates in the vicin ity of the sampl ing sites. The original 30 year old site (developed in 1 960) had a lower total P content than the other sites developed for at least 23 years that were selected in 1 990 or the additional sites of s imi lar age due to its reduced Ca-Pi accumulation (related to its early l iming history), explained further in Chapter 4. A comparison of the total P content of the sites sampled in 1 990 with samples from the same sites col lected in 1 993, at which point an additional 1 46 kg P had been appl ied to each site, reveal that P has continued to accumulate at a l l sites (Table 3.6). As expected P continued to accumulate at a greater rate in the top 7 .5 cm of the youngest site. Continued accumulation of large quantities of applied P on the 26 year old site was unexpected, however, this site does not seem to be representative of other sites of s imi lar age as explained above. Accumulation of appl ied P was clearly lowest in the two oldest sites. The appl ication of RPR and associated increases in undissolved phosphate rock residues would account for a large proportion of the total P increase in the older sites, and is investigated further in Chapter 4. Hence these results, in relation to the accumulation of P , support the hypothesis that the older sites on the Wharekohe si l t loam on the Kaikohe Research Station are approaching or have reached a maximum P storage capacity. The spring 1 993 samples from the additional sites also confirmed that the Wharekohe si lt loam on the Kaikohe station has a maximum P storage capacity, as there was no difference in the amount of appl ied P in the 0-7 . 5 cm depth between the developed sites despite the much higher P inputs to the older pasture sites 54 (Table 3 .7 ) . These resu lts ind icate that the maximum P storage capacity is reached by 1 1 years in the top 7 . 5 cm of soi l . Table 3 .6 Accumulation of P appl ied from spring 1 990 -1 993 in the top 7 . 5 cm of orig inal sites (Additional sites in brackets). Year Developed Total P in top 7 .5 cm Total P in top 7 .5 cm P accumulated 1 990- 1 990 (kg/ha) 1 993 (kg/ha) 1 993 (kg/ha) Undeveloped 43 44 (52) - 1 982 295 424 (449) 1 29 1 967 488 660 (526) 1 72 1 960 365 393 (498) 28 1 958 41 9 451 (444) 32 Table 3 .7 Accumulation of appl ied P in the top 7 . 5 cm of soi l sampled from the additional sites in 1 993. Site P Applied (kg/ha) P accumulated in top 7 .5 % applied P accumulated in Age cm (kg/ha) the soil to 7 .5 cm . 1 1 526 396 75 26 1 037 470 45 33 1 074 458 43 35 1 1 42 369 32 A comparison of the total P measured in the additional sites to those sites orig inal ly sampled in 1 990 (F igure 3.2) reveals a higher average total P content than in the original s ites. The greater total P recorded (55 kg P/ha as the d ifference in average appl ied P accumulated , original 23 year old site removed as outl ier) can be attributed to the appl ication of P solely as RPR on the additional s ites. The RPR residues were expected to have accumulated reasonably evenly across the developed sites. This aspect is investigated further in chapter 4. 600 500 ii 400 � - C) � � 300 "' '0 t- 200 1 00 o +-----4-----4-----�----�----�----_+----� o 5 1 0 1 5 20 25 30 35 Years of Development F igure 3 .2 Comparison of total P measured in orig inal 1 990 sites (0) and total P measured in additional sites in 1 993 ( • ) . (Vert ical bars = standard errors of the means). 55 A low maximum P storage capacity appears to be a feature of podzol topsoi ls and not a feature of most other agricu ltural soi ls in New Zealand. Evidence from a chronosequence study of P (extracted by 0 .5 M H2S04) in Okarito soi ls (West Coast gley podzols) also found that, wh i le appl ied P accumulated with pasture development in the top 1 0 cm of the soi l , there was no significant d ifference between the P contents of the top 5 cm of the developed sites (3, 5 and 1 3 years) ( Lee et a I . , 1 983). In contrast to the podzols, long term studies of P changes over t ime in sedimentary yel low grey earths have found that P was sti l l increasing in the topsoi l (7 . 5 cm) after 8 (Ooak, 1 942), 1 5 ( Lambert et a I . , 1 988) and 35 years (Nguyen et aI . , 1 989). Evidence col lected from several studies indicates that P also accumulates to very high levels in yel low brown loams and yel low brown pumice soi ls high in a l lophane (Jackman, 1 951 ; Saunders, 1 959a; Walker et aI . , 1 959; Perrott et a I . , 1 989). 56 3.4. 1.2 Losses of Applied P A large proportion of the P appl ied to the chronosequence sites could not be accounted for in the top 7 .5 cm of the soi l (the effective root zone), to greater depth (0- E horizon) or when losses associated with the grazing animal were included in the calculation (F igure 3 .3, Table 3 .8) . The amount of P not accounted for in the top 7 .5 cm of the soi l increased with pasture age to over 40% of the appl ied P. When P accumulation to the top of the E horizon is included, nearly al l of the P appl ied to the 8 year old site could be accounted for (F igure 3 .3 b) . 1 6 -27% of the appl ied P is sti l l unaccounted for in the older developed sites even when al l the P accumulated above the E horizon is included in the calcu lation. Such large losses indicate that P is lost from the system by lateral movement down slope above the E horizon and/or to below the E horizon. Some of the lost P may have accumulated in the E and 8 horizons (F igure 3.3 c, Table 3 .8a) . However, g iven the low permeabi l ity of the E horizon, large losses via vertical leach ing are unl ikely. High variabi l ity of native P in the 8 horizons could account for the larger P at this depth in some of the developed sites rather than the accumulation of appl ied P. P has most l ikely been lost through e ither lateral movement of soi l water above the impermeable E horizon or has moved beyond the sampled 8 horizon where vertical leaching has occurred. The former is the most l i kely scenario as l ittle appl ied P accumulated in the 8 horizon samples despite their high P retention capacity. Lateral movement of P in soi l water above impermeable horizons has been suggested to account for the high losses of P from other New Zea land podzols (O'Connor, 1 980; Lee et a I . , 1 983) and from Austral ian soi ls ( Lewis et a I . , 1 987; Khanna et a I . , 1 992) . For each developed site, the percentage soi l loss of appl ied P was calculated over the entire period from development. However, soi l losses of P in the years fol lowing in it ial pasture development are low, and hence, soi l losses of P wil l be much higher in later years. The soi l losses of P appl ied to the original sampl ing sites between 1 990 and 1 993 were calculated as 65% and 62% for the 33 and 35 year old sites a) Fate of appl ied P from the top 7 .5 em of soi l . 1 200 - 1 000 ca J: - 800 en � - 600 Q.. c;; 400 - 0 200 I- 0 0 4 8 1 2 1 6 20 24 pplied P � Soil P Loss stimated Animal P � P Accumulated in 0- 7.5 cm soil 28 32 b) Fate of appl ied P from the soi l to the top of the E horizon. 1 200 - Applied P ca 1 000 J: - .It:' Soil P Loss en 800 � --2 Estimated - Q.. 600 � Animal P Loss C;; - 400 P Accumulate.d in 0 � soil to top of I- 200 E horizon 0 0 4 8 1 2 1 6 20 24 28 32 e) Fate of appl ied P from the soil profi le to 30 em below the E horizon. 1 200 - 1 000 ca J: - en 800 � - Q.. 600 c;; 400 -0 I- 200 0 0 4 Applied P P Accumulated in � soil profile 8 1 2 1 6 20 24 28 32 Years of Development Figure 3 .3 Fate of appl ied P showing losses from eaeh depth with t ime from pasture development, a) 0-7 .5 em, b) 0 em -Top of E Horizon, e) Soi l Profi le to 30 em below E Horizon. 57 Table 3 .8 Losses of appl ied P from different aged pastures on the Wharekohe silt loam. a) Spring (1990) Sampling Site Age P Applied (kg/ha) % Applied P lost from the soil (ie not accounted for by (Years) animal loss· or accumulation in vary ing soil depths) To 7.5 cm To Top of E To 30 cm Below Horizon E Horizon° (fu l l sampling depth) 8 394 20 4 0 23 91 0 31 24 1 2 25 908 38 26 1 9 30 947 41 27 2 32 1 01 0 38 1 6 1 6 b) Spring (1993) Sampling Site P Applied (kg/ha) % Applied P not accounted for by Age animal loss· or accumulation in soi l to 7 .5 cm 1 1 526 8 26 1 037 35 33 1 074 35 35 1 1 42 42 58 *An imal loss calcu lated as 0 .5 kg P/yrlSU (Cornforth and Sinclair, 1 984) , SR=1 5.6 SU/ha.J °Calculation included an estimate for P in the unsampled 20-30 cm below E depth. respectively. If soluble P fert i l i ser, such as superphosphate, had been used throughout this period, h igher P losses than recorded would be expected. Much of the P accumulating on each site in later years cou ld be attributed to the accumulation of RPR residue as RPR was appl ied in 1 993. H igh losses of appl ied P have also been reported in other studies on podzol ic soi ls in New Zealand. It is not possible to estimate the amount of appl ied P wh ich remains unaccounted for in the soi l or by animal loss in these studies on other 59 podzols, as stocking rate data is not avai lable, and hence, appl ied P accumulation figures are presented. The accumulation of appl ied P was only 1 0% in the top 1 5 cm of a Te Kopuru sand after 9 years of superphosphate appl ication averaging 34 kg P/ha/yr (Jackman, 1 955a), 43% in the top 7 . 5 cm after 5 years where an average of 48 kg P/ha/yr was appl ied to a Maungatua podzol (F loate and Enright, 1 991 ) and 33% in the top 1 5 cm after 5 years of P appl ication averaging 44 kg P/ha/yr to an Addison s i lt loam (O'Connor, 1 980). Losses of P from the root zone, 0-7 . 5 cm, were possibly even greater in the Te Kopuru and Addison soi ls. The above studies of P loss from New Zealand podzols have been conducted on mostly young pasture of less than 1 0 years. In most of these studies, losses of P have been far higher than those recorded in the 8 and 1 1 year old Wharekohe s i lt loam pastures sampled in the present study where a large proport ion, 64% and 75% respectively, of the P was recovered in the top 7 .5 cm (80% of P accumulated in the soil above the E horizon (0-1 7 .5 cm) in the 8 year old site). The reasons for far greater accumulation of appl ied P in the early years fol lowing in it ial pasture development in the Wharekohe si lt loam in comparison to the above mentioned podzols are not clear. Phosphorus was appl ied at an average rate of 48 kg P/ha on the Wharekohe si lt loam, and hence, is s imi lar to rates appl ied to the other podzols. The other podzols may have had lower maximum P storage capacities than the Wharekohe soi l . Higher annual rainfa l l (>2500 mm) may have led to higher P losses in the South Island podzols compared to the Wharekohe soil ( 1 630 mm). However, Powel l and Taylor ( 1 980) also reported l itt le loss of appl ied P from the top 1 0 cm of the South Island Maimai g ley podzol 3 years after development due mostly to the organic immobi l isation of appl ied P. The Maimai trial was very short term and it is not possible to know if losses from the soi l would have been simi lar or greater in the ensu ing years. The coarser texture and higher rainfa l l of the Te Kopuru sand podzol , in comparison to the Wharekohe si l t loam, may have created larger losses of appl ied P through greater porosity and water movement. D ifferences in n itrogen fixation may also have effected differences in organic P accumulation. Increased n itrogen fixation leads to greater accumulation of organic matter and organic P (Walker et aI . , 1 959; Perrott and Sarathchandra, 1 987) . 60 Phosphorus losses from the sedimentary yel low grey earths under pasture are variable but general ly lower than losses from the podzols. Most of the P appl ied to a North Is land yel low grey earth was accounted for in the top 1 5 cm of soi l after 1 3 years (Saggar, 1 990b) . Of the P appl ied as dairy effluent over a 3 year period, 90% had accumulated in the top 5 cm of a yel low grey earth (MacGregor et a I . , 1 982) . On ly 40 % of the appl ied P had accumulated in the top 5 cm of another North Island yel low grey earth after 9 years of P appl ication but most was accounted for by animal losses and sampl ing to 25 cm (Ooak, 1 942) . However, larger losses have been calculated for the South Is land yel low grey earth Lismore stony s i l t under i rrigation at Winchmore. Only 27% of the appl ied P had accumulated in the top 7 . 5 cm of the Winchmore soi l fo l lowing 35 years of 35 kg P/ha/yr in comparison to 34% of the P appl ied to the 35 year age site on Wharekohe si lt loam. However a lower rate of P averaging 32 . 5 kg P/ha/yr had been appl ied to the 35 year age s ite on the Wharekohe s i l t loam possibly accounting for the lower losses. When animal loss of P was included, >53% of appl ied P could not be accounted for in the top 7 .5 cm of t�e Winchmore soi l (calculations made using data presented by Nguyen and Goh, 1 S92). I nclusion of P recovered to 30 cm soi l depth reduced the amount of appl ied P wh ich could not be accounted for to 30%. The losses of P from the top 30 cm of the yel low grey earth under i rrigation are simi lar to the losses of P from above the E horizon of the Wharekohe soi ls recorded in this study. However, the pattern of loss was d ifferent to that in the Wharekohe si lt loam. As P has accumulated in the soi l at Winchmore at a fairly steady rate and is sti l l accumulating, P losses over time may also have been at a steady 30%. I n contrast, P loss increased with t ime, from pasture development, from an average of 4% for the fi rst 8 years on the Wharekohe si l t loam. The downward movement of irrigation waters may be a factor faci l itat ing P losses at Winchmore, where the anion storage capacity of the soi ls is approximately 25% (Perrott et a I . , 1 992a) . In part icu lar, the movement of dry faecal pel lets in flood i rrigation waters may lead to losses. However, losses via i rrigation water would not be expected to be large where i rrigation is carried out correctly. 61 Accurate figures for the accumulation of appl ied P are difficult to determine on the volcanic soi ls . The chronosequence trials on volcanic soi ls have included a l imited number of different sites d iffering in pasture age rather than ongoing measurements on the same sites and the soi ls have larger native P content and variabi l ity than sed imentary soi ls . Litt le loss of P has been recorded from the top sampl ing depth in studies on volcanic soi ls . Al l of the P appl ied over a 25 year period to a Taupo sandy silt (yel low brown pumice soi l ) under pasture cou ld be accounted for by animal loss and accumulation in the top 1 0 cm of soi l (Walker et a I . , 1 959). 88% of appl ied P was accounted for by animal loss and accumulation in the top 7 .5 cm of soi l under 7 and 25 year old pastures on a yel low brown loam (Saunders, 1 959a) . Overal l , appl ied P in it ial ly accumulates in the topsoi l of the Wharekohe si l t loam, fol lowing pasture development, and soil losses of P are low. However, once the maximum P storage capacity of the topsoi l is reached , P can no longer accumulate and soi l losses of appl ied p become high. Soi l losses of appl ied P from o lder sites were measured up to 65%. Lateral movement of P in soi l water above the E horizon is the most l i kely pathway for P loss. High losses of appl ied P have also been recorded from other New Zealand podzols, but in earl ier stages of development. 3.4.2 Effect of Pasture Development on Olsen P Olsen P decreased sign ificantly (P<0.05) with depth on a l l sites (F igure 3 .4) . This finding was consistent with the findings of other New Zealand studies which investigated the effects of P appl ication on P avai lab i l ity (F loate and Enright, 1 991 ; Rowarth et a i , 1 992a) . Olsen P values in the undeveloped sites were very low, averag ing 4 in the top 7 .5 cm. Olsen P increased significantly (P<0.05) with pasture development at a l l depths as a result of fert i l iser P appl ication (Figure 3.4) . In the soi ls sampled above the E horizon in 1 990, Olsen P was related to total P (R2=0.87) . Hence, Olsen P values showed a s imi lar pattern to total P values at each depth. There was no significant difference between the Olsen P values for the 0-3 cm depth of the developed sites sampled whi le Olsen P increased with increasing pasture age and applied P unti l 35 30 25 Il. 20 c Q) In 0 1 5 1 0 5 o 'I' 0 I I S.E.D. for comparing ages within depths. S .E.D. for comparing depth==-� / - �fJ I _------------,/ fo · · . .. _ _ . _ _ ._ -ef :' - . - -,?�, - - ­ G- - - - -� - .- '- ,; "'" . _ _ • . _ . • _ . , . _ _ _ . , _ _ . . • • . _ . , >1 ,; . '<> ,; . ' '" . ' ,/ . ", ' . ' '" . ' "" . ' 4 8 1 2 1 6 20 24 28 32 Years of Development --+- 0-3 em - 13 - 3-7 .5 em . . {:.- - · 7 .5 em - E horizon �0-7.5 Figure 3 .4 Effect of pasture age on Olsen P (MAF Quicktest) for each depth and a calculated value for the 0-7 .5 cm depth . (Vert ical bars=S. E .D . s for developed sites sampled to 0-3 cm, 3-7 .5 cm and 7 . 5 cm to the E horizon) 62 between 23 and 25 years below 3 cm. Floate and Enright, ( 1 991 ) found increases in Olsen P with pasture development and P appl ication in a series of South Island upland yel low-brown earths and a podzol ised yel low brown-earth to at least 1 00 mm over the five years from development. Where the P app l ication rate is greater than animal and soi l losses, O lsen P values can be expected to increase (F loate and Enright, 1 991 ; Nguyen et aI . , 1 989; Perrott et a I . , 1 992a; Rowarth et aI . , 1 992a). In the present study, where approximately 30 kg P/ha/yr has been appl ied, Olsen P values appear to rise with pasture development in the top 7 .5 cm to approximately 26 on the 3 oldest sites. The desired range of Olsen P to maintain pasture production at 90% of Ymax on this soi l is considered to be between 20 and 25 (Edmeades et aI . , 1 991 a) . Hence, 30 kg P/ha/yr would appear to be sufficient to maintain the appropriate Olsen P levels on the Wharekohe soi ls at Kaikohe. However, assuming an ALF of 0. 5, a SLF of 0.4, 63 CC of 1 8, and a SR of 1 5.6, the mod ified CFAS model predicts that only 2 1 kg P/ha would be sufficient to maintain a stocking rate of 1 5 .6 SU/ha . If 30 kg P/ha were to be appl ied annually, the model would predict a progressive increase in O lsen P. Despite these relatively h igh P inputs, Olsen P seems to have reached an asymptote of 26. Resampling of a wider area at the study site in 1 993 confi rmed that Olsen P does not increase despite higher than calculated maintenance inputs of appl ied P , as Olsen P increased from in it ial pasture development reaching an asymptote of 22 in the older developed sites (Appendix 3 .2) . I f practice suggests that 30 kg P/ha is required for maintenance, then it appears that the model is underestimating the amount of P required for maintain ing a stocking rate of 1 5.6 SU/ha on these soi ls, the SLF of 0.4 is too low for Wharekohe soi ls . However, if there is a maximum P storage capacity in the root zone of older sites, i t is also possible that no matter how much P is appl ied above animal losses, the Olsen P may not increase beyond current levels due to high soi l losses. Hence, it wou ld be counterproductive to increase the SLF used in the model, therefore increasing requirements for carrying 1 5.6 SU/ha, if the extra P was only going to be lost from the pasture cycle to the wider environment. The SLF is investigated more fu l ly in Chapter 7 . 3.4.3 Effect of Stage of Development on Pasture Production and Composition. Statistical analysis, using FLEXI, of the relationship between pasture age and dry matter yield, N and P concentration and uptake, and botanica l composition are presented in appendix 3 .3 . 3.4.3. 1 Dry Matter Yield Over the 2 years of measurement, pasture age did not impact on pasture growth (Fig 3 .5) . On the 1 0/2/92 the 30 and 32 year old s ites grew sl ightly more pasture and on 1 6000 14000 - ta 1 2000 � - C) 1 0000 � - � Q) 8000 --ta 6000 � � 4000 C 2000 1 992 0 8 23 25 30 32 Years of Development Figure 3.5 Effect of pasture age on dry matter yield in 1 991 and 1 992. 70 60 - ta 50 � - C) � 40 - Q) � 30 ta -c.. ::> 20 Il.. 1 0 1 992 0 8 23 25 30 32 Years of Development Figure 3.6 Effect of pasture age on P uptake in 1 991 and 1 992. 64 65 the 1 8/8/92 the 23 and 25 year old sites grew sl ightly more pasture than the other sites (Appendix 3 .3a) . Other factors unrelated to pasture age may have influenced yield differences on these two occasions. For example, the two older sites retained more water, due to poor drainage, which may have depressed growth in winter. The lack of a relationship between pasture yields and pasture age was surprising as avai lable P in the root zone of the sites increased over t ime and the Olsen P of the youngest s ite, 1 6 in the top 7 . 5 cm, is considered to be below the optimum range for pasture growth (Edmeades et aI . , 1 991 a) on these soi ls . However, herbage P concentrations (Appendix 3. 3b) were considered adequate for pasture growth at a l l sites including the youngest. Other factors besides P influence pasture growth. pH was less than optimal for pasture growth on a l l but the youngest site and N is l ikely to be l imit ing pasture growth at a l l sites 3 .4 .2 .3 . C l imatic factors had a large influence on pasture growth during the trial period, with the annual dry matter yield measured in the second, more favourable, year ( 1 3000 kg DM/ha) over twice that measured in the first at some sites. Extremely dry condit ions were recorded in the fi rst year of the tria l . Hence, these factors which are l im it ing pasture growth at a l l sites may be masking any response to soil chemical factors which may differ with pasture age. Examination of pasture growth measurements recorded conti nuously for 1 1 years on the pasture developed in 1 982 revealed no pasture yield relationship with pasture age (AgResearch, unpubl ished data) . Also the annual pasture yields were closely related to rainfal l with low pasture production during very dry and very wet years. Pasture growth is severely restricted by animals pugging Wharekohe soi ls during wet years. Other authors have also recorded no dry matter yield relationship with pasture age (up to 35 years) from init ial pasture development (Nguyen et aI . , 1 989) and after oversowing to create improved pasture (Suckl ing, 1 976) on h i l l country where P was appl ied at a constant rate annual ly. 66 3.4.3.2 Botanical Composition Pasture age had no effect on the botan ical composition with respect to grass, legume or dead matter content (Appendix 3 . 3c) . The 23 and 25 year old s ite recorded a lower summer weed content than the other sites. I n contrast, another study has found that pasture composition deteriorates with time from in it ial sowing, resulting in an increased summer weed content with increasing pasture age although dry matter yield was not effected (Nguyen et a I . , 1 989). In the present study, the grass to clover ratio was highest in spring when n itrogen is more freely avai lable in the soi l , wh i le the proportion of dead matter increased in summer to 20-40% due to lack of rainfa l l . The proportion of clover in the swards on each of the developed sites was low, general ly below 20% and as low as 3 - 9% in the first spring, in comparison to other New Zealand s ites on a range of soi ls where clover content was approximately 30% where P fert i l iser was appl ied (Morton et a I . , 1 994) . Such low clover content has impl ications for the N status of the pasture on these Wharekohe si l t loam as described in more deta i l in 3 .4 .2 .3 . 3.4.3.3 Pasture N and P Concentration and Uptake Overal l , pasture N and P concentration and uptake did not vary with pasture age except for small d ifferences recorded on the 1 0/2/92 harvest (Figure 3 .6 , Appendix 3 . 3b) . The N concentration decl ined and P uptake increased in the 30 and 32 year old pastures in l ine with the sl ightly higher dry matter yield recorded at this sampl ing. Both N and P concentration were highest in autumn, winter and early spring and lowest in late spring and summer each year as has been noted by other authors for P (McNaught, 1 970; Metson and Saunders, 1 978; Rowarth, 1 987) . The highest recorded N and P concentrations were 3 .5 - 4 .7% N in the first autumn sampled on 20/5/91 and 0.48 - 0. 56% P in the 2nd winter sampled on 1 8/8/92. The lowest recorded N and P concentrations were 2 . 1 - 2 .7% N in the early summer at the end of the two years on the 1 4/1 2/92 and 0.28 - 0 .35% P in the fi rst summer sampled on 67 1 1 /2/91 . A lack of water wou ld have restricted the d iffusion of nutrients to plant roots in summer. However, a very high proportion of dead matter, 20-40%, in summer would also have contributed to such low N and P concentrations and the N and P concentrations in the l iving component of the herbage would have been higher. Herbage N concentrations ofl<4%in ryegrass and<4.5% in clover at active growth are considered l im it ing to plant growth (Cornforth, 1 984) . As herbage N concentrations were general ly far below these levels, even in spring 1 991 when the pasture was cut regularly ensuring the sampl ing of active growth, a lack of N from N fixing legumes is most l ikely l imit ing pasture growth on the Wharekohe si lt loam. Some reduction in herbage N concentration may have occurred during sample storage, however such losses are not expected to have accounted for such low herbage N concentrations. The proportion of clover in the pasture swards was found to be very low in comparison to other soi ls (Morton et a I . , 1 994) . Herbage P concentrations were general ly h igher than the 0 .34%, 0 .34%, 0 .46% and 0.49% P, considered to be l im it ing to pasture growth in a m ixed sward in spring, summer, autumn and winter, respectively (Cornforth, 1 984) . Therefore factor/s, other than P concentration, are l im it ing clover growth and hence N fixation on the Wharekohe si l t loams on the Kaikohe station. pH may be one of these l im it ing factors ,as pH had fal len as low as 5 .3 - 5.6 by 1 993 on the older developed sites. However, it is not the only l im iting factor as pH was considered optimal at the youngest site wh i le the proportion of clover and herbage N concentration were not. Molybdenum is not considered to be l imit ing clover growth on the Wharekohe station. Although Mo had not been appl ied to the Wharekohe soi ls since 1 982, herbage analysis in 1 984, 1 988 and 1 989 revealed high Mo status on the Kaikohe Research Station. The total P uptake for the two years was 90 kg P/ha (38 kg P/ha in year one and 52 kg P/ha in year two) and was marginal ly lower than the 1 00 kg of P appl ied as superphosphate and RPR during this period. This relationship is s imi lar to the findings of Rowarth ( 1 987) where pasture P uptake was found to be s imi lar to a P appl ication rate of 50 kg P/ha appl ied as superphosphate. In Rowarth's study, P uptake was higher than P appl ication on gentle s lopes at low P appl ication rates « 50 kg P) and lower at high P appl ication rates. However, Perrott et a l . ( 1 992b) 68 found that P uptake was simi lar to P appl ied at 22. 5 kg P/ha on a central yel low brown earth. The lower avai labi l ity of P from RPR in the present study on the Wharekohe si l t loam may have resulted in s l ightly lower P uptake than for s imi lar rates of appl ied superphosphate. The values for soi l P and pasture P uptake measured over the two years are used in a model showing the fate of appl ied P in Wharekohe podzols presented in chapter 8 . 3 . 5 CONCLUSIONS • Pasture development and the associated appl ication of ferti l iser P on Wharekohe podzols results in an increase in total soi l P to the top of the E horizon with appl ied P accumulating to a higher concentration in the top 3 cm. • Evidence has been presented which supports an increase in the movement of appl ied P down the profi le with increasing pasture age. The Wharekohe si lt loam appears to have a fin ite abi l ity to retain P (referred to as the maximum P storage capacity) , wh ich is reached by 8 years in the 0-3 cm depth and by 1 1 years in the 0- 7 .5 cm depth. Once the P storage capacity at each depth is reached, there is l itt le further accumulation of appl ied P , apart from smal l quantities as ferti l iser P residue. Much of the P appl ied in subsequent appl ications moves through the soi l result ing in large soi l losses of P from the pasture cycle . • Soi l losses of appl ied P from older sites of up to 65% were measured. As l itt le of the lost P could be accounted for to a depth of 30 cm below the E horizon, lateral movement of P in soil water above the E horizon is the most l ikely pathway for P loss. • Pasture development on Wharekohe podzols results in an increase in O lsen P levels to the top of the E horizon. Olsen P reached an asymptote of 26 in the root zone of the oldest sites despite annual appl ications of P estimated to be in excess of the maintenance requi rements calculated by the CFAS model . ------ - 69 • Calculated large losses of appl ied P, combined with an apparent maximum Olsen P for the Wharekohe soi l , despite P appl ication in excess of the amount calculated by the CFAS model to maintain current stocking rates, indicate that the SLF used in the CFAS model may be too low. However, i f P appl ied in excess of animal losses is lost from the soi l , a high SLF would be counterproductive, increasing P losses from the pasture cycle to the wider environment. • Pasture production, botanical composition and herbage N and P concentrations were not effected by pasture age in this chronosequence study. Legume content was low at a l l sites which impacted on the N nutrition of the pasture. Herbage P concentrations were considered adequate for maximum pasture production at a l l sites, however, low N concentrations would have been restricting pasture growth at a l l sites. • Large losses of P from these Wharekohe soi ls are of economic importance and could impact on the environment through increased P loading of waterways. C learly further studies are required to investigate mechanisms for P retention and P loss in the Wharekohe podzols (Chapters 4-7) . --------- CHAPTER 4 CHANGE IN P FRACTIONS IN A PASTU RE DEVELOPM ENT CH RONOSEQU ENCE ON A WHAREKO H E PODZOL. 4. 1 INTRODUCTION 70 Over t ime, appl ied phosphorus has been shown to be lost from the root zone of a Wharekohe si l t loam (Chapter 3) . In add ition to P lost by physical movement and leaching of d issolved P from the root zone, the other component of the Soil Loss Factor (SLF) is the loss of P from the pasture cycle through the net accumulation in the soi l of non-labi le inorganic and organic P. The accumulation of appl ied P into various P fractions is known to be effected by several factors including pasture age, soi l depth, l ime appl ication, the rate and type of fert i l iser appl ication, the orig in of soil parent material and the degree of weathering of soi l minerals. Various P fractionation schemes have been developed to characterise the distribution and avai labi l ity of Pi and Po in soi ls based on their solubi l ity in chemical extracts (Chang and Jackson, 1 957; Wi l l iams et a I . , 1 967; Hedley et aI . , 1 982; Perrott et aI . , 1 989, Tiessen and Moir, 1 993). The methods of Chang and Jackson ( 1 957) and Hedley et a ! . ( 1 982) were wel l reviewed by Tiessen and Moir and the main pOints were: The method of Chang and Jackson ( 1 957) , wh ich was later modified by Wi l l iams et al. ( 1 967) , involved the extraction of labi le P (by NH4CI ) , fol lowed by the extraction of AI-Pi (by NH4F) , Fe-Pi (by NaOH) , "occluded Pi" (by dith ion ite-citrate) , Ca-Pi (by HCI ) and residual P (after Na2C03 fusion) . Organ ic P was determined separately as the difference between acid-extractable P i before and after ignit ion. This early fractionation procedure was l imited by the precipitation of Pi in the fluoride extraction, the unrel iabi l ity of AI and Fe separation and the "occluded Pi" pool being i l l-defined (Wi l l iams and Walker, 1 969a) . Consequently, an alternative fractionation procedure was developed by Hedley et a l . ( 1 982) which divided soi l P into labi le P i and Po (extracted by Resin P and NaHC03) , Fe & AI-Pi and stable Po (extracted by NaOH), Ca-Pi (extracted by HCI ) and residual P. 7 1 Th is fractionation procedure of Hedley et a l . ( 1 982) has been further modified by Perrott et a l . ( 1 989) , to include a pre l iminary NaCI extraction to remove exchangeable Ca wh ich precipitates with P in the alkal i extracts, and by Tiessen and Moir, ( 1 993) to include a concentrated acid extraction to d ist inguish between Po and Pi in the residual P. A modified version of these latter three fractionation procedures is used in this study to indicate the extent of P accumulation into non-labi le P compounds and possible reasons for P loss from the soi l profi le. In acid soi ls , P usually accumulates as alkal i extractable P i in Fe and AI phosphates, and as Pi and Po sorbed on the surfaces of sesquioxides (Saunders, 1 959a; Condron and Goh , 1 989; Perrott et a I . , 1 989; Floate and Enright, 1 991 ; Perrott et a I . , 1 992a) , wh i le in calcareous soi ls , P usual ly accumulates as Ca bound P (Hedley et a I . , 1 982; Tiessen et a I . , 1 983) . The Wharekohe soi ls are acid soi ls , but, due to extreme weathering, they contain very l itt le iron and alumin ium, and have extremely low An ion Storage Capacities « 1 0%) in the root ing zone (New Zealand Soil Bureau, 1 968). Hence, a low level of accumulation of P i and Po associated with Fe and AI in the Wharekohe soi ls , in contrast to less weathered soi ls derived from simi lar parent materia l , may account for the high loss of P recorded from the Wharekohe s i lt loam over t ime. Accumulation of P in specific fractions with depth may also provide an indication of how P moves through the profi le of Wharekohe soi ls . Alone, however, fractionation data cannot confirm how P moves in a Wharekohe podzol , in comparison to other New Zealand soi ls , as P can move down the profi le in one form to change in situ. to another (as inferred by Haynes and Wi l l iams, 1 992) . 4.2 OBJ ECTIVES The objectives of this P fractionation study were to assess: 1 . The effect of pasture age on the accumulation of appl ied P into various P fractions in a pasture development chronosequence on Wharekohe s i lt loam. 72 2. The contribution of appl ied P accumulation into plant unavai lable fractions to the SLF for use on the Wharekohe s i lt loam. 3. Possible reasons and pathways for P loss from the Wharekohe si l t loam. 4. The influence of l ime history and fert i l iser P form on the accumulation of appl ied P into various fractions in the Wharekohe si lt loam. 5. The effect of parent material on P fractions in Wharekohe podzols to determine possible reasons for the higher apparent P requirement of the Wharekohe sandy loam in comparison to the Wharekohe si lt loam. 6. The effect of the degree of weathering of the s i lt parent material on P chemistry by characteris ing P fractions in the soi l under pasture on the yel low brown earth, Aponga clay, compared with the Wharekohe si l t loam. 4.3 MATERIALS AN D M ETHOD 4.3. 1 Soils The Wharekohe s i lt loam soi ls , used for the P fractionation analysis, were sampled from the pasture development chronosequence to the top of the E horizon in spring 1 990 as described in Chapter 3. In order to confirm conclusions reached from the soil P data col lected in 1 990 and to investigate the effect of RPR appl ication on P fractions, the original sites and an additional four s ites were resampled in 1 993 as described in Chapter 3. The Wharekohe sandy loam and Aponga clay soi ls were sampled in 1 991 from plots within the trial areas which had been under pasture for at least 30 years as described in chapter 8. 73 4.3.2 Soi l Treatment Al l soi ls were air dried and sieved to <2 mm particle s ize and then ring ground to < 1 50 um. 4.3.3 Soil Analysis 4.2.3. 1 P Fractionation Soi l P in the 1 990 samples was divided into 1 0 fractions using a sequential extraction procedure modified from the methods of Hedley et a l . ( 1 982) , Perrott et a l . ( 1 989) and Tiessen and Moir ( 1 993) and out l ined in Figure 4. 1 . So i l P in the additional Wharekohe s i lt loam samples, Wharekohe sandy loam and Aponga clay was divided into 8 fractions, NH4CI Pi & Po, NaHC03 Pi & Po, NaOH Pi and Po, H2S04 P i and Residual P. The extraction solutions removed the fol lowing P fractions: 1 M NH4CI - The fi rst solution in the fractionation procedure is considered to extract avai lable P in soi l solution and P from the death of micro-organisms when the soi l is dried prior to fractionation (Chang and Jackson, 1 957; Sparl ing et a I . , 1 985) . NH4CI also extracts exchangeable Ca2+ which cou ld precipitate phosphate in the alkal i extracts leading to an underestimation of alka l i extractable P i and Po and an overestimation of Ca-P and Hot HCI extractable Po (Chang and Jackson , 1 957; Perrott, 1 992; Mackay et a I . , 1 986) . 0 .5 M NaHC03, pH 8 .5 - changes the solution in a s imi lar way to roots (Tiessen and MOir, 1 993) and therefore extracts avai lable P i adsorbed onto the surfaces of crystal l ine compounds, sesquioxides, or carbonates (Matt ingly, 1 975; Bowman and Cole, 1 978) and easi ly mineral isable Po which contributes to plant avai lable P (Bowman and Cole, 1 978) . I 0.9 9 air dried soil « 1 50Um)1 J, 36 ml 1 M NH4CI for 1 6 hr -J.. Centrifuge at 9000 rpm for 1 0 min J, residue J, 36 ml 0.5 M NaHC03 (pH 8.5) for 1 6 hr -J.. Centrifuge at 9000 rpm for 1 0 min J, residue J, 36 ml 1 M NaOH for 1 6 hr -J.. Centrifuge at 9000 rpm for 1 0 min J, residue J, 36 ml 0 .5 M H2S04 for 1 6 hr -J.. Centrifuge at 9000 rpm for 1 0 min J, residue J, 1 0 ml Cone. HCI for 1 0 min at 80°C + 5 ml Cone. HCI for 1 hour at room temperature -J.. Centrifuge at 9000 rpm for 1 0 min J, residue J, Triaeid - 5 HN03 : 5 HCI : 7 HCI04 at 260°C Figure 4. 1 . F low chart of sequential P extraction. 74 � NH4CI P Pi/Po � NaHC03 P Pi/Pc � NaOH P Pi/Po � H2S04 Pi � Hot HCI Pi/Po � Residual Pi 75 1 M NaOH - extracts less labi le P i in iron and aluminium phosphates, P i and Po held by chemisorption on sesquioxides and Po associated with humic and fulvic acids (Chang and Jackson, 1 957; Perrott and Mansel l , 1 989). NaOH Pi can contribute to plant P especia l ly where ferti l iser P is withheld, whereas NaOH Po is not readi ly avai lable to plants even where fert i l iser P is withheld (Hedley et a I . , 1 982; Goh and Condron, 1 989; Condron and Goh, 1 989). NaHC03 and NaOH extractable P i are not considered to be d iscrete pools, representing instead a continuum of Fe and AI associated P extractable with increasing pH from the orig inal pH of the soi l to 8 .5 to 1 3 (Tiessen and Moir, 1 993) . O .5M H2S04 - extracts mostly P i he ld in calcium phosphates (apatite) and a small quantity of P released upon the part ial d issolution of secondary more crysta l l ine sesquioxides (occluded Pi) (Hed ley et a I . , 1 982; Tiessen and Moir, 1 993). The d i lute acid extractable fraction usual ly contains l itt le or no Po (Perrott et a I . , 1 989) and this was confirmed on the Wharekohe soi l in a prel iminary fractionation where less than 2% of the total P was recovered in the HCI Po fraction. D i lute HCI is most commonly used in fractionation procedures however H2S04 has been shown to be more effective at extracting P from highly weathered soi ls although the reasons for this are not clear (Tambunan, 1 992) . Some Ca-P i can be made avai lable to plants as the pH in the vicinity of plant roots fal ls lead ing to the dissolution of Ca-Pi (Hedley et a I . , 1 982) . Hot Conc. HCI - extracts Pi and Po in very stable residual pools (Tiessen and Moir, 1 993). Some of the Po may be avai lable Po from organic matter which is not a lkal i extractable (Tiessen and Moir, 1 993). Other fractionation procedures commonly use a second NaOH extraction to remove this P pool . However, in highly weathered soi ls hot HCI is preferable as i) Hot HCI extracts more P (Tiessen and Moir, 1 993) and i i ) NaOH fol lowing acid extraction removes s i l icon wh ich interferes with colour development in the phosphomolybdate method of Murphy and Ri ley ( 1 962) (H .Tiessen, personal communication) . Tri-acid digest - extracts residual P which is unl ikely to contain anyth ing but recalcitrant Pi (Tiessen and Moir, 1 993) . 76 Supernatant solutions from cold extractions were fi ltered through 0.45 um m i l lepore fi lter papers. The hot HCI solutions were fi ltered through Whatman No. 40 fi lter papers. Inorganic P was determined in each extract and d igest colorimetrical ly by the phosphomolybdate method of Murphy and Ri ley ( 1 962) . Interference to absorbence readings by discolouration from extracted organic matter i n the a lka l i extracts was accounted for by read ing the absorbence of the extracts with Murphy and Ri ley solution without the addition of ascorbic acid. Total P in each extract was determined after digestion in 2 ml of concentrated H2S04 overnight at 360°C fol lowed by the addition of 1 drop of H202 and reheating for 30 minutes to clear the d igest. Organ ic P was determined as the difference between Pt and P i . 4.2.3.2 pH Ten grams of soi l was stirred with 25 ml of disti l led water and left overnight. pH was measured in the soi l solution with a glass electrode. 4.2.3.3 Cation Exchange Capacity and Exchangeable Cations Exchangeable calcium in the orig inal Wharekohe s i lt loam samples was removed by leaching with 1 M ammonium acetate method (B lakemore et aI . , 1 987). An atomic absorption spectrophotometer was used to measure the calcium content of the leachates. Cation Exchange Capacity (CEC) and exchangeable Ca, Mg, K and Na were determined using the si lver thiourea method (Searle, 1 986) for the additional Wharekohe silt loam samples col lected in 1 993 and the bulked Wharekohe sandy loam and Aponga clay samples. 4.2.3.4 Total Calcium Total calcium was measured in the soi ls sampled from the orig inal s ites in 1 990 after d igestion in a tri-acid mixture of concentrated HN03 : HCI : HCI04 (5 :5 :7) (Bolan and 77 Hedley, 1 987}. One gram of soi l was digested with 25 ml of tri-acid in a 250 ml Erlenmeyer flask at 260°C unti l white fumes appeared. The d igest was made up to 50 ml in volume and al lowed to settle before fi ltering through 0 .22 IJm mi l lepore fi lters and total Ca determined by atomic absorption spectrophotometry. 4.3.3 Statistical Analysis As in Chapter 3, resu lts from each P extraction , the sum of the inorganic and organic P fractions (sum Pi and sumPo), pH, exchangeable Ca and total Ca for the spring 1 990 soils were subjected to analysis of variance using Genstat to determine d ifferences between sites for each depth and between depths for each site. For the soi ls col lected in spring 1 993, the relationships between each fraction, sumPi , SumPo, pH, CEC, exchangeable cations and total Ca, and pasture age were determined by using the computer program FLEXI (Upsdal l , 1 994) . 4.4 RESU LTS AN D DISCUSSION 4.4. 1 Effect of Pasture Age on the Accumulation of Applied P i nto Soil Fractions in a Wharekohe silt loam 4.4. 1 . 1 Inorganic P Inorgan ic P accumulation levels (F igure 4 .2) fol lowed a s imi lar pattern to total P accumulation (Figure 3 . 1 ) . Pasture development resulted in an increase in P i as was found in other New Zealand studies (Jackman, 1 951 ; Walker et a I . , 1 959; Perrott and Sarathchandra, 1 987; Condron and Goh, 1 989; Nguyen et a I . , 1 989) and Pi concentration decreased with depth on all s ites (Figure 4.2) . Al l of the developed sites had simi lar P i concentrations in the top 3 cm, except the 30 year old site where Pi was significantly lower. In the 3-7 . 5 cm depth, the inorgan ic P concentrations of the 8 and 30 year old s ites were Significantly lower than the other developed sites. (The early l iming history had influenced Pi 600 500 -� 400 C) � - 300 n.. a) 0-3 cm ... /,/-o 200 U) /' /..-: .... / 1 00 // o r···./ o // 5 b) 3-7.5 cm 350 300 - 250 C) - � 200 - n.. 150 ·0 U) 1 00 / ---------------- - --- /..$. .......... . ... ., .. .. ..... . .. ".. . ... ". ... ....... .. . _ • • nv . ... �··· .. ' • 1 0 15 1 0 15 // /r{0- . . w •. � • . __ ._ •• __ .� ...... . . . ........ ,"' .... . , . ....... ¥r/ .:<' 20 25 30 20 25 30 c) 7.5 cm - E horizon 1 80 1 60 o 5 1 0 15 20 Years of Development Key: --+-Inorganic P . . . 1&: . . . . Organic P 25 30 Figure 4.2 Effect of pasture age on inorganic and organic P concentration in each depth of a pasture chronosequence on a Wharekohe silt loam . 78 35 35 35 79 accumulation at the sites. Lower l ime appl ication on the 30 year old s ite had led to lower Ca-Pi accumulation, explained further in 4.4.4) . As was the case with total P data, interpretation of P i levels in the 7.5 cm to E horizon samples was compl icated by large d ifferences in soi l depth to the E horizon. P i concentration in the 8 year old s ite was significantly lower than that of the 23 and 25 year old sites. P i concentrations in the 30 and 32 year old s ites were not significantly different from the 8 year old s ite. However, the amount of inorganic P (kg/ha) increased with pasture age at this depth below 7 .5 cm (Table 4. 1 ) . Table 4. 1 Age 0 8 23 25 30 32 Inorganic and organic P content (kg P/ha) of Wharekohe s i lt loam samples. Inorgan ic P Organic P (kg/ha) (kg/ha) 0-3 cm 3-7 .5 cm 7 .5 cm-E 0-3 cm 3-7 .5 cm 7 .5 cm-E 1 1 1 4 21 1 4 1 9 24 1 1 7 82 59 50 49 56 1 34 1 57 58 84 1 06 33 1 1 3 1 1 0 73 74 1 06 75 89 82 72 79 1 04 1 1 0 1 06 1 09 1 05 77 1 26 1 62 As with total P , the abi l ity of the Wharekohe si l t loam to sorb Pi appears to reach an asymptote at 8 years in the 0-3 cm depth, and between 8 and 23 years for the 3-7 .5 cm depth . Hence, the accumulation of Pi into non-lab i le P i cannot be contributing significantly to the SLF in the older developed sites, apart from the l ikely accumulation of a small quantity of P i as insoluble Ca-Pi residues from annual P ferti l iser additions. It is also possible that Ca-Pi decreases with pasture age in older sites as Ca-P, which was associated with the in itial heavy l ime appl ications with pasture development, is lost as Ca is leached from the soi l . The calculation of P i accumulation over t ime from pasture development was restricted to differences 80 between the sites, rather than continual actual measurements over time, the extent of the smal l amount of Pi accumulation was impossible to quantify for the soi ls sampled in 1 990. The impact of the change to less soluble superphosphate and RPR on soil P fractions is discussed further in 4.4.4 . P i accumulation in the Wharekohe si l t loam fol lowed a d ifferent pattern to that reported for other New Zealand soils. Other New Zealand studies (Condron and Goh, 1 989; Nguyen et a I . , 1 989; Perrott et aI . , 1 992a; Rowarth et a I . , 1 992a) have shown that with continued P appl ication, Pi continues to accumulate in the soi l with increasing pasture age. The rate of Pi accumulation is dependent on soi l type, the rate and type of appl ied P and the rate of Po accumulation. Pi accumulates at a greater rate where P is appl ied at rates surplus to pasture requirements, where P fert i l iser contains low soluble P i or where Po accumulation has slowed or stopped. The reduction in P i accumulation in the older sites in the present study cannot be explained by differences in P appl ication or pasture P requirements between the youngest and older sites. S imi lar rates of P were appl ied to a l l the developed sites in recent years, except the site developed in 1 960 which had received ma�ginal ly lower rates of P in the 1 0 years prior to the 1 990 sampl ing. Pasture production and herbage P concentration on the developed sites were s imi lar over a 2 year period (Chapter 3). Although pasture production and herbage P figures prior to 1 990 are not avai lable for a l l the sites in this study, P requirements are l ikely to have been s imi lar for the developed sites once pasture maintenance had been reached. The rate of organic P accumulation did not increase with increasing pasture age in the top 7 .5 cm (4.4. 1 .2 ) and so this can be d iscounted as a reason for such a drastic reduction in Pi accumulation. D iscussion of the fate of appl ied P in chapter 3 indicated that appl ied P not held in the topsoi l was lost from the effective root zone of the soi l in the older developed sites (Chapter 3) . 81 4.4. 1.2 Organic P As with inorganic P , pasture development resulted in an increase in organic P with organic P concentration decreasing with depth on a l l s ites (F igure 4.2) . Organic P has also been found to increase with pasture development in other New Zealand studies (Jackman, 1 95 1 ; Walker et a I . , 1 959; Perrott and Sarathchandra, 1 987; Condron and Goh, 1 989; ) . Organic P concentration increased at each depth with increasing years from development. Po concentration in the 7 .5 cm - E depth dropped in the 32 year old site, however once again this drop was a function of depth to the top of the E horizon and expressing results in kg Po/ha indicated a higher level in the 32 year old s ite than the other developed sites (Table 4. 1 ) . Hence, it appears that the soi l 's capacity to retain 'p as organic P has not been reached by 32 years at any deptl:l and _the . ; accumulation of appl ied P as less labi le Po (extracted by NaOH and Hot HCI ) may be contribut ing to the SLF (discussed further in 4 .4 .2) . Despite increasing Po and a constant P i , the combined effect of P i and Po accumulation was a constant total P concentration beyond 8 years in the top 0-3 cm and beyond 23 years in the 3-7 .5 cm depth (Chapter 3) . The maximum P storage capacity d iscussed in chapter 3 can be solely attributed to a maximum Pi storage capacity as Po continued to accumulate. The accumulation rate of Po is in itia l ly higher than the accumulation of P i after in it ial pasture development from native vegetation in New Zealand soi ls including some podzols where OM levels are low and C :N ratios are general ly high (Jackman, 1 95 1 ; Jackman, 1 960; Walker et a I . , 1 959; Powel l and Taylor, 1 980). In the present study on Wharekohe soi ls there was a large increase in Pi in the top 7 .5 cm in the first 8 years after pasture development. However, it is not possible to precisely assess the Po accumulation pattern in the first years fol lowing pasture development due to the lack of developed sites of less than 8 years. It is possible that applied P may have accumulated as Po at the expense of Pi in it ia l ly in the Wharekohe soi ls as was the situation in a South Is land podzol (Powel l and Taylor, 1 980). Appl ied P accumulated 82 solely as Po in the first two years after pasture development in the Maimai gley podzol with both P i and Po accumulating in the thi rd year. Provided soi l Pi levels are sufficient to support pasture growth, Po accumulation is more dependent upon the growth rate of pasture and the associated inputs of nitrogen than the appl ication of P (Burgess and Davies, 1 95 1 ; Saunders, 1 959a; S impson et a I . , 1 974; Lambert et a I . , 1 988; Nguyen and Goh, 1 989; F loate and Enright, 1 991 ; Perrott et a I . , 1 992b) . However, Wharekohe soils contain very l itt le native Pi and pastures cannot be establ ished without appl ied fert i l iser P , so Po is unl ikely to accumulate at any depth unt i l the Pi levels are raised by the appl ication of fert i l iser P . After the h igh init ial immobi l isation rate, the rate of Po accumulation during pasture development usua l ly decreases with time from pasture development in the top soi l of New Zealand soi ls (Jackman, 1 95 1 ; Walker et a I . , 1 959; Jackman 1 964a; Perrott et a I . , 1 989). The average rate of Po accumulation (I-Ig Po/g) was slower in the older sites in comparison to the 8 year old site in the top 3 cm which is in agreement with the findings of the other New Zealand studies. However, at the 3-7 . 5 cm depth, the average rate of Po accumulation was simi lar in the 8 and 23 year sites with Po increasing at a s lower rate at this depth than in the top 3 cm ind icating that Po is l ikely to reach equi l ibrium closer to the soi l surface first. The rate of Po accumulation was expected to be higher in the 3-7 . 5 cm depth of the 8 year old s ite than in the older sites, as was the case in the 0-3 cm depth, because appl ied P is rapidly immobil ised in soi ls with low OM and high C :N ratios in the first years fol lowing pasture development. Possible explanations for the lower than expected Po accumulation in the years fol lowing in it ial pasture development are i ) restricted movement of appl ied P to this depth before the top 3 cm reaches its P storage capacity, combined with i i ) the recent heavy l iming h istory of the 8 year old site as l ime can encourage the mineral isation of Po (Condron and Goh, 1 989; Nguyen et a I . , 1 989; Condron and Goh, 1 990) (d iscussed further in 4 .4.4) . The average rate of Po accumulation per gram of soi l was lower in the 7 .5 cm - E horizon depth of the 8 year old site than at a s imi lar depth in the 23 year old s ite confirming 83 that l im ited movement of appl ied P to depth restricts P immobi l isation in the early years fol lowing init ial pasture development. Interestingly the amount of Po accumulat ing, up to 267 �g Po/g soi l for the top 7 .5 cm of the 32 year o ld site receiving on average 32 kg P/ha, was s imi lar to the amount of Po (240 �g Po/g) wh ich had accumulated over 32 years in the top 7 .5 cm of a weakly weathered yel low grey earth at Winchmore under i rrigation (Nguyen and Goh, 1 989) at two appl ication rates ( 1 7 .5 and 35 kg appl ied P/ha). Hence, Po accumulation with in 32 years does not appear to be restricted when compared to a less weathered sedimentary soi l with higher sesquioxide content (New Zealand Soi l Bureau, 1 968). However, Po accumulation in the Wharekohe soi l is expected to reach equ i l ibrium earl ier than in the less weathered soi l due to its lower content of AI and Fe complexes for soi l organ ic matter stabi l isation. The comparison of the extent of Po accumulation in the development of Wharekohe si l t loam in the present study with Po accumulation in soi ls in other studies is restricted by these other studies having i) unknown times from init ial pasture development, i i ) the use of different soi l sampl ing depths and i i i ) d iffering P appl ication rates. 4.4. 1.3 Po/Pi Ratio The Po concentrations measured may be at their lowest annual level in a l l sites due to mineral isation in spring leading to lower Po/Pi ratios (Perrott et a I . , 1 992b) . Organic P was higher than inorganic P in a l l depths of the undeveloped sites with Po/Pi ranging from 53:47 to 58:42 (Figure 4 .3) . Pasture development with its associated application of fert i l iser P resulted in a decrease in the Po/P i ratio in al l depths to as low as 30/70 in the top 0-3 cm of the 8 year old site. However, once Pi reached a peak, organic P continued to accumulate and hence the Po/Pi ratio increased at each depth with more than 50% of the soi l P as organic P below 3 cm in the 2 oldest sites. 65 60 55 � 50 o o � 45 40 35 30+-----�----��--_r----_+----�----�------r_--� o 4 8 1 2 1 6 20 24 28 Years of Development �0-3 em -11-- 3-7.5 em ···,u···· 7 .5 em-E Figure 4 .3 Effect of pasture age on organic P content (%) at each depth. 32 84 The Po/Pi ratios obtained for the highly modified Wharekohe soi ls, where Pi reaches equi l ibrium before Po due to a low maximum Pi storage capacity, are d ifferent from the ratios obtained on other New Zealand soi ls. Most undeveloped soi ls have very high Po/Pi ratios wh ich decrease over time under developed pastures as the Po content approaches equi l ibrium and the appl ied P continues to accumulate as Pi (Jackman 1 951 , Walker et a I . , 1 959; Condron and Goh, 1 989; Perrott and Sarathchandra, 1 987; Nguyen et a I . , 1 989). The Wharekohe soil had been highly d isturbed by gum diggers and the cut over manuka vegetation was not the cl imax­ equi l ibrium vegetation cover, but the result of the fel l i ng of native timber trees, which may have led to a lower Po: P i ratio on undeveloped soi ls . 4.4. 1.4 Changes in P Fractions Over half the P extracted at a l l depths from the undeveloped sites was found in the most labi le and easi ly extractable NH4CI and NaHC03 Pi and Po fractions (F igure 4.4) . Hence much of the P in the undeveloped podzol under Manuka vegetation o I/) C) - C) :::1 D.. - '0 I/) C) - C) :::1 D.. a) 0- 3 cm i) Inorganic P . • ,: . . .. ... . ....... . . . . . . . .. . . . . . : . .. . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . •••••••••• • . . . . . . • .. ·: . . . . • . . . . .. • . . . . . . •• . . • . . . . f 1 00 O -F---- o i i ) Organic P o 8 23 25 30 32 ..... � · ·�·��·�····· · · ·� . i···· · ·· · · · · · .... · · · · · · · · · ·v : : . . . . . . . " . . . . . . . . . • . . . . . . . · · · · · · .. · .. · · · · · · · · · ·• .. ·')'··,,· .. ···· .... · · · · · .. · .. 1 i I 8 23 25 30 32 Years of Development Figure 4.4 Comparison of i norganic and organic P fractions measured i n each site at each depth. 85 Pi Pi Po -0 I/) C) - C) ::1. - Q. - '0 I/) C) - C) ::1. - Q. b) 3-7.5 cm ( Fig. 4.4) i) Inorganic P 250 200 1 50 1 00 50 0 0 8 23 25 30 32 i i ) Organic P .. , . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . -- -- . . . . . --... . . . • . --...... -- ..... . , ... .. . . . . . . . . . . . . . . . . ...... .. �.-• • • . . . . . ..... . . . . . ...- .--.•..•. --.•.•.•. "" . . . . .... --.----: .. · , ········· · · · ·· · · · · · · · · · · · · · ·i····················· ...... , . . ... .. . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . . . l l o 8 23 25 30 Years of Development 32 86 Po 87 c) 7.5 cm - E horizon ( Fig. 4.4) i ) Inorganic P 1 80 1 60 1 40 - 1 20 0 t/) 1 00 C) - C) 80 :::1. a.. 60 40 20 0 0 8 /' 23 25 30 32 NH4CI Pi i i ) Organic P .u . ..... . . . . . __ •• u��� .. 1 80 1 60 -----------------1 140 - 1 20 0 t/) 1 00 C) - C) 80 :::1. a.. 60 Po 40 20 Po 0 0 8 23 25 30 32 Years of Development 88 appears actively involved in the cycl ing pool . The high labi le Po was unexpected as Po usual ly exists as more stable Po in highly weathered soi ls (Cross and Schlesinger, 1 995). However, as the Wharekohe podzols are amongst the most weathered soi ls in the world , thei r extremely low sesquioxide contents may not be conducive to the stabi l isation of very much Po. The high proportion of Pi as labi le P i was simi lar to that of other spodosols with very low total P contents (Pare and Bern ier, 1 989; Trasar-Cepeda et a I . , 1 990) . Calcium-P extracted by H2S04 was the smal lest fraction in a l l undeveloped samples as predicted for a highly weathered soi l (Walker and Syers, 1 976; Cross and Schlesinger, 1 995), with only 0-1 .6 I-Ig PIg being measured. H ighly residual P i , as extracted by the hot HCI and Tri-acid digest, was a larger fraction than e ither the Fe and AI-P extracted by NaOH or the acid extractable Ca-P at each depth in the' spring 1 990 samples in agreement with the findings of Walker and Syers ( 1 976) on other highly weathered soi ls . Al l inorganic and organic P fractions increased with pasture development in the top 7 .5 cm of the soi ls (Figure 4.5) . Other New Zealand studies have also shown increases in a l l P fractions in the top soi l fol lowing in it ial pasture development (Perrott et aI . , 1 989; Haynes and Wi l l iams, 1 992) . Except the most residual Pi , which was a very smal l fraction of the inorganic P , a l l i norganic and organic fractions decreased with soi l depth (Figure 4.3). Inorganic P Fractions There was l ittle difference between the Pi extracted from the developed sites for most fractions in the top 0-3 cm. Exceptions were where NH4CI Pi was lower in the 23 year old s ite, NaOH Pi was lower in the 8 and 30 year old sites and H2S04 Pi was lower in the 30 year old s ite. The very low H2S04 Pi concentration measured in the 30 year old site was not related to pasture age but was instead pecul iar to that particular s ite and related to early l iming history (explained further in 4.4.4) . In the 3 - 7.5 cm depth the Pi extracted in each fraction was lower in the 8 year old site than in the older developed sites, except the H2S04 Pi fraction in which there Inorganic P Fractions 80 c; 60 C, .a 40 a:: U z Oi 60 C, 50 .a 40 a:: 30 0 20 u J: 1 0 III Z 0 0 4 8 1 2 1 6 24 28 32 80 I A � 60 .a a:: 40 J: 0 20 III Z o 4 8 1 2 1 6 20 24 28 32 400 Oi C, 300 .a I B a:: 200� � o ______ � . U) 1 00 .- M' " £ . i .... ... • .. • .... ,,"" ..... ·,.· ........ •• ' ..... �f:.;.--.. . . ...... � o '. . . ·---r--·�·k'··"' I I I I I ..• «; o 4 8 1 2 1 6 20 24 28 32 � .a a:: U J: 0 0 4 8 1 2 1 6 20 24 28 32 Oi 30 C, 25 .a 20 a:: 1 5 iii ::::I 1 0 "C 'Uj 5 41 II:: 0 0 4 8 1 2 1 6 20 24 28 32 Years of Development 30 � 25 en 20 2- o 1 5 Q. U Z � 1 50 en .a 1 00 0 Q. 0 50 u J: III Z 250 � 200 en -; 1 50 Q. 1 00 J: o III Z � en .a 0 0.. U 35 30 25 20 1 5 1 0 5 89 Organic P Fractions 0 0 I A I B /" p------ei i/.� . . �._._/. .. ( .x ." .. . " O/, .. ,/.� . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . � "'� 4 I 4 I A I B 8 8 1 2 1 6 1 2 1 6 I I 20 24 28 32 20 24 28 32 J: 0 �. __ '-4-€) i===+==:::ii�==+::..::::�.""""., . . wl_.-.tt I I -5 4 8 1 2 1 6 20 24 28 32 Years of Development Key: -+-0-3 em -&-3-7.5 em ·····3···· 7.5-E em Figure 4.5 Effect of pasture age on each P fracti on at each depth. (Vertical Bars = S.E.D., A for comparison of ages with i n depths, B for comparisons of depths within ages.) 90 was no sign ificant difference between any of the developed sites. Inorganic P continued to accumulate in each of the other fractions unt i l some time/s between 8 and 23 years in the 3-7 .5 cm depth. Hence, only the sites older than 23 years had simi lar or larger concentrations of alkal i Pi than H2S04 P i . The largest P i fraction in both the 0-3 cm and 3-7 .5 cm depths of the developed sites was the H2S04 P i , accounting for up to 41 % of the total P in the top 7 .5. The accumulation of appl ied P as Ca-Pi is effected by fert i l iser type and l ime appl ication explained further in 4.4 .4. Once again, analysis of P fractions below 7 .5 cm was compl icated by the depth to the top of the E horizon. Increases in each inorganic P fraction with development in the 7.5 cm - E horizon were much smal ler than in the top soi l . A l l of the Pi fraction concentrations increased with pasture development below 7 .5 cm. However, only the NH4CI , NaHC03 and H2S04 Pi fractions were Significantly greater in the 8 year old site compared to the undeveloped site. Therefore, only the sites developed for at least 23 years had NaOH, Hot HCI and residual extractable Pi concentrations which were sign ificantly d ifferent from those extracted in the undeveloped sites. Inorgnaic P appears to accumulate init ial ly below 7 .5 cm in a fraction extractable by NaHC03. After 8 years smal l amounts of NaHC03 Pi continued to accumulate with associated smal l increases in NaOH Pi and residual P i , whi le a far larger proportion of applied P accumulates at this depth as H2S04 P i . Hence, the proportion of Pt as H2S04 Pi below 7 .5 cm increased init ial ly to 23 years before dropping again as H2S04 Pi decreased and Po continued to accumulate. The 23 year old site was the only site where H2S04 Pi was larger than NH4CI + alka l i Pi at this depth. When the depth to the E horizon was taken into account, the 32 year o ld site had the greatest Pi content in each fraction except H2S04 Pi and the amounts of NaHC03 and NaOH Pi were simi lar to the amount of H2S04 Pi recovered in the 23 year old s ite . Haynes and Wi l l iams ( 1 992) also found increases in a l l P i fractions, calculated as the d ifference in P fractions recovered in fert i l ised plots compared to unferti l ised controls, in the 1 0-20 cm depth of i rrigated Lismore stony s i lt at Winchmore. H2S04 Pi increased the most fol lowed by NaOH Pi in thei r study. However, interference of 91 exchangeable Ca in the fractionation procedure would have underestimated the accumulation of NaOH Pi and overestimated the accumulation of H2S04 Pi in the Winchmore soi l . Saunders ( 1 959a) on the other hand, found that P accumulated as AI and Fe-P rather than Ca-P in the 5-1 2 .5 cm depth of a yel low brown loam high in Fe and AI complexes where the Chang and Jackson ( 1 957) procedure was used to fraction soi l P . The impl ication of P accumulation in various fractions at depth to P movement in the Wharekohe si lt loam is d iscussed in 4.4 .3 . Fert i l iser P has also been shown to accumulate mostly as Fe and AI-P in the top sampl ing depth of a wide range of acid soi ls in New Zealand (Saunders, 1 959a; Steele, 1 976; Grigg and Crouch ley, 1 980; Condron and Goh, 1 989; Perrott and Mansel l , 1 989; Perrott et a I . , 1 989; Floate and Enright; 1 991 ; Perrott et a I . , 1 992a; Rowarth et a I . , 1 992a) and overseas (Sainz and Arines, 1 988; Satte l l and Morris, 1 992) studies where superphosphate had been applied using fractionation procedures modified from the methods of either Chang and Jackson ( 1 957) or Hedley et al. ( 1 982) . However, the fractionation of soi l P in the Wharekohe s i lt loam has confirmed that l ittle appl ied P accumulates as Fe and AI-P extractable by NaOH, despite the acid nature of the soi l . It is the lack of alka l i extractable P i accumulation, rather than unusually high Ca-P accumulation, which has resulted in such a high proport ion of total P being extracted by H2S04 in the top 7 .5 cm of the Wharekohe si lt loam, as the amounts of Ca-Pi in the Wharekohe soi ls are not high in comparison to other New Zealand soi ls (Table 4.2) (Perrott et a I . , 1 992a) . Alka l i P accumulation was also l imited in the top 7.5 cm of two Northern podzol ised yel low brown earths in a two part fractionation , NaOH Pi and HCI P i fol lowing removal of Ca with a NaCI buffer, in contrast to 6 other soi ls in the study (Perrott et aI . , 1 992a) . The concentrations of NaOH P i in the developed sites on the Wharekohe si l t loam were even lower than the more avai lable P i (NH4CI P i + NaHC03 P i ) at all depths (Figure 4.4) . The NH4CI extractable P i was surprisingly large, up to 64 I-Ig PIg in the top 0 - 3 cm of the developed sites, i l lustrating how weakly P is held in the soil under 92 pastures on Wharekohe si l t loam. Readi ly avai lable P i as measured by NH4CI and NaHC03 extraction was considered to be ample to meet pasture requirements on a l l of the Wharekohe s i l t loam developed sites as herbage P concentrations were above levels wh ich are l ikely to l im it pasture growth (Cornforth, 1 984). The accumulation of appl ied P into the plant unavai lable hot HCI P i and residual Pi fractions was very low (only 3% of the total accumulated P) in the top 3 cm of the 32 year age site. Organic P Fractions The increase in the sum of the Po fractions with pasture development was reflected in a l l of the Po fractions in al l soi l samples (F igure 4.5) . The rate of Po accumulation in each fraction appeared to slow with pasture age in the top 3 cm. NH4CI Po appeared to reach equi l ibrium at some point between 23 and 25 years in the top 7.5 cm. Perrott et a I . , 1 989 also found that the least stable Po fractions reach equi l ibrium first. The remainder of the P fractions were sti l l accumulating by 32 years. NaOH Po and to a lesser extent NH4CI Po increased in it ial ly with pasture development and pasture age in the 7.5 cm - E horizon depth. NaHC03 Po accumulation occurred after 8 years. NaOH Po accumulated at a faster rate and made up the largest proportion of Po at a l l depths in the developed s ites (Figure 4.5) . As i norganic P accumulation had most l ikely ceased by 23 years after pasture development on the Wharekohe si l t loam in the present study, with the exception of smal l quantities of Ca-P from fert i l iser residues, the proportion of total P as NaOH Po increased with t ime. Other studies have also shown that most of the Po accumulates in New Zealand (Condron and Goh, 1 989, Perrott et a I . , 1 989) and overseas (Sattel l and MorriS, 1 992) soi ls as the more stable NaOH Po after in itial pasture development 93 4.4. 1.5 Spring 1993 The later sampl ing (to 7 .5 cm) in spring 1 993 of additional sites which had been developed in 1 967, showed that the 23 year old site, sampled in spring 1 990, was not representative of other s imi larly aged sites due to its having a h igher level of H2S04 extractable Pi (Appendix 4. 1 ) . Samples col lected from both the orig inal and additional s ites i n 1 993 confirmed the conclusions reached from the orig inal 1 990 samples (Appendix 4. 1 ) . A maximum P i storage capacity was reached by 1 1 years i n the top 7 .5 cm of the additional sites. Organic P concentration increased with years from development. The 33 and 35 year old additional sites had s imi lar Po concentrations, however, it is not possible to determine if the Po capacity was reached between 33 and 35 years due to a lack of s ites beyond 35 years to confirm the trend. Although a significant d ifference was not detected , the sum of the inorganic P fractions appeared to be lower in the oldest developed site than in the younger sites, expla in ing why total P did not increase with increasing pasture age although organic P did. Despite the less comprehensive extraction procedure used for the 1 993 samples, the sum of the extracted Pi and Po fractions indicate a simi lar pattern of increasing Po/Pi ratios with pasture age for the developed sites. Both sets of 1 993 samples from the undeveloped sites confirmed that a large portion of P (41 - 42%) was involved in the active cycl ing pool , whi le no H2S04 Pi was recovered. Once again, a l l inorganic and organic P fractions increased with pasture development. NaOH Po was confi rmed as the largest Po fraction and H2S04 Pi as the largest Pi fraction. Comparison of appl ied P accumulation into various fractions between years is compl icated by seasonal effects on the flux of P between d ifferent fractions. However, comparison of the P recovered in the orig inal sites sampled in 1 990 with the P recovered from the same sites in 1 993 revealed that the continued accumulation of appl ied P recorded for those years and reported in chapter 3 was 94 due mostly to its continued accumulation as H2S04 P i . The accumulation of much of the appl ied P as H2S04 Pi in the older sites confirms that applied P accumulates as a fert i l iser P residue once the maximum P storage capacity has been reached, . Phosphorus continued to accumulate i n smal l quantities i n a l l other measured Pi fractions on a l l of the original sites, although the results from the 1 990 sampl ing indicated that a l l inorganic P fractions had reached equi l ibrium in the older s ites. However, the smal l increase in each of the Pi fractions (aside from H2S04 P i ) may have been effected by the large increase in P appl ication rates from approximately 30 kg P/ha per year to an average of 49 kg P/ha per year between 1 990 and 1 993. A comparison of the P fractions accumulating on the original s ites between 1 990 and 1 993 with the pattern of P accumulation in the soi ls in the National Series study by Perrott et a l . ( 1 992a) supported the earl ier conclusion that the high proportion of H2S04 Pi in the developed sites on the Wharekohe s i lt loam was due to low alka l i P accumulation rather than excessive H2S04 Pi accumulation (Table 4.2) . The accumulation rate of H2S04 Pi in the Wharekohe soi ls was within the range of accumulation rates for the soi ls in Perrott et a l . 's study where RPR had been appl ied . However, H2S04 Pi accumulation was very high in the youngest Wharekohe site compared to the sites in Perrott et a l . 's study where TSP had been applied. By 32 years, H2S04 Pi accumulation had dropped markedly on the Wharekohe si l t loam. Some of the H2S04 Pi wou ld have accumulated in the youngest Wharekohe developed site from the 37 kg of RPR appl ied in 1 993. However, much of the 54 kg H2S04 Pi/ha which had accumulated over the 3 years wou ld have been derived from superphosphate. Although H2S04 Pi accumulation may be very high in the early years fol lowing pasture development ( related to high lime inputs leading to high Ca content and pH, explained further in 4.4 and chapter 7) , H2S04 Pi accumulation is very much reduced in older sites. Table 4 .2 95 Estimated a lka l i and acid Pi accumulation in the original 8 and 32 year sites for a 3 year period compared to estimated accumulation for a 3 year period in MAF 'National Series' soi ls , TSP and RPR appl ied at twice maintenance. Soi l Soi l group Anion Alka l i Storage (�g/g) Capacity (%) Wharekohe si l t loam Northern Podzol < 1 0 28 (8 year old site) Wharekohe s i lt loam Northern Podzol < 1 0 30 (32 Year old site) Hukerenui s i lt loam* Podzol ised NYBE 1 5 29 36** Warkworth clay* Podzol ised NYBE 34 30 22** Whangaripo clay* NYBE 22 74 68** Waitoa si lt loam* Gley 70 1 72 84** Te Kuiti s i lt loam* Yel low brown 98 302 loam 80** Lismore stony s i lt loam* Yel low grey earth 25 32 20** Monowai stony si lt loam* Yel low brown 75 1 1 1 loam 44** Waikoikoi si l t loam* YBEIYGE 28 70 35** NYBE=Northern yel low brown earth YGE=Yel iow grey earth * Data from National Series Trials (Perrott et a I . , 1 992a) . ** RPR appl ied Pi Acid P i ( �g/g) 1 05 40 20 1 08** 4 1 74** 25 6 1 ** 40 1 92** 38 300** 5 41 ** 1 9 1 89** 1 5 49** 96 The estimates of appl ied P accumulating in each fraction over a 3 year period in the soi ls of Perrott et a l . ' s study were calculated as half the d ifference between their values in the fert i l ised plots and the unfert i l ised control plots. Calculation of accumulated Pi in this way wou ld most l ikely have resulted i n an overestimation of both H2S04 P i and, to a greater extent, NaOH P i which may have been uti l ised by pasture growth in the control plots receiving no fert i l iser, but comparisons are sti l l val id . The alkal i P i accumulation in the Wharekohe s i lt loam (An ion Storage Capacity < 1 0%) was considered low and ranked with those calculated for other soi ls with low Anion Storage Capacities, Hukerenui si l t loam (Anion Storage Capacity 1 5%) , Warkworth clay (Anion Storage Capacity 22%) and Lismore stony s i lt (Anion Storage Cpacity 25%) where either TSP or RPR was appl ied. Such low alkal i P i accumulation confirms that the h igh proportion of H2S04 P i in the Wharekohe s i l t loam was due to l imited alkal i P i accumulation rather than excessive H2S04 Pi accumulation in the older sites. 4.4.2 Contribution of P Fractions to the SLF for the Wharekohe silt loam The net accumulation of P into less avai lable fractions under pasture in a 'steady state' constitutes a component of the SLF (Cornforth and S inclair, 1 982) . The P fractions extractable by NH4CI and NaHC03 in the sequentia l fractionation procedure employed in this study are considered to form the labi le P pool (Chang and Jackson; 1 957; Mattingly, 1 975; Bowman and Cole, 1 978, Hedley et aI . , 1 982; Tiessen and Moir, 1 993). Net accumulation into the inorganic and organic NaOH, H2S04, Hot HCI or Residual P fractions would therefore be considered to be a component of the SLF even though they can contribute to plant avai lable P in the long term if a steady state is lost or through cycl ing where no net change in each fraction occurs (Sattel l and Morris, 1 992) . The CFAS and Outlook models assume a 'steady state' so that the flux rates between the different compartments of the P cycl ing pool do not need to be 97 considered (Cornforth and S inclair, 1 982). However, examination of the P fractions in the samples col lected in 1 990 and 1 993 reveal that the net accumulation of appl ied P into less avai lable plant fractions changes with t ime from pasture development. Consequently, a steady state does not exist on the Wharekohe s i lt loam whi le P is accumulating in the soi l , as the flux rates between the d ifferent compartments of the P cycl ing pool do change with pasture age and need to be considered in models used to predict P fert i l iser levels required to maintain pasture production. As l ittle P was lost from the youngest s ite and P was sti l l accumulating in al l fractions in the top 7 .5 cm, the SLF consists mostly of net P accumulation at 8 years. However, in the oldest sites, losses of P from the top 7 .5 cm were large and l itt le P appeared to be accumulating apart from H2S04 Pi from fert i l iser residues and some organic P, mostly NaOH extractable. Hence, in the older s ites, net accumulation of P into unavai lable fractions may play a minor role in the SLF compared to P loss from the soi l . In contrast to other acid New Zealand soi ls wh ich have been under pasture for some time, l ittle appl ied P accumulated as Fe and AI-P extractable by NaOH and so the net accumulation of Fe and AI P does not contribute large amounts to the soi l P loss. The accumulation of appl ied P into the plant unavai lable hot HC I Pi and Residual Pi fractions was only 3% of the total accumulated P in the top 3 cm of the 32 yea·r aged site and so these fractions do not contribute sign ificantly to soil P loss in the Wharekohe silt loam either. Calculation of the contribution of the net accumulation of applied P into each fraction to soil loss parameters appropriate for use in the Outlook and CFAS models is compl icated by the use of RPR in later years, the lack of sites ranging in age from 0 to 8 years and changing P appl ication rates over t ime. However, if it is assumed that i} a maximum amount of P can accumulate in each P pool i n any one year and P appl ied surp lus to this soi l accumulation and production requirements is lost from the soi l via runoff, and i i } P was appl ied at rates surplus to soi l accumulation and production requirements in each year, the contribution of net accumulation of appl ied P into each fraction to soil loss parameters can be determined for various periods 98 from the chronosequence data. The contribution of soi l P accumulation to the CFAS SLF is investigated further in chapter 7, whi le chapter 8 presents models for the accumulation of app l ied P into various soi l P pools and d iscusses the development of a future ferti l iser P requirement model wh ich would take into account changes soi l P accumulation rates with pasture age. 4.4.3 M ovement of P Through the Profi le of a Wharekohe silt loam Results from this P fractionation study support the theory that the low P storage capacity characteristic of the top soi l of the Wharekohe s i lt loam has contributed to the high loss of P recorded in this soi l . In this study, i t i s not possible to determine the form (fraction) i n which P i s moving through the profi le of the Wharekohe silt loam. The in it ial accumulation of P mostly as H2S04 Pi in the 3-7 .5 cm depth, does not necessari ly imply that P is moving as Ca-P. Phosphorus can move through the soi l profi le in one form to be transformed into another form in situ. Haynes and Wi l l iams ( 1 992) attributed the movement of P through the South Is land yel low grey earth, Lismore stony si lt, under i rrigation at Winchmore to physical movement of P through the incorporation of dung and surface soil by earthworms, along with the movement of particulate matter in water through macropores. They considered that incorporated Ca-P was then converted to a lka l i extractable Pi in situ. The results of P accumulation in both the 3-7 .5 cm and 7 .5 cm - E horizon depths of the Wharekohe s i lt loam and noticeable earthworm activity support a s imi lar pathway for P movement through Wharekohe podzols. However, physical movement of P alone cannot account for al l of the very large loss (up to 65%) of P from the top 7 .5 cm of the Wharekohe silt loam (Chapter 3). As P is so weakly held in Wharekohe soi ls , the movement of d issolved P in subsurface runoff water must be contributing sign ificantly to the movement of P both down the profi le and latera l ly through the top soi l . Haynes and Wi l l iams ( 1 992) also found that although the highest percentage increase in Po was in the labile NaHC03 Po, the highest absolute increases were in the NaOH extractable fractions. They therefore concluded that labi le NaHC03 Po may have leached down the profi le to be 99 converted to more stable NaOH extractable Po in situ. Such movement of labile Po and also Pi is also possible in Wharekohe topsoi ls with their extremely low Anion Storage Capacities. I ncreases in accumulated NaHC03 and H2S04 Pi below 7 .5 cm were much larger in the Wharekohe soi l than below 1 0 cm in the Lismore stony si lt . Some of the extra P measured at depth in the Wharekohe soi l , in comparison to the Lismore soi l , can be attributed to the Wharekohe samples being taken closer to the soi l surface ( ie 7 .5 cm - E horizon rather than 1 0-20 cm for Lismore) and to the movement of RPR fert i l iser P particles in soi l water or by earthworms. However, such large levels of accumulated P to depth in the Wharekohe si l t loam indicates large levels of dissolved P movement down the profi le of this soi l . Chapters 5 further investigates the movement of P in so i l water through Wharekohe profi les 4.4.4 The Influence of P Fertiliser Form and Liming History on P Fractions in a Wharekohe silt loam Both P fert i l iser form and l ime appl ications have influenced the accumulation of P into soi l fractions, in particular H2S04 P i , in the pasture development chronosequence on the Wharekohe si lt loam. It is l ikely that some of the H2S04 Pi in the Wharekohe soi ls accumulated as fert i l iser P residues in the years prior to soil sampl ing on all the developed sites. Acid extractable P i accumulation in acid soi ls low in Ca is often attributed to insoluble P fert i l iser residues (Condron and Goh, 1 989; Rowarth et aI . , 1 992a) . The use of 30% RPR in the fert i l iser appl ied in the 5 years prior to the 1 990 sampl ing would have increased the proportion of P recovered in the H2S04 Pi fraction on the Wharekohe s i lt loam in the present study (Grigg and Crouchly, 1 980; Perrott et aI . , 1 992a; Kumar et a I . , 1 993). A further 3 years of 1 00% RPR appl ication on the additional s ites resu lted in h igher H2S04 Pi accumulation than was recorded for the original sites in 1 993, where P was appl ied mostly as superphosphate in the previous 3 years. Acid II 1 00 extractable P i was also substantial ly higher than alkal i P i in the 8 soi ls in the study by Perrott et a l . ( 1 992a) where RPR (Sechura) had been used annual ly for 6 years. Although the actual amount of H2S04 Pi d id not change with pasture age, the proportion of Pt as H2S04 Pi decreased with age in the developed sites in the top 7 . 5 cm at both sample times. The younger sites had received a higher proportion of their appl ied P as RPR and less soluble Ca-P in the low solubi l ity superphosphate used in New Zealand from the late 1 960's. The decreasing proportion of H2S04 Pi was balanced mostly by an increasing proportion of Pt as Po. Changes in the nature of Pi accumulation in a Lismore stony si l t over the 35 years from pasture development fol lowed the sequence of in it ia l ly accumulating as NaOH Pi , then as NaHC03 and NaOH Pi , and final ly as HCI P i (Condron and Goh, 1 989; Haynes and Wi l l iams, 1 992) . Although the change to HCI extractable P i may have been enhanced by exchangeable Ca levels interfering with the fractionation procedure, Condron and Goh ( 1 989) cited two other possible reasons for the change to acid extractable Pi accumulation, namely i} changes in P fert i l iser to less water soluble P and i i } l ime addition. Trends towards the accumulation of P as acid P with the addition of l ime have also been noted by Pratt and Shoemaker ( 1 955) , Laverty and McLean ( 1 961 ) and Chang and Chu ( 1 961 ) . In contrast, l ime resulted in a decrease in NaHC03 P i and 0 . 1 M NaOH I P i and no change to H2S04 Pi in the top 2 .5 cm of Kokotau s i lt loam near Masterton (Perrott and Mansel l , 1 989) where the pH of 5 .9 was lower than the pH of 6 .6 recorded after the addition of l ime at Winchmore (Quin and Rickard, 1 98 1 ) . The accumulation of H2S04 Pi in the Wharekohe si l t loam in this study appears to be closely related to l ime history (Lime History outl ined in 3 .3 .2) part icularly their l ime history in the early years fol lowing pasture development. The sites which had received the highest rate of l ime in their first 7 years (8, 23 and 32 year old s ites) had the h ighest H2S04 Pi in the top 7 .5 cm. The site (30 years) which had received the lowest rate of l ime in the first 7 years had the lowest H2S04 Pi in the top 7 .5 cm. The more recent heavy l iming h istory of the youngest site and the extremely high l ime appl ication rate in the first seven years on the 32 year old site was also 1 01 reflected in the higher pH (F igure 4.6) , CEC, exchangeable Ca and total Ca contents of these two sites (Appendix 4.2) . 6 - . - - - -5.5 . - . to 5 � Q. 4.5 4 3 .5 +------+----+-----+-----+------1---+-----+---; o 4 8 1 2 1 6 20 Years of Development • 0-3 em • 3-7.5 em . . -& • . 7 .5-E em Figure 4.6 Effect of pasture age on pH at each depth . 24 28 32 The lower P i in the oldest sites compared to the younger sites in spring 1 993 can mostly be attributed to lower H2S04 P i , a lthough there was no sign ificant difference between the developed sites for each fraction (Figure 4.4) . L iming history may also account for the higher and lower H2S04 Pi measured in the 1 993 samples from the sites developed in 1 960 and 1 958 respectively compared to the sites sampled in 1 990. L ime appl ication varied enormously over the areas developed in those years due to the requirements of l ime trials . The 1 960 sites sampled in 1 993 received a higher average l ime rate than the site sampled in 1 990, whi le the 1 958 sites sampled in 1 993 had received a lower average lime rate than the site sampled in 1 990. The accumulation of H2S04 Pi in the top 7 .5 cm of the developed sites most l ikely results from reactions of appl ied soluble monocalcium phosphate with the undissolved CaC03 from the appl ied l ime to produce less soluble Ca-P compounds, which become more stable over time, combined with the accumulation of low 1 02 solubi l ity fert i l iser Ca-P residues. Some of the H2S04 extractable P i would contribute to plant avai lable P through the dissolution of Ca-P over time, part icularly from RPR residues. However, Ca-P residues from the unreactive phosphate rock used in the manufacture of superphosphate and more stable Ca-P formed in situ . , are not l ikely to contribute significantly to plant growth. Organic P accumulation has been shown to cease where the appl ication of l ime increases pH values to above 5 .9 (Condron and Goh, 1 989; Nguyen et a I . , 1 989; Condron and Goh, 1 990). Soil pH increased with pasture development from 4.0 in the top 7 .5 cm of the undeveloped sites, reflecting the large l ime inputs ( Figure 4.6) . Increases in pH occurred at a l l depths, but were general ly greater in the top 7.5 cm where the surface appl ied l ime had the greatest influence. A simi lar result was found in a pasture development chronosequence on the South Is land Okarito podzol where increases in pH with pasture development were confined to the top 1 0 cm and greatest in the top 5 cm (Lee et aI . , 1 983). There was no significant d ifference in pH between the 0-3 cm and 3-7 .5 cm depths on the Wharekohe sites. The highest pH was recorded in the top 7 .5 cm of the youngest site developed in 1 982 at both sampl ing t imes (pH 5 .95 in spring 1 993). This result was consistent with the more recent, large appl ication of l ime with pasture development on the youngest site as l ime may have a shorter term effect on pH in podzols in comparison to other sed imentary soi ls. Surface applied l ime had a shorter-term effect ( lasting 5 years) on the pH of a Maungatua podzol in the South Is land in comparison to the yel low brown earths in the study (Floate and Enright, 1 991 ) . H istoric pH measurements reveal that the youngest site developed in 1 982 had a pH in the top 7 .5 cm of over 5 .9 for the two years prior to sampl ing (Appendix 4.3) , a lthough this had fal len below 5 .9 by 1 990. Hence, Po accumulation may have been suppressed at this site during that time in both the 0-3 cm and 3-7 .5 cm depths providing an explanation for the s imi lar average rates of Po accumulation recorded in the 3-7 .5 cm depth of the 8 and 23 year sites, which was unexpected. H istoric pH values were not avai lable for other sites, but it is possible that if the pH of the other sites had remained below 5 .9 in recent years (which was l i kely due to the lower 1 03 recent l ime history of the older sites) , Po accumulation may not have been impeded by the pH level . Lime induced changes in the retention of added P are d iscussed further in chapter 6. 4.4.5 Effect of Parent Material on P Fractions in Wharekohe Podzols Comparison of the P partit ioning in a Wharekohe s i lt loam with a Wharekohe sandy loam, both under pasture for at least 30 years, provides an insight into the effect of parent material on the accumulation of P into various fractions. The Wharekohe sandy loam contained less total P than the Wharekohe si lt loam soil (Table 4 .3) . Unfortunately an accurate P fert i l iser history was not avai lable for the Wharekohe sandy loam site and so calculations of the quantity of appl ied P which had accumulated in the soil and consequently losses of appl ied P from the top 7 .5 cm could not be made. However, the site was on a dairy farm which had received regular high dressings of P fert i l iser (Table 2 . 1 ) and as the total P content was low, some P would have been lost from the soi l . A larger proportion of total P , 1 1 . 5% (63 �g PIg) , was measured in the NH4C I P i fraction in the Wharekohe sandy loam compared to the Wharekohe si lt loam (Table 4 .3 , F igure 4.7) . The sandy loam had received only one dressing of RPR and NH4CI Pi is expected to be lower where RPR is used (Kumar et a I . , 1 993). The sl ightly lower surface area of the sandy loam may also have contributed fewer sites for the sorption of appl ied P. The high proportion of Pi in the NH4CI fraction i l l ustrates just how weakly avai lable P is held in both Wharekohe soi ls, in particu lar the sandy loam, and therefore how prone the avai lable P is to leaching. A high proportion of the P appl ied in superphosphate, 9 to 1 5%, was also extracted as NH4CI P i in the top 1 0 cm of three sandy soi ls in Western Austral ia known to leach P where a mod ified Chang and Jackson procedure (Wi l l iams et a I . , 1 967) was used to fractionate P (Kumar et a I . , 1 993). 1 04 Table 4 .3 Effect of parent material on the P fractions measured in Wharekohe podzols (% in brackets) . S ite Wharekohe si lt loam Wharekohe sandy loam Olsen P NH4CI P i (1-I9/g) NaHC03 Pi (1-I9/g) NaOH Pi (1-I9/g) H2S04 Pi (1-I9/g) NH4C I Po (1-I9/g) NaHC03 Po (1-I9/g) NaOH Po (1-I9/g) Residual P (1-I9/g) Total P ( 1-I9/g) - o II) C) - C) :::l. - Q. Inorganic p 22 32 (5%) 47.5 (7%) 53 (8%) 1 83 .5 (28%) 25 (4%) 7 1 . 5 ( 1 1 %) 1 68 .5 (26%) 77 .5 ( 1 2%) 658.5 Organic p Inorganic p 26 63 23 42 1 2 1 3 1 55 1 43 67 545 Organic p ( 1 2%) (4%) (8%) (22%) (6%) ( 1 0%) (25%) ( 1 2%) Wharekohe silt loam Wharekohe sandy loam • Residual P 6 H2S04 o NaOH II NaHC03 . NH4CI Figure 4 .7 Effect of parent material on inorganic and organic P fractions in Wharekohe podzols. The proportion of total P as NaHC03 Pi was much lower i n the sandy loam than in the si l t loam. However, the sum of the easi ly extractable NH4CI and NaHC03 P i 1 05 fractions was simi lar for both soi ls . Hence, avai lable P i appears to be more weakly held in Wharekohe sandy loam. In both soi ls NaOH Pi was low, only 8% of the total P , ind icating how few Fe and AI P retention sites are avai lable in the Wharekohe soi ls . The actual amount of NaOH Pi was lower in the Wharekohe sandy loam supporting a lower number of s ites in the sandy loam for P sorption. Calcium-P extracted by H2S04 was the largest fraction in both the Wharekohe soi ls . However, once again this was not due to an unusual ly high affin ity for Ca-P pecul iar to Wharekohe soi ls but rather to a lack of a lka l i extractable P i accumulation. The h igher proportion of P extractable by H2S04 in the Wharekohe si lt loam was most l i kely a reflection of the RPR use on those sites. Organic P d ifferences showed a simi lar pattern to avai lable Pi between the two Wharekohe soi ls although the differences were not quite as pronounced as for P i . Most of the organic P was in the NaOH Po fraction for each soi l . As Po stabi l isation is d i rectly related to P sorption sites (Saunders, 1 959a) , l im ited sorption sites are restrict ing Po accumulation in both Wharekohe soi ls . A lower number of sorption sites in the Wharekohe sandy loam due to a lower surface area may have resulted in the lower Po accumulation recorded in the sandy loam. At such an extreme degree of soi l weathering, parent material has had only a minor influence on the accumulation of P into various fractions. However, it is l ikely that P losses from the soi l would be s l ightly higher from the sandy loam due to the easi ly extractable nature of the avai lable P and to less sites for the sorption of P i and stabi l isation of Po. The abi l i ty of both Wharekohe soi ls to retain added P is investigated further in Chapter 6 . 4.4.6 Effect of the Degree of Weatheri ng of Silt Sediments on P Fractions 1 06 The degree of soi l weathering has had a marked effect on the P chemistry of the two soi ls derived from s imi lar parent materials under older pasture, the yel low brown earth, Aponga clay, and the podzol , Wharekohe si l t loam. The Aponga clay contained substantial ly more total P , more than twice that found in the Wharekohe s i lt loam (Table 4 .4) . As an accurate P fert i l iser h istory was not avai lable for the Aponga clay s ite, calculations of the quantity of appl ied P wh ich had accumulated in each soi l and consequently losses of appl ied P from the top 7 .5 cm cou ld not be made. Once again the use of RPR on the Wharekohe s i lt loam and sampl ing t ime compl icates comparisons of P fractions between the two d ifferent soi ls . However, d ifferences are large and a greater proportion of the P measured in each fraction in the Aponga clay was l ikely to be derived from native so i l P rather than P fert i l iser appl ication in comparison to the more strongly weathered Wharekohe soi l (Table 4.4, F igure 4 .8) . Table 4 .4 Effect of the degree of so i l weathering on the P fractions measured in the moderately leached yel low brown earth, Aponga clay, and the podzol , Wharekohe si l t loam, derived from simi lar si lty parent materia l (% in brackets) . S ite Wharekohe si lt loam Aponga clay Olsen P (MAF) 22 25 NH4C I P i (�g/g) 32 (5%) 0.9 I ltracej NaHC03 Pi (�g/g) 47.5 (7%) 61 (4%) NaOH P i (�g/g) 53 (8%) 437 (27%) H2S04 P i (�g/g) 1 83 .5 (28%) 346 (22%) NH4C I Po (�g/g) 25 (4%) 8 (0 .5%) NaHC03 Po (�g/g) 71 . 5 ( 1 1 %) 95 (6%) NaOH Po (�g/g) 1 68 .5 (26%) 482 (30%) Residual P (�g/g) 77 . 5 ( 1 2%) 1 68 ( 1 0 .5%) Total P (�g/g) 658 .5 1 597 1 200 1 000 - '0 800 I/) en 600 -en ::1. - Q. 400 200 0 Inorganic Organic p p Wharekohe silt loam Inorganic p Organic p Aponga clay • Residual P El H2S04 O NaOH II NaHC03 . NH4CI Figure 4 .8 Effect of degree of weathering of s i lt sediments on inorganic and organic P fractions in the soi l . 1 07 Only 0.86 �g PIg or 0.05% of the total P recovered in the yel low brown earth Aponga clay was in the NH4CI Pi fraction, in contrast to the 32 I-Ig PIg measured in the Wharekohe si lt loam. The proportion of total P as NaHC03 Pi was lower than in the Wharekohe s i lt loam, although the actual amount was h igher. Phosphorus is much more t ightly held in the Aponga clay than in the Wharekohe soi l . This was confirmed by the much larger size of the NaOH Pi extractable P i in the less weathered Aponga clay due to the presence of more Fe and AI sorption sites, than in the Wharekohe s i lt loam. The Ca-P measured in the Aponga clay was far higher than the amount measured in the Wharekohe s i lt loam, despite RPR appl ication in the latter. The presence of residual primary Ca-P in the less weathered soi l is the most l i kely explanation of the difference. The Aponga clay contained a larger quantity of organic P than the Wharekohe soi l . Most of the organic P was in the NaOH Po fraction for each soi l and although the 1 08 proportions in the NaOH fraction were simi lar, the amount in the Aponga clay was far h igher than in the Wharekohe soi l (9x). Such high levels of NaOH Po in the less weathered soi l are most l i kely due to the stabi l isation of Po against mineral isation by AI and Fe complexes. Consequently, as was the case with P i , weakly held NH4CI Po was very low in the Aponga clay compared to the Wharekohe soi l . The degree of weathering on the s i lty sediments has resulted in large differences in P accumulation, with both P i and Po held more strongly through reactions with and sorption on the surface of Fe and AI complexes, in the less weathered Aponga clay in comparison to the Wharekohe si lt loam. The accumulation of non-labi le P would contribute much more to the SLF in the Aponga clay than in the Wharekohe si lt loam. The abi l ity of the Aponga clay to retain added P in comparison to the Wharekohe si l t loam is investigated further in Chapter 6. 4.5 S U M MARY AN D CONCLUSIONS 4.5. 1 Effect of Pasture Age on the Accumulation of Applied P into Soil Fractions in a Wharekohe silt loam • The maximum P storage capacity reported in chapter 3 can be attributed solely to a maximum Pi storage capacity. I norganic P accumulation had stopped by 8 years in the top 3 cm and by 1 1 years in the 3-7 .5 cm depth of the Wharekohe si l t loam, apart from the continued accumulation of small quantities of Ca-P from insoluble ferti l iser residues. The Pi accumulation in the Wharekohe silt loam fol lowed a d ifferent pattern to other New Zealand soi ls where P i continues to accumulate where P is app l ied surp lus to pasture requirements. • In contrast to P i , Po continued to accumulate over time to at least 32 years at each depth to levels s imi lar to those recorded by other researchers in a less weathered South Island yel low grey earth. Restricted movement of appl ied P under young pasture contributed to delayed organic P immobi l isation at depth. 1 09 • In it ia l ly pasture development resulted in a decrease in the Po/P i rat io. However, in contrast to other New Zealand soi ls, the Po/P i ratio increased with t ime after 8 years as P i had stopped accumulating whi le Po continued to accumulate. • Over half of the P extracted from a l l depths of the undeveloped sites was in the plant avai lable and easi ly extractable NH4CI and NaHC03 Pi and Po fractions ind icat ing that a large proportion of the total P is actively involved in the cycl ing pool . The amounts of P i extracted in the other inorganic fractions were as expected for a highly weathered soi l , with occluded P extracted by hot acid larger than both the neg l ig ib le Ca-P extracted by H2S04 and the small quantity of NaOH Pi associated with AI and Fe. In contrast to other highly weathered soi ls, the more stable NaOH Po fraction was low in comparison to the more labile Po, which was perhaps due to the low Fe and AI content of the Wharekohe si lt loam. • All inorganic and organic P fractions increased with in it ial pasture development. In contrast to other acid New Zealand soi ls, Ca-P extracted by H2S04 was the largest fraction in the top 7 . 5 cm of a l l the developed s ites. Calcium-P was sti l l accumulat ing in the top 7 .5 cm of the Wharekohe s i lt loam s ites from the accumulation of P fert i l iser residues and reactions of soluble monocalcium P over t ime with Ca added to the soi l in l ime. Such a high proport ion of P as Ca-P was not due to the preferential accumulation of Ca-P pecul iar to Wharekohe soi ls , but rather to a l imit to the accumulation of P associated with Fe and AI (NaOH P i ) in the Wharekohe soi l in comparison to other New Zealand soi ls including the Aponga clay in the present study. • The Wharekohe silt loam contained a large quantity of easi ly avai lable Pi of which a large amount was very weakly held in the soi l (NH4CI P i ) and therefore prone to leach ing in comparison to other less weathered soi ls. • The rate of Po accumulation decreased over t ime in the top 3 cm. By 25 years, the most labi le Po fraction extracted by NH4CI had reached equi l i br ium. Organic P accumulated predominantly as NaOH extractable Po, that is Po stabi l ised against 1 1 0 mineral isation by sorption on sesquioxides and Po associated with humic and fulvic acids, at all depths. The low Fe and AI content of the Wharekohe soi ls should result in a l imit to the amount of Po which can be stabi l ised against minera l isation, and hence Po is expected to reach equ i l ibrium sooner than in other soi ls . 4.5.2 Contri bution of P Fraction to the SLF for a Wharekohe silt l oam • The relative contribution to the SLF of P accumulation into non-labi le P fractions decreases with t ime from in itial pasture development. The quantity of appl ied P accumulating in plant unavai lable compounds in the rooting zone (top 7 .5 cm) of Wharekohe si l t loam is not large in comparison to the losses recorded from the soi l profi les of the older developed sites (Chapter 3 ) . Hence, non-labi le P accumulation only forms a minor component of the SLF on soils wh ich have been under pasture for more than 1 1 years where further P accumulation is mostly smal l amounts of insoluble Ca-P fert i l iser residues and the more stable forms of Po. The amounts of accumulated P recorded in this chapter are used to calculate values for the SLF in chapter 7 and to model the fate of P in chapter 8. 4.5.3 Movement of P Through the Profi le of a Wharekohe silt loam • It is the l imit to the accumulation of appl ied P as Fe and AI phosphates and P sorbed on the surfaces of sesquioxides which has led to the high P losses recorded from the Wharekohe si l t loam in comparison to other soi ls . • Phosphorus accumulates in it ial ly in the first 8 years as NaHC03 Pi and NaOH Po, then predominantly as Ca-Pi and NaOH Po to 23 years and then as NaHC03 and NaOH Po after 23 years below 7 .5 cm in the Wharekohe s i lt loam. I t is not possible to determine in which form P is moving through the profi le of the Wharekohe si lt loam due to possible changes in P forms in situ. The resu lts of P accumulation in both the 3-7 .5 cm depth and 7 .5 cm - E horizon support the theory that some P moves physical ly as Ca-P and Po in dung and surface soi l particles ( including RPR) via earthworms and soi l water, some of which is then converted to a lkal i P i at depth. Physical movement of P alone cannot account for the vary large losses of P 1 1 1 recorded in the Wharekohe si lt loam. As P is so weakly held in Wharekohe soi ls, the movement of d issolved P in soi l water must be contributing significantly to the movement of P both down the profi le and latera l ly through the top soi l . Chapter 5 investigates whether P is moving in soil water as P i or Po. 4.5.4 Influence of P Fertil iser Form and Historic Lime Appl ication on P Fractions i n Wharekohe Podzols • The use of RPR and l ime history have contributed to H2S04 extractable Ca Pi accumulation. • Calcium-Pi , extractable by H2S04, was closely related to l iming history, in particular the l ime appl ication in the years immediately fol lowing in it ial pasture development. • The more recent heavy lime appl ication on the youngest s ite has led to a higher pH which may have restricted Po accumulation in recent years. 4.5.5 Effect of Parent Material on P Fractions in Wharekohe Podzols • At such an extreme degree of weathering, parent material has had only a smal l influence on the accumulation of P into various fractions in Wharekohe soi ls with avai lable Pi and Po held less strongly in the soi l derived from sandy sediments. It is l ikely that P losses from the soi l would be s l ightly higher from the sandy loam due to the eas i ly extractable nature of the avai lable P and to less s ites for the sorption of P i and stabi l isation of Po. 4.5.6 Effect of Degree of Weathering of Silt Sediments on P Fractions • The degree of weathering on the si lty sediments has resulted in large amounts of P in the slowly avai lable and unavai lable P fractions in the less weathered Aponga clay in contrast to the Wharekohe s i lt loam. Applied P accumulation, particularly as 1 1 2 NaOH extractable Fe and AI P i , would be expected to make a greater contribution to the SLF of the Aponga clay in comparison to the Wharekohe s i lt loam. CHAPTER 5 P MOVEM ENT IN S U BSU RFACE RUNOFF 5.1 INTRODUCTION 1 1 3 Runoff from intensively grazed pastoral systems has resulted i n the eutrophication of many lowland and coastal lakes in New Zealand (Wi lcock, 1 986) . Much of the research into nutrient losses in runoff water has focused on P as the key nutrient l imiting eutrophication, as C and N can exchange with the atmosphere, but greater control can be exerted over P enrichment (Sharpley et aI . , 1 995) . Although P is considered to be relatively immobi le and losses of P from New Zealand soi ls are general ly considered to be low from an agronomic point of view, the appl ication of fert i l iser to pastures has resulted in losses of P in runoff waters wel l in excess of the 0 .02 to 0. 035 I-Ig P/ml (OECD, 1 982; Vol lenweider, 1 968) considered l im iting to eutrophication (McCol l et aI . , 1 977; Bargh, 1 978; Lee et a I . , 1 979; McCol l and Gibson, 1 979 a&b; Sharpley and Syers, 1 979a&b; C lose and Woods, 1 986; Sharpley and Syers, 1 983; Lambert et a I . , 1 985; Cooper and Thomsen, 1 988). Such losses appear to be simi lar to those reported overseas (Olness et a I . , 1 975; Jawson et aI . , 1 982; Schepers and Francis, 1 981 ) . However, data presented in Chapter 3 shows that P losses from Wharekohe soils are so large as to not only possibly impact negatively on eutrophication, but to also represent a large economic loss to the farmer. Up to 65% of P appl ied over a 3 year period (an average of 32 kg P/ha/yr) could not be accounted for in the top 7 .5 cm and up to 27% (8 .5 kg P/ha/yr) of P appl ied since pasture development could not be accounted for above the E horizon in older developed sites on Wharekohe soi ls. As l itt le P was found to accumulate at depths below the E horizon, P is most l ikely being lost in water movement down slope, and most l ikely into the wider environment. Both inorganic and organic P were found to accumulate at depth in the Wharekohe s i lt loam (Chapter 4) . However, due to possible transformations of P fractions in situ . , it was not possible to determine from the fractionation data presented in 1 1 4 chapter 4 , whether P was moving in soi l water as dissolved inorganic P , organic P or both forms. Phosphorus is lost from the soi l in surface runoff, in subsurface runoff and in groundwater runoff. The relative contribution of subsurface and surface flow to P losses depends on both the concentrations of P and the volume of water in each (Sharpley et aI . , 1 976) . Surface runoff usua l ly contributes the greater proportion of P losses from finely textured mineral soi ls , despite contributing lower volumes of water (Burwel l et a I . , 1 974; Hanway and Laflan, 1 974; Baker et aI . , 1 975; Sharpley et aI . , 1 976, 1 995; Sharpley and Syers, 1 979a). Subsurface losses of P are usually low in both New Zealand and overseas except in sandy soi ls with low P sorption capacities (Ozanne, 1 961 ; Hogg and Cooper, 1 964; Diggle and Bel l , 1 984; Ruprecht and George, 1 993; Gi l l iam et a I . , 1 994) and in organic soi ls (Hogg and Cooper, 1 964; Fox and Kamprath , 1 97 1 ; Cogger and Duxbury, 1 984; Deal et a I . , 1 986) . The low concentration of total P in subsurface waters in finely textured mineral soi ls is due to the greater soi l contact leading to increased sorption of d issolved P and the fi ltering nature of the soi l removing particulate P (Sharpley and Syers, 1 979a) . In developed Wharekohe soi ls, however, the soluble P concentration should be higher in subsurface runoff than i n other New Zealand soi ls, due to a lack of P retention sites in the A horizon . The loss of P in subsurface runoff is expected to be h igher from older developed sites than from younger sites on Wharekohe soi ls. The older s ites have become saturated with P to a greater depth, contain ing P close to their maximum P storage capacities through the soi l profi le above the E horizon, where subsurface runoff is l ikely to flow. Consequently, there are few remaining sites for P storage, whi le at the younger sites, the lower soil P concentration at depth is l i kely to lead to the more effective removal of P from subsurface runoff water. 5.2 OBJ ECTIVES The main objectives of this study were to determine: 1 1 5 1 . If freshly appl ied P can move through Wharekohe soi ls as dissolved P in sub­ surface water movement and whether this is effected by; i) t ime under developed pasture, or i i ) d ifferences in parent material . 2 . I f d issolved P movement occurs as inorganic and/or organic P 5.3 MATERIALS AND M ETHOD 5.3.1 Glasshouse Trial 5.3. 1 .1 Collection and Preparation of Soil Cores S ixteen intact soi l cores, 1 4 cm in d iameter by 9 cm deep, were col lected ·from 'ea<;l1 -. . t · of the fol lowing soi ls using the method of Cameron et a l . ( 1 990) and i l lustrated 'i�� figure 5. 1 ; . - Wharekohe si l t loam Wharekohe silt loam Wharekohe sandy loam Aponga clay ( 1 0 years under developed pasture) , (32 years under developed pasture) , (>30 years under developed pasture) , (>30 years under developed pasture) , Aponga clay cores were included for. comparative purposes. It is a soi l with parent material s imi lar to the Wharekohe but has a sign ificantly higher anion storage capacity. The anion storage capacities at each of the sites were 1 0 % for the Wharekohe si l t loam, 7 % for then Wharekohe sandy loam and 54% for the Aponga clay. Galvanised steel core casinQ PVC cutting edge Undisturbed soi l core 1 1 6 Vasel ine seal 1 3. 5 cm diameter plastic container 20 cm diameter plastic container Figure 5. 1 Intact soi l cores, with petrolatum (vasel ine) seal between galvan ised casing and soi l , and the dual leachate col lection system. 1 1 7 Perennial ryegrass and white clover were the dominant pasture species at each site. Pasture was in it ial ly trimmed to 2 cm height and then trimmed to this height at regular intervals during the leaching trial to avoid excessive transpiration in the glasshouse. The bottom of each core was careful ly picked with a nai l to expose an uncut surface and to open the pores wh ich had been smeared when cut by the knife. The bottom was then covered with nylon gauze to prevent sed iment fal l i ng into the leachate. The soi l cores were wel l watered over several days and then left to drain for 2 hours before weighing to establ ish a watering weight close to saturation but from which no soi l water wou ld drip from the base of the cores. 1 00 kg P/ha (superphosphate <2 mm) was appl ied to 8 cores of each soi l leaving 8 cores of each soi l as 0 kg P/ha (controls) on Day O . The soi l cores were arranged randomly in four b locks, 2 cores of each soi l x treatment in each b lock. 5.3. 1.2 Leaching Technique The cores were watered to weight prior to each leaching event and between leaching events to prevent soi l water loss from over watering. Adjustments were made to the watering weight during the trial to account for a reduction in the hydrau l ic conductivity of some of the soi ls and their increased water retention. 1 1 .4 mm of s imulated rain was appl ied on four separate occasions (3 , 5 , 1 0 and 1 3 days from P app l ication) over a four hour period (35 ml container of water spread over the core surface each hour). Water did not accumulate on the surface except in a very few cores where hydraul ic conductivity had markedly deteriorated. The cores were then kept watered for a further 40 days before 3 further leaching events on days 53, 81 and 94. The cores had become dry during the 40 days and required reseal ing with vasel ine and re-wetting before further leaching events. Some water (not more than 200 m l ) had passed through the cores during this period and was d iscarded. 1 1 8 Leachate samples were col lected via a dual col lection system (shown in F igure 5. 1 ) to check for preferential flow down the sides of the cores after each rainfal l event. A 1 3. 5 cm diameter container col lected leachate from the bul k of the cores wh i le a second container col lected leachate from the 0.25 cm perimeter. Conditions in the glasshouse for the duration of the trial in summer were hotter than experienced in the fie ld . I n itial leaching events, prior to fert i l iser appl ication, revealed that most of the cores col lected from the Wharekohe soi ls displayed substantial flow from the 0.25 cm perimeter, particu larly those cores col lected from the Wharekohe sandy loam site and the 1 0 year old Wharekohe si l t loam site. In order to investigate reasons for this preferential flow, a core from each of these two sites was leached with a bright pink marker dye and then dissected to examine water movement before the trial commenced. At the end of the leaching tria l , methylene blue was appl ied to four cores from each soi l in the same manner as the simu lated rainfa l l . The soi l cores were then removed from the cyl inders and the vasel ine peeled back to examine macropores. Each soi l core was s l iced at one centimetre intervals from the top and an acetate grid divided into 1 cm squares was used to determine soi l water penetration cover at each depth. 5.3. 1.3 Chemical Analysis of Leachate \ Leachate was fi ltered through a <0.45 IJm mi l lepore fi lter. D issolved inorganic P (D IP) was measured in the leachate colorimetrical ly by the phosphomolybdate method of Murphy and Ri ley ( 1 962) . D issolved organic P (DOP) was determined as the difference between total d issolved P (TOP) , measured after H2S04 IH202 d igestion (Thomas et a I . , 1 967) and dissolved inorganic P. 1 1 9 Leachate was analysed straight away, except those col lected on day 1 3. These were fi ltered and then approximately 30 ml samples were frozen overnight after the addition of one drop of concentrated He ! . 5.3. 1.4 Statistical Analysis Differences and s imi larities between the quantities of P leached through the d ifferent soi ls were read i ly apparent and consequently analysis of variance of the data was unnecessary. Mean standard error bars are included where appropriate. ( 5.3.2 Field Trial 5.3.2. 1 Field Sites and Fertiliser Treatments Six 25 m2 plots, with slopes of 9°, were selected at 2 Wharekohe s i lt loam sites, 1 1 and 33/35 years under developed pasture, on the Kaikohe Research Station. The plots were fert i l ised with 1 00 kg P/ha (superphosphate) on the 5th of September (Day 0) . 5.3.2.2 Soil Solution Collection Porous ceramic cups were used to col lect water samples at two depths, 2-7 .5 cm and 7 . 5-1 3 cm, with in and at 3 distances away from the fert i l ised p lots ( 1 0 cm down slope, 20 cm down slope and 1 0 m away from fert i l ised plots (contro l ) ) . Soi l solution was sampled, from each cup, 6 days prior to fert i l iser appl ication and after each of five major natural rainfa l l events by placing the cups under suction for 24 hours. To min imise P sorption on the surface of the ceramic cups, cups were soaked in a P solution prior to use and then rinsed thoroughly in d ist i l led water. 1 20 5.3.2.3 Chemical Analysis of Soil Solutions DIP in fi l tered water samples « 0.45 um) was measured colorimetrica l ly straight away using the phosphomolybdate method of Murphy and Ri ley ( 1 962) . DOP was determined as the d ifference between TDP, measured colorimetrical ly after H2S04 IH202 d igestion (Thomas et a I . , 1 967) and d issolved inorganic P . 5.3.2.4 Statistical Analysis Differences between D IP in the control and each of the other three d istances were tested for significance (P=0.95) at each sampling using Dunnett's procedure after log transformation. DOP was only a smal l proportion of TDP in the 2-7 .5 cm depth under the fert i l ised plots and often below the l im its of detection. A comprehensive statistical analysis was not possible due to unequal variances wh ich could not be overcome by transformation. T-tests (unequal variance) were used to test for the significance of d ifferences measured between the control and under fert i l ised plots on the first day after ferti l iser appl ication. 5.4 RESU LTS AN D DISCUSSION 5.4. 1 Glasshouse Trial 5.4. 1 . 1 Amounts of DIP and DOP Lost by Leaching Movement of D IP through Wharekohe soi ls after superphosphate appl ication was substant ial . Concentrations of DIP in the Wharekohe leachate 3 days after P appl ication were very high, 37.5, 45.6 and 3 1 .2 IJg/ml for the 1 1 year si l t loam, 33/35 year si l t loam and >30 year sandy loam sites respectively. After just four, 1 1 .4 mm rainfa l l events, a total of at least 8% of the appl ied P had leached through each of the three Wharekohe soi ls as D IP (Figure 5 .2 ) . 9 8 -ca .c -CI � 6 't:I GI .c 5 u ca GI ...J 4 0.. .!:! 3 c ca E' 2 0 .= 1 0 Aponga clay Wh. slit loam 10 yr Wh. silt loam 32 yr • -P Fertil iser 0 +P Fertiliser Wh. sandy loam Figure 5 .2 Sum of dissolved inorganic P leached through soi l cores after four 1 1 .4 mm rainfa l l events on days 3, 5, 1 0 and 1 3. 1 21 The appl ication of ferti l iser P resulted in no additional movement of OOP (Figure 5 .3) . The proportion of TOP leached as OOP in the Wharekohe soi ls where fert i l iser P had been appl ied was low ranging from 0 to 5.5%. The proportion of TOP leached as DOP was higher in the unfert i l ised sites where l ittle P movement occurred. Both New Zealand and overseas stud ies have found D IP to be the major component of TOP in subsurface (Sharpley and Syers, 1 979b; Turner et a I . , 1 979; Cul ley et a I . , 1 983; Heckrath et a I . , 1 995) and surface (Sharpley and Syers, 1 976; Sharpley and Syers, 1 979a; Sharpley et a I . , 1 978; Sharpley et a I . , 1 982) runoff from m ineral soi ls where fert i l iser P has been applied. McCol l ( 1 978) found that 93 % of the TOP was lost as D IP in post-fert i l iser floods from a podzol ised yel low brown earth catchment, which included Wharekohe soi ls in Northland . In contrast to the Wharekohe soi ls , P equivalent to less than 0.27% of the P applied ( 1 . 07 IJ/ml ) had leached through the yel low-brown earth, Aponga clay, as D IP . Neg l ig ible P movement has also been measured through repacked soi l cores in a a) Day 3 - CU .c - en � - "tJ G) .c (.) cu G) ..J Q. b) Day 1 3 - CU .c - en � - "tJ G) .c (.) CU G) ...J Q. I " c ) Day 94 - CU .c - en � - "tJ G) .c (.) cu G) ...J Q. Figure 5.3 1 22 4.5 4 3.5 3 2.5 2 1 .5 1 0.5 0 -p +p -p +P -p +p -p +p 1 .4 1 0.8 0.6 0.4 0.2 0 -p +p -p +p -p +p -p +P 0.45 0.4 0.35 0.3 0.25 0.2 0.1 5 0.1 0.05 0 -p + Aponga c lay sandy DIP and DOP leached through cores after three 1 1 .4 mm rainall events, a) 3, b) 1 3 and c) 94 days after P ferti l iser application. 1 23 range of laboratory and field studies on New Zealand soi ls including yel low brown earths, yel low brown pumice soi ls , granular brown loams and yel low brown loams despite high P appl ication rates ranging from 45 to 79 kg P/ha/yr (E l l iott, 1 973; Mul ler and McSweeney, 1 974; McSweeney and Mul ler, 1 979). Studies on repacked cores can lead to lower nutrient losses from P sorbing soi ls than would be found by using intact soi l cores or in the fie ld , as natural ly occurring macropores, which result in preferential flow, are removed (Cassel et a I . , 1 974; White, 1 985; Scotter and Kanchanasut, 1 981 ) . The effect of preferential flow on P leached from soi ls with low P sorption capacities is d iscussed further i n 5 .4 . 1 .4 . The repacked cores used in the above studies were also longer, 30 cm, than the cores used in the present study al lowing for greater soi l contact and hence P sorption (Gerritse, 1 995). However, despite the removal of macropores and the use of long cores, large losses of appl ied P have been recorded through repacked cores of the New Zealand podzols Wharekohe s i lt loam, Te Kopuru sand (up to 36%) and an Addison gley podzol (42%) , and through a Northland peat soi l ( Hogg and Cooper, 1 964, McSweeney and Mul ler, 1 979). In an Austral ian study, considerable movement of P occurred in repacked columns of a gley podzol in comparison to no recorded P movement in a red earth (Lefroy et a I . , 1 995). Freezing can reduce the amount of D IP measured in both fi ltered and unfiltered runoff samples . (Nelson and Romkens, 1 972; Kl ingaman and Nelson, 1 976; Haygarth et a I . , 1 995). In the present study, DOP in the leachate sampled on day 1 3, which had been frozen, was higher than in leachate sampled on days 3 and 94. As DOP is calculated as the d ifference between TOP and D IP , some loss in recovery of DIP may have occurred in the day 1 3 samples upon freezing. However, any such changes were not large enough to impact significantly on the results. The results from the podzol leaching experiments described above, combined with those of the present study, i l lustrate how readi ly P is leached from podzo l ic soi ls with low anion exchange capacities such as the Wharekohe podzols. 1 24 5.4. 1.2 Effect of Pasture Age on P Leaching Time under developed pasture had no significant effect on P movement i n either the unfert i l ised or fert i l ised Wharekohe si lt loam (Figure 5 .2 ) . In contrast, an Austra l ian laboratory leaching study found that more P was leached through a sandy soi l col lected from a site which had been under pasture for 20 years, receiving on average 33 kg P/ha/yr, compared to a soi l col lected from a site 1 0 years under pasture, receiving on average 1 8 kg P/ha/yr (Weaver et a I . , 1 988). However, it is d ifficult to conclude from the Austra l ian study if the higher P loss in the soi l col lected from the older developed site was a function of fert i l iser appl ication rate or pasture age. The rapid movement of water through macropores in the present tria l may have masked d ifferences in P movement with pasture age which may occur in the field, where the presence of the pan leads to much slower movement of subsurface runoff. Rapid movement of water preferential ly through macropores a l lows for less contact of P in subsurface runoff with the soi l surface and hence lowers P sorption. Alternatively, smal l (but undetected) d ifferences in flow rates between the two pasture ages in the glasshouse study may have masked differences in P loss which may occur under field conditions. The effect of preferential flow on the retention of P and hence leaching losses is d iscussed more ful ly in 5 .4 . 1 . 5 . Pasture age in relation to P saturation and P losses is d iscussed further i n 5 .5 .2 . 5.4. 1.3 Effect of Parent Material on P Leaching There was no detectable difference between the amount of P leached from the Wharekohe sandy loam and Wharekohe s i lt loam cores for the duration of the trial . The sandy loam at the col lection sites was not particularly more coarse textured than the si lt loam. The texture is strongly influenced by a few large quartz particles and the matrix is a s i lt loam. Hence, d ifferences in porosity may not be as marked as one would expect between a si l t and sandy loam leading to more simi lar infi ltration and flow rates. Certain ly no d ifference in infi ltration rate and water flow through the 1 25 soi l cores was observed between the two Wharekohe variants, a lthough the rate of water movement was not measured d i rectly. However, once again, small (but undetected) differences in flow rates between the two Wharekohe variants in the glasshouse trial may be masking differences in P loss which may occur under field cond itions. 5.4. 1.4 Effect of Time on P Leaching The amount of D IP leached per leaching event decreased exponential ly with t ime from fert i l iser appl ication (F igure 5.4). An exponential decl i ne in P leached from soi l cores was also found by Weaver et a l . ( 1 988) in sandy soi ls with low anion storage capacities. A greater proportion of freshly appl ied P wi l l be susceptible to leaching in soi ls with low anion storage capacities resulting in large in it ial P losses. P losses are reduced dramatica l ly over time as the P wh ich was susceptible to leach ing has already been lost from the soi l . Hogg and Cooper's ( 1 964) Te Kopuru sandy loam leaching resu lts support a decl ine in the amount of P leached over t ime in soils with low P sorption capacities. The amount of P leached each week only reduced with time in the highest P leaching soi l with the lowest P retention. Interestingly, the amount of P leached increased with time from the higher P retention soi ls col lected from a l l other Te Kopuru sites where P losses were much lower. Assumed loss of 501 1 structure with continued leach ing may have led to waterlogging in the repacked cores. Increased waterlogging over time leads to both an i ncrease in pH and the reduction of Fe ( I I I ) to Fe ( I I ) , and consequently the release of increasing quantities of P from these higher P retention soi ls (Ponnamperuma, 1 972; Gotoh and Patrick, 1 974; Khal id et aI . , 1 977; Kirk et aI . , 1 990). The effect of waterlogging on the retention of P is d iscussed further in the general d iscussion in 5 .5 .3 . The concentration of DIP leached from the fert i l ised Wharekohe cores was approaching the concentration leached from the unfert i l ised cores after 94 days (F igure 5.5) , a lthough it was sti l l higher. As much of the fresh ly appl ied P remained in the soi l , the P must have been becoming less accessible for leaching with time from appl ication as an equi l ibrium concentration was approached. If the equi l ibrium concentration was dependent upon the volume of water leached through the cores, 5 ---- -- �---- - - --- - - - - - - - - .- - - _ . . - . - - - - - - - - -- - -0 O +-·�-�-�- �- � - - - - -r- - - -�-�- -�- -r-�- -�- -�-�- -�-�- -�- -�-4--�----+_---f�--4_� o 10 20 30 40 50 60 70 80 90 Days - - - � - - Aponga clay �Wh. silt loam 10 yr -___ -Who silt loam 32 yr - -0- - Who sandy laom 1 26 F igure 5.4 Effect of t ime on the amount of DIP leached from fert i l ised cores during each 1 1 .4 mm rainfa l l event. (Vert ical bars = S .E .D . s for Day 3) . greater rainfa l l would not have led to substantia l ly higher losses of P . The amount of simulated rainfal l applied in the g lasshouse study was low in comparison to that encountered in the field in the months fol lowing an autumn P appl ication. However, if the equi l ibrium concentration was time dependent, greater rainfa l l during the period of the trial would have led to larger losses of appl ied P. The latter scenario is the most l i kely as time al lows for the diffusion of solution P into soi l aggregates and away from preferential water flow, reducing P losses in low P sorb ing soi ls as discussed in 5 .4. 1 .4 . a) Aponga clay 'C Q) � u lIS Q) ­..J lIS Q. :E u CI .- .:.:: c: -lIS � 0.1 0.08 0.06 0.04 0.02 o c: 0.· · � - - - - - - - - - - - - - - . . - - - - - - - - - - - - - - - - - - � - - - - - -o +-��----r---�--�----+----+----+----F----r- o 1 0 20 30 b) Wharekohe silt loam, 1 0 years 'C 4 Q) � U lIS Q) ­..J lIS a. :E .� � c: ­lIS � o c: 3 2 1 o 1 0 20 30 c) Wharekohe silt loam, 32 years 'C Q) � U lIS Q) ­..J lIS a. :E u CI .- .:.:: c: ­lIS 5 4 3 2 40 50 60 70 80 90 40 50 60 70 80 90 CI ... o c: 1 o ���������--+-���--+---��--�--�� o 1 0 20 d) Wharekohe sandy loam 'C Q) � U lIS Q) ­..J lIS Q. :E u CI .- .:.:: c:: ­lIS CI ... 3.5 3 2.5 2 1 .5 1 30 40 50 60 70 80 90 o c: 0.: ���---+�'---"-I----�+-----�;::==:=::;====::==t=3 o 1 0 20 Key: • F�rti l ised 30 40 50 - - � - . Unfert i l ised 60 70 80 Days 90 1 27 Figure 5.5 Comparison of DIP leached from fertil ised and unferti l ised cores with time from P application for the cores collected from each site. ----- -��---- 1 28 5.4. 1.5 Water Movement Through Intact Cores Water had broken through the soi l cores a lmost immediately after s imulated rainfa l l at each leaching event. The leachate would not on ly consist of freshly appl ied water, but may also have contai ned older displaced water higher in solution P. In the two soil cores through which pink marker dye flowed prior to trial commencement, the water was found to have penetrated evenly into the top 2 cm and then flowed preferential ly down earthworm channels. The earthworms had moved down through the cores coming out to the sides in the top 6 cm and then turn ing back into the soil core when they encountered the vasel ine l i n ing the cores. However, where the earthworms had burrowed to the sides of the soi ls cores in the bottom 3 cm, where there was no vasel ine to hinder them, they had burrowed straight down the edge of the cores to the bottom resu lt ing in most of the water being col lected from the perimeter. There was no flow of soi l water between the soi l , vasel ine and container sides apart from through the earthworm channels. There was l itt le flow from the perimeter of the Aponga clay cores as any macropores located down the perimeter of the cores had been fi l led with vasel ine when the cores were col lected and earthworm activity was l imited. At the end of the tria l , the methylene blue stained simulated rainfa l l in it ial ly penetrated even ly, and was recorded in 44 and 62% of the centimetre squares at 1 cm depth (Table 5. 1 ) . At the 2 cm depth, appl ied water penetration was only recorded in 1 3-25% of the squares. By 4 cm depth, water movement was mostly restricted to pores. The number of pores conducting water decreased with depth so that by the base of the cores, the bulk of the water movement was restricted to just a few macropores as was found in Iysimeter studies conducted by Scotter and Kanchanasut ( 1 981 ) and Munyankusi et al. ( 1 994) . Hence, leachate col lected from the bottom of the cores had moved through the cores via preferential flow through macropores, mostly earthworm channels. Preferential flow through macropores occurs when the soi l is sufficiently saturated or when macropores are open at the soi l surface and the appl ication of water exceeds the 1 29 infi ltration rate (Scotter and Kanchanasut, 1 981 ; Beven and Germann, 1 982; Munyankusi et a I . , 1 994) . The soi ls in the present study were close to saturation and received simulated rainfa l l at a rate faster than infi ltration due to the method of appl ication. Table 5. 1 Pattern of methylene blue stained water infi ltration through intact soi l cores at the end of the leaching tria l . Water Infiltration (% squares (1 &2 cm from surface) or mean no. of pores (4 cm - core base) occupied by methylene blue) Distance from Aponga clay Wharekohe silt Wharekohe silt Wharekohe core surface loam 1 0 yr loam 32 yr sandy loam (cm) 1 54 44 53 62 2 25 1 9 1 3 25 4 7 .25 7 .75 5 .75 8 .75 6 3 2.25 3 .34 4 8 0 .75 3 1 .67 3 .5 10 (base) 0 .75 1 . 75 3.75 5 Relating P movement through the soil cores to the field situation is compl icated by several factors including the effect of preferential flow through soi ls with low anion storage capacities, the removal of the pan influence when the intact cores were col lected and the methods of simulated rainfa l l appl ication employed in the tria l . Phosphorus adsorption below 6 cm may have been s l ightly restricted in cores where water flowed from the perimeter, as some of the earthworm channels were edged with PVC result ing in less soi l contact for sorption of P from the leaching solution. However, l itt le difference between the movement of non-sorbed and adsorbed ions occurs during preferential flow (Scotter, 1 978; Kanchanasut et a I . , 1 978; Scotter and Kanchanasut, 1 981 ) and hence, the reduced soil contact at depth for soi l water flowing down the perimeter of cores is expected to have l itt le impact on the amount 1 30 of P leached. Cameron et a l . ( 1 990) did not find perimeter flow as their leaching experiment was conducted shortly after core col lection. This l imited t ime did not a l low earthworms, if present, to create fresh channels by burrowing. Rapid movement of P through macropores does not al low for very much soi l contact for sorption of P from the flowing water as a l im ited surface area is exposed to moving water and the water is moving faster than it wou ld through the remainder of the soi l mass (Scotter and Kanchanasut, 1 98 1 ) . This explanation is used to indicate the reason for varying nutrient losses from surface appl ied fert i l isers i n soi ls with high anion storage capacities. However, in soi ls with very low anion storage capacities and/or those soi ls which are P saturated, such as the Wharekohe soi ls, preferential flow may eventually reduce P losses. Freshly appl ied P which has diffused into soi l aggregates would largely be protected from the water flowing preferential ly through macropores (McClay et aI . , 1 991 ) . In contrast, where water moves through the soi l mass, soi l solution of high P concentration, due to l ittle P retention, may be displaced from the soi l mass and lost from the soi l . The removal of the pan influence from the base of the intact cores has most l i kely led to greater preferential flow through the Wharekohe soi l cores than would be observed in the field. Therefore, it is not possible to extrapolate the quantity of P leached through cores in the laboratory to the field situation. The method of leaching employed in the present study may have led to lower losses than wou ld have been encountered with other methods used. As P is 50 easi ly desorbed from Wharekohe soi ls , the delayed and intermittent rainfa l l app l ication would have decreased losses compared to immediate or continuous rainfa l l appl ication. Upon appl ication, P had time to diffuse into soi l aggregates in which it wou ld have been protected from leaching with the bulk of the water flowing through macropores. Losses of P in surface runoff have also been found to be higher when leaching is commenced closer to P appl ication (Sharpley, 1 982) . The intermittent leaching employed in the present study would have resulted in lower losses of P than continuous leaching, as P would have been able to d iffuse into aggregates between 'ra infa l l ' appl ications where it would be protected from subsequent ' ra infal l ' 1 31 events as shown for S and N on a yel low grey earth s i lt loam with low S adsorption (McClay et a I . , 1 991 ) . 5.4.2 Field Trial 5.4.2. 1 Movement of Applied P Rainfa l l recorded for the duration of the field trial is presented in F igure 5.6. Freshly appl ied P (as DIP) moved to the 2-7 .5 cm depth at both sites from the first day after P appl ication when 1 0 mm of rain had been recorded ( Figure 5.7a) . D IP concentrations were high, 1 8.65 jJg/ml and 1 3.87 jJg/ml for the 1 1 and 33/35 year old sites, respectively. McAul iffe et al. ( 1 979) also measured high in it ial concentrations of D IP from pasture i rrigated with casein waste appl ied at 1 44 kg total P/ha (95 kg D IP/ha) on an a l luvium soi l derived from greywacke material and volcanic ash. The top soil had become saturated with P resulting in downward movement of the appl ied P and D IP concentrations of up to 20 ppm were recorded at 60 cm depth. Interestingly, the highest D IP measurements were at 60 cm depth in this free drain ing soi l . Heavy rainfa l l on day 16 resulted in freshly appl ied P moving down slope in the 2-7 .5 cm depth on the younger s ite (Figure 5. 7a). In contrast no movement of P down slope at this depth was recorded on the older s ite (F igure 5 .7 b) . Inorganic P a lso moved down the profi le to the 7 .5-1 3 cm depth and down the slope at that depth on the younger site after the heavy rainfa l l on day 1 6 (Figure 5 .8 a). Only sl ight movement of freshly appl ied P was recorded down the profi le or the s lope at the lower depth on the older site (Figure 5 .8 b). Concentrations of D IP were lower in the 7 .5 to 1 3 cm depth. Wheeler and Edmeades ( 1 995) a lso found P concentration in soi l solution extractions general ly decreased with depth in a range of pastoral soi ls which had not received P fert i l iser for 4 .5 years. 0 0 0 0 0 0 0 0 0 0 0 0 m CO l"- to L() '-IV C iU 'i: - " a; It: .... 0 " 0 'i: Q) c. � Q) > 0 " Q) " � 0 CJ Q) � :! c 'cu D:: CD II) ! ::::s C) u: -E - Cl ::1. - Q. CJ .- c C'G Cl ... 0 c - - E - Cl :t. Q. CJ c C'G Cl ... 0 c a) 1 1 year old sites 20 1 8 1 6 1 4 1 2 1 0 8 6 4 2 0 -6 1 1 3 1 6 24 Sampling time (days) b) 33/35 year old sites 20 1 8 1 6 1 4 1 2 1 0 8 6 4 2 0 -6 1 1 3 1 6 24 Sampling time (days) 56 56 1 33 1 0 years (within plot) 1 0em down-slope 20cm down-slope 1 0 years (control) 32 years (within plot) 1 0cm down-slope 20cm down-slope 32 years (control) Figure 5.7 Dissolved inorganic P measured in water samples collected from 2-7.5 cm depth, a) 1 1 years under developed pasture, and b) 33/35 years under developed pasture. (s-significantly different from control). - E �I - Q. .� c "' en ... 0 c - E - � - Q. .� c "' en ... 0 .E a} 1 1 year old sites 1 .4 1 .2 1 0.8 0.6 0.4 0.2 0 � 1 1 3 1 6 24 Sampling time (days) b} 33/35 year old sites 1 .4 1 .2 1 0.8 0.6 0.4 0.2 0 � 1 1 3 1 6 24 Sampling time (days) 56 56 Within plot 1 0cm down-slope 20cm down-slope 1 0 m away (control) Within plot 1 0em down-slope 20em down-slope 1 0 m away (control) Figure 5. 8 Dissolved inorganic P measured in water samples collected from 7.5-1 3 cm depth, a} 1 1 years under developed pasture, and b} 33/35 years under developed pasture. (s-significantly different from control). 1 34 1 35 Movement of P to the 7 .5-1 3 cm depth may have been restricted by the water saturated nature of the soi ls at the t ime of P appl ication which could have led to much water and appl ied P being lost as surface runoff. Hence, less movement of P both to depth and latera l ly could have been recorded in the present study than if the trial had commenced in autumn when P is normal ly appl ied . However, lower D IP concentrations below 7 .5 cm may not necessari ly indicate low levels of D IP movement to this depth. I t is quite l ikely that the soi l at the 7 .5-1 3 cm depth is yet to be saturated with P (Chapter 3) . Hence, D IP which moves below 7 .5 cm may have been removed from solution through soi l P retention at both sites. The appl ication of P appeared to result in an in it ial flush of DOP at least to the 2-7 .5 cm depth in both sites (Figures 5.9 and 5. 1 0) . However, examination of data reveals that the flush was restricted to only one out of the 6 rep l icates at both sites and t­ tests showed that the d ifference was not significant. Once again the amount of DOP was general ly smal l in relation to D IP under fert i l ised plots « 5% of TDP for the 2-7 .5 cm depth, Day 1 ) . D issolved organic P may sti l l play a role in P leach ing as the D IP concentration reduces with time after P appl ication (F igure 5. 1 1 ) , and for a large part of the year, DOP is l i kely to form a large proportion of TDP. However, as P is mostly appl ied in autumn before the wettest part of the year, the bulk of the TDP lost from sites is expected to be D IP at the Kaikohe station. McAul iffe et al. ( 1 979) also found that the D IP concentration decreased with time fol lowing the in it ial increase encountered whi le the freshly appl ied P was moving to the depths where D IP concentration was measured. D issolved inorganic P concentrations in the field may not necessari ly decrease exponential ly, as occurs in laboratory leaching studies where simulated rainfal l is general ly appl ied at a constant level at each rainfa l l event. In the field solution P concentrations over time wi l l be effected by changes in soi l water volume, due to natural variation in the rainfal l pattern. Although the areas surrounding the suction cups were protected by pasture cages (2 x 1 m), rotational grazing by cattle may have influenced P concentration to depth. Grazing has been shown to increase losses of P substantia l ly in surface and E - � :::1. - Q. (,) c: "' � � 0 - :e - � :::1. - Q. (,) c: "' � � 0 a) 1 1 year old sites 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 1 3 . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . .................. : ....... . . . . . ...... . . . . .. .. , . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1" ... 1 6 56 With i n plot 20cm down-slope 1 0 m away (control) b) 33/35 year old sites 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 1 3 ,·· · · · ·'1· · · · · ·· · · · · · · ·· · · · ·· · · · · ·+ · · · · · · ······ ··· · · · · ..... � .......... ........... . , . . . . . . . :� . ... . ...... . . . .. . ... .. .. + .... . . . . . . . . . . . . . . . . . . . . . ; ........ ............ . 1 6 56 Withi n plot 20cm down-slope 1 0 m away (control) Figure 5.9 Dissolved organic P measured i n water samples collected from 2·7.5 cm depth, a) 1 1 years under developed pasture, and b) 33/35 years under developed pasture. 1 36 1 37 a) 1 1 year old sites 0.6 0.5 E - 0') 0.4 ::1. Q. 0.3 (J c:: tIS 0.2 0') ... 0 0.1 0 b) 0.6 0.5 E - 0') 1 ::1. 0.4 Q. 0.3 (J c:: tIS 0.2 � 0 0.1 0 Figure 5. 1 0 1 1 3 1 6 56 Within plot 20cm down-slope 1 0 m away (control) 33/35 year old sites 1 Within plot 1 3 1 6 56 Dissolved organic P measured in water samples collected from 7.5-1 3 cm depth, a) 1 1 years under developed pasture, and b) 33/35 years under developed pasture. 20 1 8 1 6 1 4 =- 1 2 E c;, 1 0 -= � 8 C 6 4 2 0 0 8 1 6 24 32 40 Days F rom P Application _ 32 years (with in plot) � 32 years (control) -.- 1 0 years (with in plot) � 1 0 years (control) 1 38 48 56 Figure 5. 1 1 Effect of t ime on D IP concentration in the 2-7 .5 cm with in the fert i l ised plots and 1 0 m away (controls) . subsurface runoff both in New Zealand (Sharpley and Syers, 1 976; Turner et aI . , 1 979; McCol l and Gibson, 1 979a; Lambert et a i , 1 985) and overseas (Olness et aI . , 1 975; Schepers and Francis, 1 981 ; Jawson et a I . , 1 982) . Grazing by cattle has been found to have a greater impact on P concentrations in runoff waters than grazing by sheep (Lambert et a I . , 1 985). As surface pugging and destruction of drainage channels by animals reduces infi ltration , total losses of fert i l iser P (kg/ha) via subsurface water wi l l not be increased by as much as the P concentrations. Grazing animals increased P concentration in ti le drainage 1 50 t imes for D IP and 400 times for PP but the actual amounts only increased by 50 and 1 00%, respectively, in a Tokomaru si l t loam (Sharpley and Syers, 1 979b). 5.4.2.2 Effect of Pasture Age on P Movement On the basis of P concentration data, P appeared to move down slope from the trial sites and to greater depth after heavy rain on day 1 6 in the youngest s ite, compared 1 39 to l itt le movement of P in the oldest site, suggesting greater movement of appl ied P at the young site. This result contrasts with the expectation that the movement of P in soi l water would be greater at the oldest site. However, volumes of runoff waters were not measured and may effect both the total P movement and P concentrations at each depth and each site. Therefore, it is not possible to use the soi l water P concentration field trial data directly as a basis for comparing the extent of P movement in subsurface runoff between the two sites. 5.5 GEN ERAL DISCUSSION Results from the glasshouse leaching experiment and field tria ls indicated that the movement of d issolved inorganic P through Wharekohe soils, after superphosphate appl ication, appeared to be substantia l . Inorganic P added to the soi l from sources such as l iv ing pasture, l itter and dung are expected to undergo the same t e fate. Some of the P which moved through the Wharekohe soi ls in the above experiments would have been freshly appl ied ferti l iser P, whi le some may have been derived from the store of accumulated soi l P displaced by the freshly appl ied P. In most other New Zealand soi ls, P movement is not large enough to warrant concerns from an agronomic point of view, although they are sti l l considered to have considerable impact environmental ly (Gregg et a I . , 1 993). However, in the Wharekohe soi ls and other New Zealand podzols, losses of P were so high that, not only may the lost P play a role in the eutrophication of water bodies, but the losses of P in soi l water reduces the potential effectiveness of the appl ied P, representing a significant economic loss to the producer. 5.5. 1 Comparison of the P Concentration in Leachate from the Intact Soil Core and Soil Solution Collected in the Suction Cups Phosphorus concentrations were higher in the leachate col lected from the intact cores in the g lasshouse study (up to 45.6 j.Jg/ml ) than in the soi l solution col lected in the suction cups in the field study (up to 1 8.65 j.Jg/ml) . This difference may be due to 1 40 d ifferences in soi l moisture at the time of P appl ication. The amount of simulated rainfa l l applied to the intact soi l cores in the days prior to leachate col lection was lower than the rainfa l l recorded in the days prior to the col lection of soi l solution from suction cups in the field study. Suction cups have been found to provide a better estimate of the stagnant soi l solution, whi le field Iysimeters provide a better estimate of mobi le solution in soils which exh ibit preferential flow (Magid, 1 991 ) . In Magid's study, the solutions col lected by the field Iysimeters contained 4.6 times more inorganic P and 2 times more organic P on average than the suction cup solutions in the field. The removal of the pan influence in the present g lasshouse trial would have in it iated greater preferential flow, than occurs in the field, result ing in the higher P concentrations observed in the g lasshouse tria l . The rapid movement of soi l water from the soi l surface, where P concentrations are high, through macropores reduces soi l contact with solution P and hence, opportunities for P retention. At the time of the present field study, movement of soil water in the Wharekohe soi l was expected to be very slow, due to poor drainage. Therefore suction cups may provide a better indication of the concentration of P moving through the soil subsurface than the leachate col lected in the g lasshouse study (where flow rates were artificial ly increased) or in the suction cups in Magid's study of a free drain ing soi l . 5.5.2 Effect of Pasture age and P Saturation on P Losses New Zealand podzols genera l ly have very low anion storage capacities. The appl ication of fert i l iser P fi l ls the avai lable P sorption sites so that, with t ime under developed pasture, the soi l P content approaches a maximum P storage capacity and P saturation (Chapter 3). Both overseas (Logan and McLean , 1 973; Adriano et a I . , 1 975; van Riemsdijk et a I . , 1 987; Lefroy et aI . , 1 995) and New Zealand (Ooak, 1 942; McAul iffe et a I . , 1 979) studies have demonstrated that P moves through soi l profi les once P saturation has occurred) . Lefroy et a l . ( 1 995) demonstrated that 32p movement through repacked cores was increased in a g ley podzol in contrast to no P movement recorded for a red earth (which had a considerable h igher capacity to sorb P) where the soi ls had received previous fertil iser P appl ications. 1 41 Even in coarse textured podzols, P leaching is very much dependent on the soi ls abi l ity to retain P . In a laboratory study using the Te Kopuru podzol col lected from six dairy farms, the amount of appl ied P leached through columns of sieved soi l , which had been sampled to 3 cm, after the appl ication of 400 mm of s imulated rainfa l l over 4 weeks, varied from < 1 % to 36% of the appl ied P (Hogg and Cooper, 1 964) . The amount of P leached was indirectly proportional to the P retention abi l ity of each soi l sample. Less than 1 % of appl ied P was lost from the soi l with the highest anion storage capacity, 47%, whi le 36% of the appl ied P was lost from the soi l with an anion storage capacity of 0%. It was expected that the amount of P leached would be greater in the older developed Wharekohe site which has reached saturation than in the younger 1 0 year old s ite. No significant effect of pasture age on P movement could be detected in the glasshouse experiment in the present study for the two pasture sites examined . The Wharekohe si l t loam was shown to be close to P saturation in the top 7 .5 cm by 1 1 years under developed pasture (Chapter 3). The rate of appl ied P in the leaching experiment was most l i kely too h igh to detect the smal l differences in the abi l ity of the soi ls to retain added P (reported in chapter 6) . In addit ion, P loss via surface runoff may have been a more important component of soil loss than subsurface runoff losses in the field. The P movement field trial results were inconclusive i n relation to the effect of pasture age as explained in 5 .4 .2 .2 . Results from a catchment study in Northland on podzol ised yel low brown earths and Wharekohe soi ls (McColl et aI . , 1 975; McCol l , 1 978) supports low P loss from freshly developed pastures on these soi ls . These losses which were very low included both surface and subsurface runoff losses. Only 1 .44% and 0 .55% of the superphosphate (appl ied at 48 kg P/ha/yr) was lost from the smal lest catchment after the first and second appl ications respectively, and negl ig ib le losses were reported for the larger catchments. Phosphorus losses from the whole basin were lower during the first 2 .5 years from development than in some natural and undisturbed catchments. Much of the P lost from the smal lest catchment could be accounted for by the d irect appl ication of fert i l iser P to the waterways, which only contained water 1 42 in the months with high rainfa l l . The soi ls in the catchment study would not have reached P saturation and hence P losses were negl ig ible. Losses of P from this catchment wou ld be expected to be considerably higher once the soils become saturated with P after continued fert i l iser P appl ication. 5.5.3 Effect of Waterlogging on P Loss Waterlogging is a soi l forming process wh ich can eventual ly lead to lower anion storage capacities. Hogg and Cooper ( 1 964) attributed d ifferences in the P sorption capacity recorded between the Te Kopuru podzols in their laboratory P leaching study to the varying degree of waterlogging experienced at the soi l col lection sites. In addition, P leach ing has been shown to increase where soi ls become water logged (H ingston, 1 959). Waterlogging a previously dry soi l results in a decrease in the anion storage capacity and hence an increase i n water soluble P as a) the increase in pH bought about by the reduction of soil compounds leads to the hydrolysis of Fe ( I I I ) and AI phosphates, and the release of P held by anion exchange on clay and hydrous oxides of Fe ( I I I ) and AI , and b) Fe( l I l ) is reduced to Fe( l I ) re leasing sorbed and chemical ly bonded P (Ponnamperuma, 1 972; Gotoh and Patrick, 1 974; Khal id et a I . , 1 977; Kirk et a I . , 1 990). Wharekohe soi ls are natural ly poorly drained due to the presence of both the s i l ica pan (E horizon) found within 30 cm of the soi l surface, and a poorly structured topsoi l . Waterlogging wi l l have contributed to the low anion storage capacities recorded for these soi ls . Prolonged waterlogging in winter could result i n the above mentioned processes further reducing the already low abi l ity of the soi l to retain P whi le the soi l is saturated, result ing in the release of P into the soi l solution from which it would be prone to loss in runoff waters. Although the soi l cores in the glasshouse study were kept wel l watered, they would not have been as waterlogged as in the field in winter and hence, P losses may not have been so great. Most New Zealand podzols are prone to waterlogging and hence P losses are expected to be enhanced by the reduction of soi l compounds including Fe in winter. 5.5.4 Loss of P in Subsurface and Surface Runoff 1 43 As al ready mentioned , the relative contribution of subsurface and surface flow to P losses depends on both the concentrations of P and the volume of water in each component of runoff. Poor drainage and the relatively flat to rol l ing areas on which the Wharekohe soi ls have developed results in waterlogging and most l i kely very slow lateral movement down slope (M. Richardson, pers. comm. ) . Hence, surface runoff may contribute a greater portion of P loss in Wharekohe soi ls i n comparison to other soi ls . Losses of P via surface runoff from Wharekohe soi ls would be expected to be far higher than the 5.63 kg P/ha/yr reported for a Tokomaru s i lt loam (Sharpley and Syers, 1 979a) as the Wharekohe soi ls have lower anion storage capacities, receive a higher rainfa l l , have low surface infi ltration rates, are more waterlogged, and are more prone to pugg ing leading to surface seal ing and higher sediment losses. Concentrations of D IP in surface runoff col lected 6 months after the appl ication of 5 1 -56 kg RPR in 1 991 on the Kaikohe Station during this research project were high, ranging from 0.24 to 0 .54 !-Ig/ml during heavy rainfa l l . In contrast, concentrations of appl ied P in surface runoff from a Maimai podzol under high rainfa l l condit ions 6 months after the appl ication of 42 kg of P as superphosphate were low, <0. 1 IJg/ml (only 6% of appl ied P had been lost in runoff to 5 months) (Lee et aI . , 1 979). However, the lower than expected P loss was attributed to a higher anion storage capacity (39%) at the trial s ite than is normal (5%) for this podzol . In contrast to the findings of McAul iffe et a l . ( 1 979), surface appl ied P d id not appear to penetrate very deeply into the soil in the present field study. Although the low D IP concentrations cou ld be explained by the removal of D IP from the solution by soi l P retention at depth (5.4.2 . 1 ) , it is l ikely that l itt le downward movement of water occurred at either site. The Wharekohe soi l was already saturated with water at the commencement of this fie ld study and consequently surface runoff was l ikely to be high for the trials duration. A s imi lar pattern of soi l water movement and P loss exists on duplex soi ls where 1 0-20 cm of sand overl ies an impermeable clay horizon, in Western Austral ia . Surface runoff contributes 75% of the P lost from shal low duplex soi ls , as lateral subsurface dra inage is slow and approximately 80% of the 1 44 drainage is surface runoff (Ruprecht and George, 1 993). The relative contribution of surface and subsurface flow to P losses in the duplex soi ls changes with position on the slope. Lateral flow is more important up-slope and mid-slope, whereas down slope, overland flow becomes more important as the soi ls are more saturated. Position on a slope is l ikely to effect runoff from Wharekohe podzols in the same way and may have influenced the resu lts in the present field tria l , a lthough it was not specifica l ly studied. 5.5.5 Loss of Particulate and Dissolved P Losses of total P in subsurface runoff would be underestimated i n this study as particulate P was not measured. Subsurface runoff is dominated by dissolved P but can sti l l contain significant quantities of particulate P (Sharpley and Syers, 1 979a&b) . Eroded soi l is usually richer in P than non eroded soi l due to the selective removal of finer part icles which have a greater abi l ity to sorb P (Sharpley, 1 980). However, Wharekohe soi ls are high in fine s i l iceous material which are prone to movement but do not have a great abi l ity to sorb P. Particulate P is not expected to play as large a role in P losses from Wharekohe soi ls compared to other soi ls, a lthough much P is sti l l l i kely to be lost as PP in surface runoff. TOP (mostly as D IP ) is l i kely to be the more important mechanism for P losses from Wharekohe podzols in both subsurface and surface where soluble P fert i l isers are applied. 5.5.6 Predicting Runoff P Losses from Soil P Tests Load ings of dissolved i norganic P in surface runoff waters have not only been found to be closely related to P storage capacity (Hogg and Cooper, 1 964) , but also to various measures of soi l P status: 0. 1 M NaCI extractable P (Sharpley et a I . , 1 977, 1 978), Olsen P (Heckrath et a I . , 1 995; Greenhi l l et a I . , 1 983), water extractable P (Romkens and Nelson, 1 974; Sharpley et aI . , 1 977, 1 978, 1 982), Bray-I P (O lness et aI . , 1 975; Romkens and Nelson, 1 974; Schreiber, 1 988; Sharpley et aI . , 1 985), 1 45 Mehl ich-3 P (Sharpley, 1 995), i ron oxide resin strip extractable P (Weaver et aI . , 1 988; Sharpley, 1 995), total inorganic P (Weaver et a I . , 1 988) and total P (Greenhi l l et a I . , 1 983; Sharpley et aI . , 1 985). The amount and concentration of P in surface runoff is wel l correlated to the P status of surface soi l , wh i le P in subsurface runoff is wel l correlated to P status at depth (Sharpley et a I . , 1 977; Hanway and Laflan, 1 974). No soi l P status data was col lected in the present studies at the t ime of soi l core or soi l water col lection. However, comparison of various P fractions extracted from the soi ls sampled in the chronosequence study (Chapters 3 and 4) revealed that the d ifferences in P lost between the unfert i l ised moderately leached yel low brown earth, Aponga clay, and the Wharekohe soi ls in the g lasshouse study was better correlated with d ifferences in the easi ly extractable NH4CI Pi fraction than any other fraction or Olsen P. Thirty six times more NH4CI Pi was extracted from the top 7 .5 cm of the two oldest Wharekohe s i lt loam areas than from the Aponga clay, whi le 33 t imes more appl ied P was leached from the oldest Wharekohe s i lt loam cores compared to the Aponga clay cores. Hence, NH4CI extractable Pi is l i kely to provide a better indication of potential runoff P losses than many other soi l tests , as it extracts P which is i n solution and very weakly held in the soi l , and therefore prone to loss in runoff water. 5.5.7 M i nimising P Losses Clearly it is very important to min im ise P losses from Wharekohe podzols to reduce losses of P to the wider environment and the uneconomic use of P by the farmer. Effective measures to reduce losses of P in runoff waters from pastures include the appl ication of a smal l amount of soluble P more frequently, the appl ication of slowly soluble P fert i l isers (Weaver et a I . , 1 988), the appl ication of P at levels which meet plant requirements but do not result in surplus P (Weaver and Prout, 1 993; Yeates, 1 993), drainage to reduce surface runoff (where surface runoff contributes the greater amount of P to runoff P losses) , riparian zones and buffer strips (Smith 1 989; Chambers et aI . , 1 993; Weaver and Prout, 1 993), impoundments or small 1 46 reservoirs (Weaver and Prout, 1 993) and the addition of soi l amendments such as the red mud waste (derived from the production of alumina from bauxite) to increase P adsorption (Ho et aI . , 1 989; Vlahos et a I . , 1 989, Summers et a I . , 1 993; Scheffer et a I . , 1 986; Summers et a I . , 1 996) Modifying fert i l i ser practices wou ld most l i kely have the largest impact and be the most cost effective means of reducing P losses on Wharekohe soi ls . On these soi ls farmers are currently applying P at rates surp lus to plant requirements and, as l itt le P accumulates in the soi l (Chapter 3) ,much of the appl ied P is being lost in runoff water. Phosphorus is usually appl ied as one autumn dressing on sheep and beef farms. At this t ime the soi l is dry enough for spreading and the app l ication t ime takes advantage of the flush of autumn growth with the first seasonal rains whi le the ground is sti l l warm. Alternatively, P is appl ied in spl it dressings ( in autumn and spring), wh ich is the more common practice on dairy farms. The app l ication of a large amount of P just before winter most l i kely leads to large losses of P in winter runoff, whi le the appl ication of P to largely saturated soi l in early spring could lead to large losses of appl ied P in surface runoff. Applying smaller quantities of soluble P more frequently may help to meet plant demands whi le avoiding such large losses of appl ied P. Alternatively the use of less soluble P fert i l isers may achieve the same purpose, meeting plant requirements through slow d issolution but restricting the amount of soluble P avai lable for leaching or loss as dissolved P in surface runoff. Slowly soluble P fert i l i sers have been found to reduce the quantity of P leached through sandy soi ls and New Zea land podzols (Ozanne et aI . , 1 961 ; Hogg and Cooper, 1 964; G i l lman, 1 973; McSweeney and Mul ler, 1 979). Of the 30 kg of concentrated superphosphate appl ied to repacked cores of Addison gley podzol , 36% had leached from the cores i n 2 1 weeks. In contrast, none of the Chatham Rise phosphorite or Reno Hypophosphate (proprietary granu lated Gafsa phosphate rock from North Africa) and only 3% of the superphosphatel Calciphos mix ( 1 8% Calciphos added to superphosphate at den cut) had been leached (McSweeney and Mul ler, 1 979) . Phosphorus losses by leaching from repacked cores of Te Kopuru sandy loam were much lower where 1 47 basic slag (P lost 6%), serpentine super (P lost 5%) or Gafsa PR (P lost O%), rather than superphosphate (P lost 1 5%) , were appl ied (Hogg and Cooper, 1 964). Whi le the use of slowly soluble P may reduce P losses dramatical ly in the short term, reductions in P losses may not be so marked in the long term if P appl ication in these form exceeds plant requirements. Weaver et a l . ( 1 988) found in a laboratory experiment that, whi le superphosphate treated cores lost more P in i n itial leach ing cycles (one cycle representing one years rainfa l l ) , losses from coastal superphosphate were greater in the 4th to 7th leaching cycles because the superphosphate had a lready been leached. The total amount of P leached from the coastal superphosphate treated laboratory cores never equal led the amount leached form the superphosphate treated cores. However, continual annual appl ication of slowly soluble P in the field could result in the annual amount of less soluble P becoming avai lable for plants and leaching, through slow dissolution, eventual ly being equal to the amount app l ied annually as soluble P . Therefore, P appl ication at rates which do not exceed plant requirements wou ld be more important in reducing P losses than the form of appl ied P . Less soluble P fert i l isers may sti l l reduce P losses in the fie ld , as the P released by slow dissolution over the year may be taken up by plants, in comparison to a once a year large appl ication of soluble P which is prone to leaching before being required for plant uptake. The effectiveness of less soluble P in comparison to soluble P in restrict ing P losses from Wharekohe soi ls may depend on the relative role of surface and subsurface runoff. Whi le the app l ication of P as less soluble d icalcium phosphate reduced D IP in surface runoff compared to superphosphate in plot trials on Tokomaru si l t loam, losses as PP were much higher as the less soluble fert i l iser part icles were in the surface runoff component (Sharpley et aI . , 1 978) . A total of 7 .09 kg P/ha/yr was lost in surface runoff from the dicalcium phosphate plots in comparison to 5 .63 kg P/ha/yr from the superphosphate plots. Losses amounted to 1 1 . 5% and 8.8% of the dicalcium phosphate and superphosphate applied, respectively. The slowly soluble dicalcium phosphate was not expected to offer any advantage over superphosphate, as the subsurface movement of P was not a major loss on this soi l . Even if surface runoff contributed the larger part of P loss from Wharekohe soi ls , less soluble P may 1 48 sti l l restrict P loss in comparison to more soluble P forms, as d issolved P may dominate P loss in surface runoff where soil P retention is so low. Resu lts from field studies investigating the agronomic effectiveness of slowly soluble P fert i l isers a lso support larger losses of P from soluble P fert i l iser on high P leaching soi ls . Herbage P concentrations decl ined over a three year trial period on an Addison podzol to below adequate levels for pasture maintenance (0 .35%) where 50 kg P/ha was appl ied as superphosphate or l ime-reverted superphosphate (Morton and Quin , 1 980). These results indicated that these P fert i l iser forms were inadequate to maintain pasture production at this s ite, where leaching losses of P were substantial , compared to the preceding annual appl ications of less soluble serpentine superphosphate. Herbage P concentrations and Olsen P levels in Sechura Phosphate Rock (SPR) treated plots were equal to or higher than those recorded in triple-superphosphate (TSP) treated plots over a five year period from the second year of a field trial on an Okarito si l t loam, where P leaching is l ikely to have occurred (Smith et a I . , 1 990, 1 991 a) (although h igher dry matter yields were measured under TSP than under either SPR or North Caro l ina Phosphate Rock (NCPR) unti l year 4) . Western Austral ian studies have found that low solubi l ity P fert i l isers are as agronomical ly effective as superphosphate on highly leached sands, including podzols, in contrast to the low effectiveness of RPR on other Western Austral ian soi ls (Wright, 1 975; Yeates et aI . , 1 984; Yeates et a I . , 1 986; Bol land et a I . , 1 995b). Even unreactive Florida phosphate rock has been found to be equal ly as effective as superphosphate on a high P leaching sandy podzol (Alston and Chin , 1 974). Austra l ian studies have also found that the low solubi l ity P fert i l isers can stimulate pasture production to a greater extent than superphosphate on high P leaching sands (Yeates and C larke, 1 993; Bolland et aI . , 1 995b). The relative response of the low solubi l ity PAPR ("Coastal Super" ) has been found to vary from year to year, depending on cl imatic conditions (Bol land et a I . , 1 995b). "Coastal Super" was considered to be more effective than superphosphate in wet years at stimulating plant growth wh i le in d ry years, superphosphate was considered to be the more - ---_.- -------------- 1 49 effective P fert i l iser. However, runoff losses from RPR may sti l l be high where RPR is appl ied surp lus to plant requirements where particulate P losses are high and dissolved P from RPR d issolution is lost. High runoff losses of P have even been reported from an Austra l ian sandy podzol fert i l ised with unreactive F lorida rock (Alston and Chin , 1 974). Hence, even with an RPR it is necessary to accurately calculate the P requirements to min imise P losses in runoff. Studies are needed to investigate strategies for min im is ing P losses in runoff water from Wharekohe podzols. Such studies should include the determination of the relative role of surface runoff and subsurface runoff to P loss, the contribution of less soluble P fert i l isers or frequent smal l appl ications of soluble fert i l i sers to meet plant demand but reduce losses, and the determination of the amount of P required annual ly to meet plant requirements but avoid appl ication surplus to requirements. P lant P requirements and the contribution of less soluble fert i l isers to reducing P losses are considered more ful ly in chapter 7 . 5.6 CONCLUSIONS • The leaching study indicated that substantial quantities of P can be transported in subsurface water movement through Wharekohe podzols (up to 45.6 jJg/ml ) in contrast to the yel low brown earth, Aponga clay (up to 1 . 07 jJg/ml ) . The levels of P wh ich move through Wharekohe podzols are so high, that it is l ikely that they contribute significant quantities of P to waterways creating the potential for eutrophication. Additional ly the losses may represent a large economic cost to the producer . • The large d ifferences i n P lost between the Wharekohe soi ls and the yel low brown earth, Aponga clay, were more closely related to the large d ifference in the easi ly extractable NH4CI P i fraction (32 jJg/g vs 0 .9 jJg/g, for the Wharekohe s i lt loam and Aponga clay, respectively) rather than either Olsen P (22 vs 25) or other soi l P fractions. 1 50 • Movement of d issolved P occurs mostly as D IP after the appl ication of fert i l iser P . D IP concentrations decreased with t ime from P appl ication in both the g lasshouse and field tria ls with the amount of P leached through intact cores at each leaching event decreasing exponential ly with time from ferti l iser P add ition. • No d ifference in P movement could be detected in relation to development history in these trials . The younger site may have been close to P saturation and the rate of P appl ication was most l i kely too high to detect the smal l d ifferences in P retention recorded between the soi ls and reported in chapter 6. In addition d ifferences in P loss via surface runoff between the two sites may have been a more important component of soi l loss. • No d ifference in the amount of P leached could be detected between the Wharekohe sandy loam and Wharekohe si lt loam in the glasshouse tria l . • Further fie ld studies a imed at quantifying P losses in both surface and subsurface run off, determining the effect of less soluble P fert i l isers in reducing P loss and determin ing the optimum amount of P required for plant uptake are required if appropriate fert i l iser practices are to be developed to min im ise such losses. CHAPTER 6 P RETENTION IN WHAREKOH E SOILS 6.1 INTRODUCTION 1 51 The precipitation and adsorption of P from fresh ly appl ied soluble P fert i l iser wi l l occur s imultaneously in the soi l fo l lowing dissolution of P fert i l iser and its diffusion outwards from the fert i l iser granule. Precipitation reactions dominate at high P concentrations in the vicin ity of the freshly appl ied fert i l iser granules, whi le adsorption reactions become dominant at low P concentrations (between 0.0001 and 0.001 molar P) further from the fert i l iser granule (Sample et a I . , 1 980; Holford , 1 989). In it ial ly, only a smal l portion of the fert i l iser P wi l l be adsorbed in the soi l compared to retention in products of precipitation reactions. However, with t ime, as the concentration of P in the vicin i ty of the P fert i l iser granule slowly decl ines, a larger proportion of the fert i l iser P wi l l be adsorbed (Holford, 1 989). In acid weathered soi ls, P retention is most closely related to the presence of high surface area amorphous inorganic and organic Fe and, in particu lar, AI compounds (Wi l l iams et a I . , 1 958; Saunders, 1 965; Syers et a I . , 1 97 1 ; Laverdiere and Karam, 1 984; Borggaard et a I . , 1 990; S ingh and Gi lkes, 1 991 ; Gi lkes and Hughes, 1 994) . Although P wi l l also be sorbed on the surfaces of s i l icate clays, the abi l ity of the alumino-si l icates such as kaol in ite and vermicul ite, found in the Wharekohe soils, to retain P is very low in comparison to amorphous F·e and AI compounds (Mol loy, 1 988). The Wharekohe soi ls have extremely low Fe and AI contents and mineralog ical analysis of Wharekohe si lt loams from the Kaikohe Research Station revealed that they contained over 90% quartz, with small quantities of crystophi l ite, vermicul ite and anatase (J .Whitton, pers. comm. ) . This explains their lower Anion Storage Capacity in comparison to other less weathered sedimentary soils. Hence, the maximum P storage capacity of Wharekohe soi ls wi l l be reached sooner than in other soi ls . The capacity of the Wharekohe si lt loam to retain added P has been shown to decrease substant ial ly with pasture age (Chapter 3) . In it ia l ly, most of the appl ied P 1 52 is retained in the top soi l (0-7 .5 cm) , but by 1 1 years under developed pasture, only a smal l proportion of appl ied P is retained in the top 7 . 5 cm as the soi l appears to reach a maximum P storage capacity. As there is a l im it to the soi ls capacity to retai n P , i t is l ikely that over time the amount of appl ied P requi red to maintain production wi l l be reduced. P appl ied surp lus to soil accumulation and production losses wi l l be lost from the soi l via runoff waters. Several studies have related P retention characteristics of soi ls to plant P requirements (Woodruff and Kamprath, 1 965; Morris et a I . , 1 992; Bol land et a I . , 1 994) and the suscept ib i l ity of appl ied P to loss in soi l water runoff (Sawhney, 1 977; Diggle and Bel l , 1 984; Sharpley, 1 995) . The abi l ity of a soi l to retain added P over a range of P concentrations in solution (known as "P sorption isotherms" in the l iterature) can be used to i l lustrate d ifferences in the P storage behaviour of d ifferent soi ls . "Phosphorus sorption isotherms" provide a useful tool for investigating, under control led conditions, the P retention characteristics of the Wharekohe podzols. Although stud ies of the nature presented in this chapter are often termed "P sorption isotherms" in the l iterature, the term is inappropriate for the description of the relationsh ip between solution P concentration and the amount of P sorbed. F irstly, the term has been adopted from surface chemistry where it is used to describe adsorption that depends only on temperature and concentration, wh ich is clearly not the case with soi ls (Barrow, 1 989). Secondly, P sorption may not be the only mechanism operating for the retention of added P. At the low concentrations usual ly employed in studies investigating P retention over a range of P concentrations, the adsorption of P is considered to be the dominant mechanism for the retention of added P. However, in the present study, the highest P addition was in the order of 0.004 M P at which sign ificant precipitation may also be occurring (Holford, 1 989) . The present study was not designed to d ifferentiate between the mechanisms for P retention in Wharekohe soi ls , but rather to investigate d ifferences in P retention between soi ls of d ifferent type and pasture age. The term used in this d iscussion of results is P retention to reflect that added P is retained in the soi l by not only adsorption and absorption, but also by precipitation. 6.2 OBJ ECTIVES The objectives of this part of this PhD study were to: 1 53 i ) confirm that the abi l ity of the Wharekohe si lt loam to retain added P decreases with pasture age, and i i ) investigate the effect of parent material texture and degree of weathering on P retention characteristics. 6.3 MATERIALS AN D M ETHODS The soi ls used in this study were the Wharekohe silt loam (0, 1 1 and 35 years under developed pasture col lected in 1 993), Wharekohe sandy loam and Aponga clay samples described in Chapters 3 and 4. The four repl icate Wharekohe si lt loam samples col lected for each of the three pasture ages were bulked , taking into account the bulk density of each repl icate. Dupl icate 0 .9 g samples of soi l were shaken with 36 m l solutions of 0 .01 M CaCb containing chloroform and increasing amounts of P (0, 250, 500, 1 000, 5000 I..Ig PIg soi l ) in 40 m l polypropylene centrifuge tubes on an end over shaker at 20°C for 3 periods ( 1 6, 40 and 1 36 hours) . 0 .03 M NaCI was also included as a background electrolyte at 40 hours. The soi l solutions were then centrifuged at 9000 rpm for 1 0 minutes before the supernatants were fi ltered through 0.45 I..Im mi l lepore fi lter papers. I norganic P was determined in the solutions colorimetrical ly by the phosphomolybdate method of Murphy and Ri ley ( 1 962) . The amount of P sorbed by the soil was calculated as the difference between the amount of P added to the solutions per gram of soi l prior to shaking and the amount of inorganic P in the solutions per gram of soi l after shaking. pH was also measured in the supernatants, after filtering, with a g lass electrode. 6.4 RESU LTS AND DISCUSSION 6.4. 1 Effect of Pasture Age on P Retention 1 54 The amount of added P retained by the Wharekohe soi l decreased with increasing pasture age after 1 6 and 40 hours of shaking in 0.01 M CaCb, except at the highest P concentration in the samples shaken for 1 6 hours (F igures 6 . 1 a & b and 6.2) . Although the differences in P retention between sites were smal l they were consistent across a l l concentrations (except the highest) and laboratory dupl icate errors were negl ig ib le. D ifferences in P retention could not be detected in the highest final P concentrations. Caution should be taken in interpreting the resu lts obtained at the highest added P concentration. The release of P from the soi ls at 0 added P indicates that the undeveloped soi l conta ined l itt le releasable inorgan ic P and that inorganic P was more strongly retained in the soi l from the 1 1 year old pasture than in the soi l from the 35 year old pasture (F igure 6.2) despite the higher inorganic P content of the younger soi l . Not surprising ly, the pattern of P release was simi lar to that encountered in the NH4CI extracts in the P fractionation study (Chapter 4) . The results from the 1 6 and 40 hour shaking periods support the hypothesis that the abi l ity of the Wharekohe s i lt loam to retain added P decl ines with pasture development as the P retention sites are fi l led with appl ied P. Anion storage capacity has also been shown to decl ine with increasing pasture age in New Zealand yel low brown loams. During ( 1 968) found that whi le P retention remained h igh on two sites which had been under pasture for 6 and 8 years, the P retention of s imi lar soi ls with much longer histories of intensive farming and fert i l iser appl icat ion were medium to low. However these differences in P retention could possibly have been explained by natural variation in an ion storage capacity between sites. In another yel low brown loam study investigating P retention over a range of pH levels, the amount of added P retained at low pH (pH 1 . 5 - 3 .5 ) decreased with increasing P status in soi l col lected from 3 sites of varying farming history on New Plymouth black loam, includ ing an undeveloped site (Saunders, 1 959b). The amount of added P 1 55 a) 1 6 hours 400 '0 300 III en -en 200 .:: 'C Q) 1 00 c::: 'j; -Q) 0 0:: .• . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - - - 0 c... 30 40 50 60 70 80 90 1 00 1 1 0 1 20 1 30 -1 00 b) 40 hours 400 - '0 300 III en -en 200 � - 'C Q) 1 00 c::: '; -Q) 0 0::: c... -1 00 . - . - - - - - - - - . - - - - - - _ _ _ _ _ _ _ _ -0 - - - 30 40 50 60 70 80 90 1 00 1 1 0 1 20 1 30 c) 1 36 hours - 0 III en -en .:: 'C Q) c::: '; -Q) 0:: c... 400 300 200 1 00 0 -1 00 .. ... .. .. _ _ _ _ _ _ _ _ _ _ _ _ - -0 ... ... .. ... 30 40 50 60 70 80 90 1 00 1 1 0 1 20 1 30 Fi nal P Concentration (J,lg/m l ) • Who si lt 0 Yrs D Who silt 1 1 Yrs M Who silt 35 Yrs - - 0 - . Who sandy Figure 6.1 Effect of increasing solution P concentration and shaking time on the retention of added P from 0.01 M CaCh py Wharekohe silt loam developed for 0, 1 1 and 35 years and Wharekohe sandy loam during a) 1 6 hour, b) 40 hour and c) 1 36 hour shaking periods. 250 5000 P 200 - 1 50 0 1/1 O! -O! .: 1 00 'a CI.I .5 1 000 P CIS .. CI.I 0::: a.. 50 500 P 1 00 P 0 5 25 30 35 O P -50 Years of Development F igure 6.2 Effect of pasture age on P storage by Wharekohe s i lt loam at various solution P concentrations. 1 56 (436 I-Ig/g soi l ) in Saunder's study was too low to detect d ifferences in P retention between the soi ls at the pH range usual ly encountered in soi ls. Further evidence for a chang ing P retention with fert i l iser P appl ication can be found in P retention studies on a range of Austral ian soi ls (Barrow, 1 984) , where the retention of added P was lower when the soi ls had been incubated with l ime and P than when they were incubated with l ime alone, suggesting that some of the previously added P was occupying P retention sites and blocking them from reta in ing further P . The soi l from the 1 1 year old pasture in the present study has the potential to retain more added P than the soi l from the 35 year old pasture, indicating that appl ied P 1 57 may continue to accumulate in topsoi ls which have been under pasture for up to at least 1 1 years. However, the greater retention of added P by the soi l from the 1 1 year old pasture than by the soil from the older developed site was surpris ing given that both soi ls appeared to have reached a simi lar maximum total P content (Chapter 3 and Appendix 3 .2) . After 1 36 hours, the amount of P retained by the soi l from the 1 1 year old pasture was even marginal ly higher than that retained by the undeveloped soi l at final P concentrations in excess of 5 I-Ig P/ml (F igure 6. 1 c). Although changes in the amount of P retained with time were smal l , the soi l from the 1 1 year old pasture appeared to retain P with continued shaking beyon� 40 hours at a sl ightly higher rate than the other Wharekohe si l t loam soi ls (Figure 6.3) . The effect of prolonged shaking time on P retention is discussed further in 6 .4. 1 . 1 . The present study investigates the retention of added inorganic P . However, organic P molecules could wel l be retained by simi lar mechanisms and are l ikely to compete with inorganic P for P retention sites in the fie ld . Likewise, added inorganic P may have displaced organic P in the present study, leading to an overestimation of P retention, as organic P was not measured in the final shaking solution. 6.4. 1 . 1 Possible Explanations for Differences in P Retention Between Developed Site While the present study was not designed to determine the specific mechanisms (sorption or precipitation) involved in P retention, some possible mechanisms for the increase in the amount of P retained by the soi l from the 1 1 year old pasture in the present study are described below. The greater than expected P retention of the soi l from the 1 1 year old pasture may be due to a combination of factors induced by the higher pH and exchangeable Ca content of the younger developed soil as a result of its more recent heavy l iming history, an integral part of the development procedure for a podzol (Chapters 3 and 4) . The appl ication of l ime with pasture development can affect P retention in several ways. 1 58 a) Wharekohe silt loam, 0 yr b) Wharekohe silt loam, 1 1 yr 400 - 400 '0 350 '0 350 / In 300 In 300 en en - 250 - 250 en en ::l. 200 ::l. 200 - " K 1 50 K " 1 50 � CI) ,. CI) : c:: 1 00 .. • c:: 1 00 r:: ra • • • ra - 50 - 50 CI) CI) D:: 0 D:: 0 D.. -50 20 40 60 80 1 00 1 20 1 40 D.. -50 20 40 60 80 1 00 1 20 1 40 c) Wharekohe si lt loam, 35 yr d) Wharekohe sandy loam - 400 0 350 In 300 en - 250 en ::l. 200 - " CI) 1 50 c:: 1 00 � 'cu - 50 CI) D:: a D.. • 4� -50 20 K 60 80 1 00 1 20 t40 e) Aponga clay 2500 - 2000 '0 In 1 500 en - en 1 000 ::l. - �M " 500 CI) • • c:: • • 'cu a - CI) 20 40 D:: -500 D.. 200 0 In 1 50 en - 1 00 en ::l. - 50 " CI) c:: 'cu 0 - � :'! CI) D:: -50 D.. -1 00 K ,. • 60 80 1 00 1 20 1 40 Time ( hours) SQ 8R 10Q 120-r0 �O P -2S0 P -.-SOO P �1000 P ___ SOOO P Figure 6,3 Influence of shaki ng time on the storage of added P by a) Wharekohe silt loam, 0 years, b) Wharekohe silt loam, 1 1 years, c) Wharekohe silt loam, 35 years, d) Wharekohe sandy loam and e) Aponga clay, 1 59 Rising pH increases the negative charge on soil surfaces reducing P sorption (Bowden et a I . , 1 980; Uehara and Gi l lman, 1 981 ; Barrow, 1 984; Naidu et a I . , 1 990a) . However, r ising pH in very acid soi ls can also increase P retention through the precipitation of exchangeable Fe and AI as AI or Fe phosphates (White and Taylor, 1 977) and amorphous AI and Fe compounds capable of both sorbing P (Amarasiri and Olsen, 1 973; White et a I . , 1 976; Robarge and Corey, 1 979; Haynes and Ludecke, 1 981 ) , and increasing surface charge (Rengasmy and Oades, 1 978; Oades, 1 979) . At higher pH levels increasing the pH above 6 can lead to increased P retention through the precipitation of calcium phosphates (Naidu et a I . , 1 990b; Condron et a I . , 1 993) . Increases in exchangeable Ca concentration wi l l i ncrease P sorption as the adsorpt ion of d ivalent cations leads to the charge on soi l surfaces becoming less negative (Ryden and Syers, 1 975; Barrow and Shaw, 1 979a; Barrow et a I . , 1 980; Stoop, 1 983; Curtin et a I . , 1 992) . Calcium, in particu lar, stimulates P sorption on oxide surfaces (Helyar et a I . , 1 976; Barrow et a I . , 1 980). Increased P sorption where Ca is the dominant exchangeable cation is supported by the higher P retention recorded when the soi ls in the present study were shaken with CaCI2 in contrast to NaCI of equivalent ionic strength (F igure 6.4) . The greater pH recorded in the NaCI shaking solutions (F igure 6.5) wi l l also have contributed to reduced P sorption. Increased exchangeable Ca also increases the precipitation of calcium phosphates (White and Taylor, 1 977; Barrow et a I . , 1 980; Sanchez and Uehara, 1 980; Naidu et a I . , 1 990b) and discourages the dissolution of existing calcium phosphate products at high pH (Mackay et a I . , 1 986) . An increase in the ionic strength of the soi l solution, induced by l iming, may also lead to increased P sorption (Ryden and Syers, 1 975; Helyar et . , 1 976). The effects of l im ing on P retention in the soil are usually smal l and dependent on the relative roles of each of the above processes and the measuring method employed. Liming of acid soi ls has been shown to both increase (Fox et a I . , 1 974; Haynes, 1 983; Holford, 1 983; Haynes and Swift, 1 985) and decrease (Lopez­ Hernandez and Burnham, 1 974; Friesen et a I . , 1 980; Smyth and Sanchez, 1 980; Haynes, 1 983; Holford et a I . , 1 994) the retention of P. Much of this variabi l ity in a) Wharekohe silt loam, 0 yr b) Wharekohe silt loam, 1 1 yr 350 - 350 '0 en 250 Cl 250 -Cl ::1 1 50 -� 1 50 CI.) .Q � 50 0 50 en a.. -50 20 40 60 80 1 00 1 20 -50 20 40 60 80 c) Wharekohe silt loam, 35 hr d ) Wharekohe sandy loam 300 '0 en 200 Cl -Cl .: 1 00 � CI.) .Q � 0 0 en a.. 0 40 - 1 00 60 80 1 00 1 20 e) Aponga clay 1 900 - 0 1 500 en Cl - 1 1 00 Cl .: � 700 CI.) .Q � 0 300 en a.. - 1 00 200 1 50 1 00 50 0 -50 - 1 00 Ca Na 0 20 40 60 80 1 00 Final P Concentration (J,lg/ml) Figure 6.4 Effect of cation species on the storage of added P by a) Wharekohe silt loam, 0 years, b) Wharekohe silt loam, 1 1 years, c) Wharekohe silt loam, 35 years, d) Wharekohe sandy loam and e) Aponga clay. Ca Na 1 00 1 20 Ca 1 00 1 20 Na 1 60 1 61 results has been attributed to the use of air dried soil compared to field moist soi ls (Haynes, 1 982) . L iming of soi ls fol lowed by air drying has been shown to decrease P retention in contrast to the increase in P retention encountered when field moist soi ls were used to investigate P retention (Haynes, 1 983). This d ifference was attributed to the crystal l isation, upon drying, of the amorphous-AI polymers which had formed in the moist soi l upon l iming as g ibbsite, and hence a reduction in P sorptive surface area (Oades, 1 979; Sims and El l is , 1 981 ; McLaughl in et a I . , 1 981 ; Haynes, 1 983). On the other hand, Baskaran et a l . ( 1 994) found an increase in P retention upon air drying in a yel low brown loam and yel low grey earth col lected from undeveloped and non­ l imed pasture, respectively. In the present study, the retention of P in both the soi ls col lected from under developed pasture, which had received l ime, was lower than that in the undeveloped soi l . Reduced P sorbing surface area due to crysta l l isation of amorphous AI compounds upon air drying is unl ikely to have had much impact in the present study due to the extremely low AI content of the Wharekohe soi l . The precipitation of A I and Fe compounds with increasing pH wi l l have contributed to increased P sorption when the soi ls were l imed during pasture development, as the pH of the undeveloped Wharekohe soi l is very low (pH 4.25) . Some of the possible i ncrease in P retention upon l iming with pasture development wi l l have been offset by the retention of appl ied P over the years in both of the developed sites. The fractionation study revealed that the effect of the more recent heavy l iming history of the 1 1 year old s ites was the greater accumulation of calcium phosphates rather than any increase in sorbed P compared to the 35 year old sites. The lack of d ifference in sorbed P between the two developed sites indicates that differences in exchangeable Ca (which decreases negative charge) may not have had a large impact on P sorption or have been offset by the high pH (which decreases negative charge) in the field. The pH of an 1 1 year old pasture had exceeded 6 in recent years encouraging the precipitation of calcium phosphates and d iscouraging the dissolution of fert i l iser P and in it ial calcium reaction products. 1 62 However, in the present P retention study, the pH of the shaking solution did not rise above 6 where P was added (F igure 6 .5) . Despite the use of CaCb as the background electrolyte, increased P sorption , due to decreasing negative charge on soi l surfaces ( induced by greater exchangeable Ca adsorption), may have played a larger role than precipitation in the higher P retention recorded in the soi l sampled from the 1 1 compared to the 35 year old pasture. Barrow ( 1 984) found that the use of 0 .01 M CaCh overwhelmed d ifferences in P retention which could have been attributed to Ca status d ifferences between the soi l treatments in his study. This did not appear to be the case in the present study where exchangeable Ca levels were much higher than those reported in Barrow's study. Greater ionic strength in the shaking solution of the developed soi ls may also have increased P sorption and some precipitation of calcium phosphates may sti l l have occurred. Whichever of the l iming effects induced the higher P retention in the soi l from the 1 1 year o ld pasture, they had a greater effect on P retention than the effect of the higher pH which would have increased negative charge and lowered P sorption. The increased P retention at higher added P concentrations encountered in the soi l from the 1 1 year old pasture after 1 36 hours is most l i kely a function of the longer shaking time combined with the effects of the higher pH of the shaking solution and exchangeable Ca content of the soi l . Prolonged shaking can lead to the breakdown of soi l part icles exposing a greater surface area for P sorption (Barrow and Shaw, 1 979b) . The higher exchangeable Ca content of the soil from the 1 1 year old pasture may have resulted in greater exchangeable Ca adsorption on the freshly exposed surfaces lead ing to a decrease in negative surface charge. Hence, the increase in P retained with prolonged shaking was greater than in the other soi ls. For the sorption of added P to have increased , the decreasing negative charge induced by the exchangeable Ca adsorption must have been greater than the increasing negative charge induced by the smal l rise in pH with continued shaking. The differences in P retention measured between the soi ls of d ifferent pastoral age in the present study were not reflected in the resu lts from the laboratory leaching study, reported in chapter 5. The amounts of P leached through the intact soi l cores 1 63 a) 1 6 Hours, 0.01 M CaCh b) 40 Hours, 0.01 M CaCI2 6 .5 6 .0 5 .5 J: 5 .0 Co 4 .5 4 .0 3 .5 0 6 .5 6 .0 5 .5 J: 5 .0 Co 4 .5 4 .0 3 .5 1 000 2000 3000 4000 5000 1 000 2000 3000 4000 5000 0 P added to shaking solution (ug/g P added to shaking solution (ug/g soil soil c) 1 36 Hours, 0.01 M CaCI2 d) 40 Hours, 0.03 M NaCI 6.5 6 .5 6.0 5.5 i 5 .0 4 .5 4 .0 o 1 000 2000 3000 4000 5000 P added to shaking solution (ug/g soil J: Co 6.0 5 .5 5 .0 4 .5 4 .0 3 .5 0 1 000 2000 3000 4000 5000 P added to shaking solution (ug/g soil --.- Aponga clay -Who sandy �Wh. silt 0 Yrs �Wh. silt 1 1 Yrs -+-Who silt 35 Yrs Figure 6.5 Effect of added P, pasture age and soil type on the final pH of the shaking solution in a) CaCI2 shaken for 1 6 hours, b) CaCI2 shaken for 40 hours, c) CaCI2 shaken for 1 36 hours, and d) NaCI shaken for 40 hours. 1 64 col lected from 1 0 and 32 year old pasture were s imi lar. The high concentration of appl ied P in the leaching study may have prevented the detection of the smal l measured d ifferences in P retention between the Wharekohe si l t loam soi ls of d ifferent pasture age. Alternatively, the added P retained by the developed soi ls in the present P retention study may be retained in a form susceptible to loss under leach ing cond itions. . These factors may also have accounted for the lack of an expected d ifference in the amount of P leached between the Wharekohe si lt and sandy loam despite large measured differences in P retention between the two soi ls and reported in 6.4 .2 . 6.4. 1 .2 Implications of Decreasing P Retention with Pasture Age on Modelling P Requirements Changing P retention, with pasture development, has important ramifications for model l ing P fert i l iser requirements, from the points of view of meeting plant P requirements, using appl ied P efficiently and min imis ing potential losses of P in runoff. Decreasing P retention with increasing pasture age is not taken into account in New Zealand models (CFAS or Outlook) used to calculate P fert i l iser requirements. Both the accumulation of P into plant unavai lable compounds in the soi l and P lost from the soi l via runoff waters contribute to the soi l loss parameters used in the models. The decrease in P retention in the Wharekohe soils, as they become saturated with P with increasing pasture age, results in an increasing proportion of appl ied P and the total soi l P loss being lost via runoff water to the wider environment (Chapters 3 and 5) . If the P surplus to soi l P accumulation and plant requirements is washed from the soi l , it is inefficient to continue applying P at a constant annual rate as pasture age increases. To reflect the decreasing P retention and to reduce the loss of appl ied P from the soi l , the soi l loss parameters used in both the CFAS and Outlook models should be reduced as the soi ls approach P saturation. Phosphorus accumulation is maxim ised at even lower levels in the top 7 .5 cm of soi l fo l lowing pasture development in another New Zealand podzol , Okarito s i l t loam 1 65 (Lee et a I . , 1 983). Hence, reducing the soi l loss parameters with increasing years pasture age and associated annual P appl ication is also l ikely to be necessary on this podzol also. In the Netherlands, an estimate of P saturation, determined as the ratio between exchangeable soi l P and the P sorption maximum, is used as a factor in models used to calculate appl ied P fert i l iser rates for free drain ing, low P retention soi ls in order to l imit P loss to runoff waters (Sharpley, 1 995). In non-calcareous soi ls, extractable soi l P and P sorption maximum are determined from oxalate extractable P, AI and Fe (Sharpley, 1 995). In the free draining soi ls in the Netherlands, a P saturation level of 25% has been identified as the critical ratio above which P appl ication should not exceed crop removal rates. In laboratory studies, Sharpley ( 1 995) used a s imi lar approach to that used i n the Netherlands to relate P saturation to P lost in surface runoff from Okalahoma soils. For each soi l , exchangeable soi l P was measured as Mehl ich-3 P and the P sorption maximum was calculated from up sorption isotherms" using the Langmuir equation (P sorption maximum = the reciprocal of the slope of the plot of the final P concentration against final P concentration/amount of P sorbed (Olsen and Watanabe, 1 957)) . Currently in the United States, runoff P losses are predicted by simple soi l tests for exchangeable P such as Mehl ich-3 P or resin-P (Gartley and Sims, 1 994) . Loadings of dissolved inorganic P in surface runoff waters have also been found to be related to other measures of soi l P status including Olsen P (see 5 .5 .6) . Sharpley ( 1 995) found that P saturation better described d ifferences in runoff P losses than either Mehl ich-3 P or Resin-P. An investigation of h istoric Olsen P levels on the youngest and oldest pastures on the Kaikohe Research station (Appendix 4.3) reveals that Olsen P does not provide a good indication of potential leaching losses from the Wharekohe silt loam. Olsen P levels varied between consecutive years and there was no trend for increasing Olsen P levels with fert i l iser appl ication rates or increasing pasture age on the youngest pasture once developed. Nor was there a consistent d ifference between the pastures of d ifferent age despite large d ifferences in potential runoff P losses as predicted from the chronosequence study in chapter 3. Likewise, Olsen P did not 1 66 provide a good indication of expected leaching losses between contrasting soi l groups (Chapter 5) . These losses were more closely related to the various chloride extracts used in the chronosequence and present P retention study (NH4CI , CaCh and NaCI) . Total P wou ld provide a better indication of the potential for P losses from the Wharekohe s i lt loam than Olsen P, but its appl ication in a fert i l iser P requirement model is restricted by variation in the maximum total P levels which would be reached with continued P appl ication between sites and soi ls. The use of soi l P fractions in predicting P saturation, and hence runoff P losses, is also restricted by variation i n maximum P levels between sites and soi ls . The inclusion of a measure of P saturation in New Zealand models that predict fert i l iser requirements is considered the most suitable indice of P status for improving P recommendations on low P retention soi ls, such as podzols. The calcu lation of P saturation for use in New Zealand models from "P sorption isotherms" using the Langmuir equation is restricted by several factors. F i rstly, the procedure is t ime consuming and a faster and cheaper method would be required. Secondly, the val id ity of the Langmuir equation to describe "P sorption isotherms" has been questioned by several authors (Bowden et a I . , 1 977; Parfitt, 1 978; Sample et a I . , 1 980; Barrow, 1 989). The sorption of P changes surface charge affecting the rate of further P sorption and P retention may not necessari ly be restricted to sorption reactions. It also impl ies an equ i l ibrium condition rarely encountered in soil P retention studies, that adsorption is reversible (some of the P is nearly a lways i rreversibly adsorbed), and that a single sorption mechanism steadi ly adsorbing P in a monolayer across the soi l surface exists (obviously not the case in soi ls ) . Other authors have suggested the use of two and three term Langmuir equations to describe the different sorption regions encountered in soi ls (Holford et a I . , 1 974; Rajan and Fox, 1 975; Ryden et a I . , 1 977a&b) and soi l constituents (Muljadi et a I . , 1 966) over a wide final P concentration range to overcome the last problem. However, these are sti l l not a suitable means for determin ing an absolute P sorption maximum or P storage capacity for a soi l . 1 67 An absolute measure of P storage capacity is not necessari ly required to predict P runoff losses. A more simple extraction method may result in an arbitrary measure of P storage capacity which can then be cal ibrated for the prediction of runoff P losses from at risk soi l groups, such as the approach used in the Netherlands where 3 elements, P, AI and Fe, are measured from a single oxalate extraction procedure and used to estimate the P storage maximum and P saturation. Phosphorus retention is closely related to oxalate extractable AI in acid soi ls includi ng Canadian podzol ic soi ls (Laverd iere and Karam, 1 984) and New Zealand podzols (Saunders, 1 965), and to the amount of a lumin ium and i ron oxyhydroxides on the surface of quartz grains in heavi ly weathered Austral ian sands, includ ing a podzol , which were prone to leaching (Diggle and Bel l , 1 984) . Therefore, the oxalate extraction wh ich extracts amorphous Fe and AI and the P retained by these Fe and AI compounds could also be used to estimate P saturation in New Zealand soi ls prone to P losses in runoff waters. When determin ing the degree of P saturation in podzols, it is important that oxalate extractable Po is also measured. In contrast to many other soi ls , Po continues to accumulate in the Wharekohe s i l t loam once inorganic P accumulation has ceased. Another approach to predicting the suscept ib i l ity of a soi l to P runoff losses would be the inclusion of a measure of the soi ls abi l ity to retain added P . The standard anion storage capacity test (previously cal led the P retention test) used in New Zealand, where 5000 jJg of P is shaken with 1 gram of soi l , is not sensitive enough to indicate changes in P retention in low P retention soi ls where P saturation is most l ikely to be encountered. The use of a lower level of added P to measure P retention, such as the P Retention Index (PRI ) used by the Austral ians (200 jJg PIg soi l ) (Bol land et a I . , 1 994), may be a more suitable measure as P retention from a low added P solution concentration wi l l change with pasture development as soi l P approaches saturation levels . Alternatively, a simple Pasture Development Index (POI ) could be incorporated into the model based on pasture age where regular topdressing of P has occurred at levels equal to or in excess of soi l P accumulation and animal production. Pasture age could be cal ibrated with runoff P losses. However, P saturation is l i kely to be 1 68 site and fert i l iser P history dependent and so a POI is un l ikely to be as sensitive an indicator of P saturation as that measured by an oxalate extraction. The main advantage of a POI is that it does not require a soi l test to gain an estimate of P saturation. Such an approach is employed to model the fate of P in the Wharekohe si lt loam in Chapter 8. As already mentioned, the abi l ity of other New Zealand soi ls to retain appl ied P may also decrease with increasing pasture age and fert i l iser appl ications. However, in contrast to podzols, appl ied P becomes less strongly retained and the continued appl ication of P surplus to production requirements is l ikely to result in an increase in the Olsen P. Hence, a low P requirement wi l l result. Decreasing P retention with pasture development may become increasingly important in New Zealand soi ls of lower P retention such as the brown grey earths, yel low grey earths, strongly leached and podzol ised yel low brown earths, recent and organiC soils in future years with time under developed pasture. The latter two soi l groups are expected to be particularly prone to P losses in runoff waters. C learly, further investigations of P retention and runoff P losses in New Zealand soi ls would be required to relate any measure of P saturation with expected runoff P losses. Only then could appropriate P saturation levels be determined for each soil and the P saturation factor be incorporated into New Zealand models. A P saturation factor would be incorporated into the models not only to reduce the quantity of P appl ied as P saturation is approached, but would also indicate when the more frequent appl ication of smal l quantities of soluble P fert i l iser or the appl ication of slowly soluble P ferti l iser, such as RPR, would be required to reduce runoff P losses and increase P fert i l iser efficiency. The model l ing of P requirements for Wharekohe podzols is d iscussed in more detai l in chapters 7 and 8. 6.4.2 Effect of Soil Weathering and Parent Material on P Retention 1 69 The Aponga clay retained substantial ly more P than either of the Wharekohe soi ls across the final P concentration range (F igure 6.6) . Such a result is not surprising given the higher Fe and AI content of the less leached yel low brown earth . The Wharekohe sandy loam retained less P than the Wharekohe s i lt loam of simi lar pasture age (F igures 6 . 1 and 6.6) . Both the Wharekohe soi ls retained substantial ly less P than other New Zealand soi ls in simi lar stud ies where 0 .01 M CaCb was used as the background e lectrolyte (Ryden and Syers, 1 975; Baskaran et a I . , 1 994) . The Wharekohe s i lt loam, which had been under pasture for 35 years, retained approximately half the amount of P retained by a Tokomaru si lt loam across a s imi lar final concentration range. Tokomaru soi ls have been recorded as loosing up to 5.63 kg P/ha/yr in surface runoff (Sharpley and Syers, 1 979a) and 1 .2 kg P/ha/yr in accelerated subsurface runoff (Turner et a I . , 1 979). The much lower retention of added P by the Wharekohe soi ls supports the potentia l for much higher P losses from the Wharekohe soi ls . The Aponga clay, on the other hand, sorbed substantial ly more P than the Tokomaru si l t loam (Baskaran et a I . , 1 994) or the sedimentary subsoi ls in the study of Ryden and Syers ( 1 975), but far less P than the al lophane conta ining volcanic soi ls in both stud ies. The finding in this study that Wharekohe soi ls retain only smal l amounts of P relates to other studies outside New Zealand. The amounts of P retained by the Wharekohe soi ls were simi lar to that retained by Austral ian podzol ic soi ls of low P retention in a s imi lar P retention study (Holford et aI . , 1 994) . The Wharekohe sandy loam in the present study retained simi lar quantities of P to those recorded from a range of P concentrations by the A horizon material from Queensland sands which leached substantial quantities of P from repacked soi l cores (O iggle and Bel l , 1 984) . An indication of the P retention figures which wou ld be expected from the standard New Zealand anion storage capacity test can be obtained from the percentage of 1 70 a) 1 6 hours 2200 - 0 1 800 C/) CI ""- 1 400 CI 2; 1 000 't:I G.I c:: 600 IU -G.I 200 D::: Q. -200 0 1 0 20 30 40 50 60 70 80 90 1 00 1 1 0 1 20 1 30 b) 40 hours - 2200 0 1 800 C/) CI 1 400 ""-CI 2; 1 000 't:I G.I c:: 600 'n; -G.I 200 D::: Q. -200 0 1 0 20 30 40 50 60 70 80 90 1 00 1 1 0 1 20 1 30 c) 1 36 hours - 2200 '0 1 800 C/) CI C, 1 400 2; 't:I 1 000 G.I c:: 600 'n; -G.I 200 D::: Q. -200 0 1 0 20 30 40 50 60 70 80 90 1 00 1 1 0 1 20 1 30 Final P Concentration (!-Ig/ml) - W h , si lt 3 5 Yrs � Wh, sandy � Aponga clay Figure 6.6 Effect of i ncreasing solution P concentration on the storage of added P from 0.01 M CaCI2 by Wharekohe silt loam, Wharekohe sandy loam and Aponga clay during a) 16 hour, b) 40 hour and c) 1 36 hour shaking periods. 1 7 1 added P retained by the soi l at 5000 jJg PIg soi l . 4 .2%, 5 .0%, 4.3% of the added P was retained by the undeveloped, 1 1 year old and 35 year old Wharekohe s i lt loam soi ls, respectively, and 1 . 3% and 28.6% by the Wharekohe sandy loam and Aponga clay in the present study confirming the extremely low P retention of the Wharekohe soi ls, in particular the sandy loam. These values are l ikely to d iffer from those which would be obtained from the standard Anion Storage Capacity test. The standard An ion Storage Capacity test involves the shaking of soi l samples at a 1 :5 soi l : solution ratio in a background solution of NaOAc-HAc buffered at pH 4.6 for 24 hours (Saunders, 1 965). A higher retention of added P is expected in the presence of CaCb whereas the higher pH of the shaking solutions in the present study would have led to increased negative charge on soi l surfaces reducing P sorption. The h igher solution to soi l ratio employed in the present study would have resulted in less soi l breakdown lowering the amount of P retained in comparison to the 5 : 1 ratio employed by the standard An ion Storage Capacity test. Soi l breakdown has been shown to be the major factor influencing d ifferences in the amounts of P sorbed by soi ls in different solution to soi l ratios (Barrow and Shaw, 1 979b). Phosphorus was only released at 0 jJg added PIg soi l from the Wharekohe s i lt loam. In contrast, P was being released at up to 500 jJg added PIg soi l in the Wharekohe sandy loam after 1 6 hours of shaking and at up to 1 000 jJg PIg soi l after 40 and 1 36 hours. The fact that P was released from the Wharekohe sandy loam at a l l rates of added P in the NaCI solutions (F igure 6.3) i l lustrates just how weakly P is held by the sandy loam. These P retention results wou ld seem to indicate that P would be more prone to leaching from the sandy loam than from the si lt loam. However, although the leaching column results presented in chapter 5 reflected the large d ifference in the P storage capacity of the Aponga clay in comparison to the Wharekohe soi ls , substantial amounts of P could be leached from intact soi l cores of both Wharekohe soi ls with l ittle d ifference in the amounts leached between soi l types. At such a large number of years from in it ial pasture development, and consequently high level of P saturation, both soi ls were expected to retain l itt le further added P. 1 72 Factors which may have influenced the differences in measured P retention between the two Wharekohe soi ls include i ) a larger surface area in the Wharekohe s i lt loam leading to greater P sorption in the artificial conditions of the present study, i i ) a lower pH in the Wharekohe si lt loam leading to a smal ler negative charge and hence greater P sorption and i i i ) higher exchangeable Ca in the Wharekohe sandy loam which should have resulted in higher P sorption as Ca was sorbed onto new surfaces fresh ly exposed by shaking. In particular, the use of considerable RPR on the Wharekohe s i lt loam soils in recent years has most l i kely reduced the amount of easi ly released P in comparison to the sandy loam where P was continual ly appl ied as soluble superphosphate and monocalcium phosphate. 6.5 CONCLUSIONS • The abi l ity of the Wharekohe si lt loam to retain added fert i l iser P decl ines with pasture development as the P retention sites become fi l led with appl ied P. The greater retention of added P by the soi l under the 1 1 year old pasture was surprising g iven that both developed soi ls appeared to have reached the same maximum total P content. This effect was attributed to the more recent heavy l iming history of the youngest developed sites. • Changing P retention with pasture development has important ramifications for model l ing P fert i l iser requirements in relation to the quantity required and effects on water qual ity. It was suggested that a P saturation factor, as used overseas, cou ld be included in New Zealand P recommendation models to reduce P appl ication to levels wh ich meet plant requirements but l im it losses in soi l water as the abi l ity of the soil to retain added P d iminishes with increasing pasture age and P appl ication. • Further research is requi red to relate any measure of P saturation to expected runoff P losses in order to determine appropriate P saturation levels for use in models to restrict P losses from at risk soi l groups such as podzols, recent gley soi ls and organic soi ls . • A P saturation factor could be incorporated into the P fert i l iser requirement models, not only to reduce the quantity of P appl ied as P saturation is approached, 1 73 but also to indicate when the more frequent appl ication of smal l quantities of soluble P fert i l iser or the appl ication of slowly soluble P fert i l iser, such as RPR, would be required to reduce runoff P losses and increase P fert i l iser efficiency. • The Aponga clay, with its higher Fe and AI content, retained substantial ly more P than both Wharekohe soi ls . • The Wharekohe s i lt loam retained more P than the Wharekohe sandy loam which released P to the shaking solution at quite high rates of added P, especia l ly when the background electrolyte was NaCI . The use of RPR on the Wharekohe si l t loam in comparison to the soluble P appl ied to the sandy loam may have led to the release of a higher amount of P to the shaking solutions from the latter. • The far greater P retention recorded in the Aponga clay in comparison to the Wharekohe soi ls was reflected by the substantia l ly higher quantity of P leached from the latter and recorded in chapter 5 . However, the smaller d ifferences in P retention between the Wharekohe s i lt loams of different pasture age and the Wharekohe sandy loam were not reflected in differences in P leaching in the study presented in chapter 5 . CHAPTER 7 QUANTI FYING THE SLF FOR WHAREKOHE SOILS 7 . 1 INTRODUCTION 1 74 Olsen P levels have been found to fal l on some Northland paddocks contain ing Wharekohe podzol soi ls, despite annual P inputs being higher than the estimated maintenance rates calcu lated by the CFAS model (Chapter 2) . In contrast, Olsen P levels increased on at least one adjacent paddock contain ing volcan ic soi l . It was concluded that the SLF (0.4) for the podzol used in the CFAS model underestimates the soi l P loss on the h ighly-weathered, low P retention, Wharekohe podzols. The maintenance P rates for any level of pasture production, calculated by the CFAS model , are sensitive to change in the SLF. For example, a change i n the SLF from 0.25 to 0.4 would result in a change in the calculated P requirement to maintain 1 5.6 stock units, on the Kaikohe Research station, from 1 6 to 2 1 kg P/ha, a 32% increase. The use of an SLF which is too low for a site can lead to a loss in production, whi le overestimating the soi l loss parameter, and consequently the P requirement, is l ikely to lead to large losses of appl ied P in soil water from the low P retention Wharekohe soi ls. From both an economic and environmental view point, it is important that the SLF can rel iably predict potential soi l P losses on the Wharekohe podzols. Soi l P is lost from the cycl ing P pool as P accumulates in the root zone in non-labi le forms (such as strongly adsorbed and absorbed P, Fe-, AI- and Ca-P precipitates and stable organic P compounds), or is lost from the root zone in runoff waters. Both these types of soi l P loss comprise the soi l loss factor in the CFAS model . As the Wharekohe topsoi ls are low in Fe and AI, the accumulation of strongly sorbed P and Fe- and AI-P precipitates is lower than in less weathered soils. Therefore, the component of the SLF which can be attributed to soil P accumulation (SLFsPA) is expected to be lower on the podzols than on the less weathered sedimentary soi ls . However, the potentia l for runoff P losses is much h igher from the podzols (chapter 5) . 1 75 It has general ly been assumed that P losses in runoff from New Zealand soi ls are smal l and, although they may impact on the environment, are of l ittle on farm economic productive sign ificance (McCol l et a I . , 1 977; 8argh, 1 978; Lee et a I . , 1 979; McCol l and Gibson, 1 979a&b; Sharpley and Syers, 1 979a; C lose and Woods, 1 986; Sharpley and Syers, 1 983; Lambert et a I . , 1 985; Cooper and Thomsen, 1 988). S ituations where the soi l P loss is l ikely to be dominated by losses of P in runoff have not been fu l ly considered in the CFAS model . In these situations, the concept of using a constant SLF regardless of pasture age in determining P fert i l i ser requirements is inappropriate if non-labi le soi l P accumulation decreases with an increase in pasture age. As less P accumulates in non-labi le forms with increasing pasture age on the Wharekohe s i lt loam (Chapter 4), the amount of P required to maintain a part icular pasture production level decreases, provided the fert i l i ser P strategy can maintain an adequate avai lable P pool over the year. Consequently, the SLF shou ld decrease with increasing pasture age. Much of the P appl ied surplus to animal losses and soi l P accumulation in the root zone is lost from the soi l via runoff. A smal l quantity is l i kely to accumulate in the soi l profi le below the rooting zone. If P is appl ied annual ly at the same maintenance rate, despite increasing pasture age and decreasing SLFsPA, runoff P losses wi l l i ncrease with pasture age. Some runoff P loss is unavoidable under present P fert i l iser appl ication strategies. Even where the SLF and P appl ication rates are reduced with pasture age, runoff P loss wi l l sti l l contribute to the SLF constitut ing a greater proport ion of the SLF as soi l P accumulation decreases. Phosphorus has been demonstrated to be less strongly retained in the Wharekohe sandy loam compared to the Wharekohe s i lt loam (Chapters 4 and 6) . Local farmer experience has indicated the possib i l ity of h igher P requirements to maintain s imi lar levels of pasture production on the sandy loam podzols compared to the silt loam podzols. Consequently the SLF may differ between these two Wharekohe soi ls of differing parent materia l . The use of slowly soluble P fert i l isers have been shown to reduce runoff P losses on soi ls prone to P leaching (Ozanne et aI . , 1 961 ; Hogg and Cooper, 1 964; G i l lman, 1 973; McSweeney and Mul ler, 1 979). Therefore, the SLF and consequently the 1 76 calculated P requirements may be less under P ferti l isers of lower solubi l ity, such as RPR, on the Wharekohe podzols. The SLF values were orig inal ly determined from pasture P response trials by div id ing annual soi l P loss, calculated from the rate of P required to maintain a steady Olsen P level m inus simulated animal P loss, by pasture P uptake at 90% of maximum yield for each trial site (Cornforth and S inclair, 1 984). The in it ial fert i l ity of trial s ites used to determine the SLF values does not appear to have been considered as a factor which may influence calculated SLF values. The in it ial soi l nutrient fert i l i ty status of the tria l sites used to determine SLF values for each soi l group may have influenced yield response to appl ied P, P uptake, animal P loss and consequently the calculated SLF value. A soi l loss parameter is also used in the new Outlook model to calculate maintenance P requ i rements (Metherel l et a I . , 1 995) . The Outlook soi l loss parameter represents losses of P from the cycl ing P pool due to non-labi le P accumulation in the root zone and losses of P from the root zone in runoff water. This soi l loss parameter is a constant proportion of P which wi l l be lost from the labi le P pool for each of four categories of soi l P loss (as opposed to the proportion of pasture P uptake which wi l l be lost from the soi l in the CFAS model) . The Outlook soil loss parameters were also determined from pasture P response trials. The factors wh ich effect the calculation of CFAS SLF values, and therefore P requirements, which have been described in this introduction, wi l l a lso impact on the Outlook model 's soi l P loss parameter and consequently calculated P requirements. 7.2 OBJ ECTIVES The objective of the study reported in this chapter was to determine if the SLF (0.4) assigned to podzols, in the CFAS model , was appropriate for calculating the P fert i l iser requirements of the Wharekohe soi ls by examining the effect of: i ) soi l fert i l ity status on the SLF for the Wharekohe si lt loam. i i ) a d ifference in parent material (s i lt vs sandy loam) on the SLF for Wharekohe soi ls . i i i ) the degree of soi l weathering (YBE vs Podzol ) on the SLF . 1 77 iv) fert i l iser solubi l ity (soluble P in MCP vs slowly soluble P in P RPR) on the SLF on the Wharekohe si lt loam. v) pasture age on the SLF for the Wharekohe si l t loam. 7.3 MATERIALS AN D M ETHODS The SLF was determined by two methods in the present study. The first method uti l ised four smal l-plot field trials to determine the SLF associated with the use of a soluble P fert i l iser for Wharekohe soi ls varying in fert i l ity and the physical nature of the parent materia l . The degree of soi l weathering was assessed by comparing SLF values for the Wharekohe si lt loam and the yel low brown earth, Aponga clay. The effect of the solubi l ity of P fert i l iser (MCP vs SPR) on the SLF was also investigated at one site. The second method uti l ised data presented in the chronosequence study (Chapters 3 and 4) to calculate the SLFsPA for various periods of pasture development on the Wharekohe s i lt loam. 7.3.1 Small-Plot Field Trials 7.3. 1. 1 Trial Sites The smal l-plot field trial s ites were selected on a Wharekohe si l t loam with a history of continuous P fert i l iser appl ication (Ft), a Wharekohe si l t loam which had not received P fert i l iser for the previous three and a half years (NFt) , a Wharekohe sandy loam, and an Aponga clay. S ites were selected on the basis that they had good qual ity pasture and, with the exception of the NFt site, Olsen P levels considered optimum for maintain ing pasture production at 90% of Ymax, 20-25 on Northland sedimentary soi ls (Edmeades et a I . , 1 991 a) . None of the sites had received any fert i l iser in 1 991 prior to becoming tr ial sites. The two Wharekohe s i lt loam sites were located on the Kaikohe AgResearch 1 78 Station. Both had been under developed pasture for 31 years at the trials' commencement and had simi lar P fert i l iser histories up until 1 988, which i ncluded Longl ife superphosphate from 1 986. The Ft site was in the same paddock and adjacent to the 30 year old pasture site sampled in 1 990 in the chronosequence study (Chapters 3 and 4) . The NFt site was 200 m up slope in a separate paddock. The Wharekohe sandy loam site was located on Rakawahia Road, near Taheke, 9 km west of Kaikohe and the Aponga clay site was located on the Hupara Road near Moerewa, 1 8 km east of Kaikohe. Both of these latter s ites had been under pasture for at least 30 years but thei r fert i l iser histories were unknown. At each field trial site a uniform area, with respect to the physical surface and pasture composition (ryegrass/white-clover) was selected. Some paspalum « 5% of total grass growth) was present on the Aponga clay site . Some characteristics of each trial site are out l ined in Table 7. 1 . Table 7. 1 Soi l properties and slope at each smal l-p lot fie ld tria l s ite, samples col lected in May 1 991 (pH and anion storage capacity) and in late July 1 991 (Olsen P) . Site O lsen P pH Anion Storage Slope (MAF in H2O Capacity (%) Quicktest) Aponga clay 23 6 .0 54 7° Wharekohe sandy loam 24 5 .9 7 < 1 ° Wharekohe si l t loam NFt 1 8 5 .7 Not Determined 9° Wharekohe si l t loam Ft 22 5 .8 9 9° 7.3. 1 .2 Trial Design and Establishment The field tria ls were designed to run for two years. Fert i l iser P was appl ied at each trial site in late July 1 991 and August 1 992. Phosphorous was appl ied at a l l 4 field 1 79 sites as monocalcium phosphate (MCP), the main form of P in superphosphate, at the rates outl i ned below: Site MCP Application rates ( kg P/ha) Aponga clay 0 20 30 40 50 60 80 1 00 Wharekohe sandy loam 0 20 30 40 50 60 70 80 1 00 Wharekohe si lt loam NFt 0 20 30 40 50 60 70 80 1 00 Wharekohe si l t loam Ft 0 20 30 40 50 60 70 80 1 00 To investigate the effect of P solubi l ity on the SLF, Sechura phosphate rock (SPR) was appl ied at 20, 40, 60, 80 and 1 00 kg P/ha on the Wharekohe si l t loam Ft s ite. Each rate of P was repl icated 4 times in a randomised complete b lock design (F igure 7 . 1 ) . Fert i l iser was appl ied to plots 3 .86 m long and 2 .75 m wide at the Wharekohe si l t loam NFt, Wharekohe sandy loam and Aponga clay sites and 2 .42 m wide at the Wharekohe si l t loam Ft s ite. Pasture and soi l sampl ing was restricted to a 3 x 2 m area within each plot. All plots received a basal fert i l iser (Table 7 .2) in 1 991 and 1 992 at both P appl ication times. Additional dressings of potassium and sulphur, as potassium sulphate at 62 .5 kg/ha, were appl ied in late spring and autumn in the first year and in the summer of the second year. Animals were excluded from the field trial sites. The tria l areas were mown to 3 .5 cm above ground level at the commencement of the tria ls before fert i l iser treatments were applied. Fol lowing the trials' commencement, when P fert i l iser was appl ied in 1 991 , pasture growth was measured at intervals determined by growth rates. At each harvest, each plot was mowed to 3 .5 cm above ground level and 46% of the pasture cl ippings (herbage samples and discarded cl ippings) removed to represent animal loss�1 ) The remaining 54% of cl ippings were returned to each plot. In it ial ly the pasture cl ippings were returned evenly to half the plot to avoid the accidental sampl ing of ( 1 ) Calculated to represent animal P loss on a dairy farm with a stocking rate of 1 6 S U on the basis of predicted herbage P concentrations of 0. 35% and an average �nnual dry matter ?roduction of 9000 kg. a) Aponga clay site 1 0 1 30 1 80 1 40 1 20 1 1 00 1 50 1 60 1 1 b ff t · m u er s np 1 00 50 40 0 80 60 30 20 1 20 1 60 1 80 1 50 1 40 1 0 1 1 00 1 30 1 1 40 1 80 1 30 1 50 1 20 1 0 1 1 00 1 60 1 b) Wharekohe sandy loam 1 0 1 70 1 1 00 1 50 1 80 1 60 1 40 1 30 1 20 1 1 b ff t · m u er s np 50 20 1 00 40 60 70 80 0 30 1 30 1 50 1 1 00 1 80 1 20 1 70 1 40 1 0 1 60 1 1 30 1 80 1 0 1 1 00 1 70 1 20 1 60 1 50 1 40 1 Figure 7 . 1 Layout of smal l-plot field trials. 1 80 (F ig 7 . 1 ) c) Wharekohe si l t loam Ft 30 0 1 00 50 40 1 00 60 40 80 80 70 20 20 60 MCP MCP MCP SPR SPR SPR MCP SPR MCP MCP SPR MCP MCP 1 m buffer strip 20 80 1 00 80 60 50 60 0 70 1 00 30 20 40 40 MCP MCP MCP SPR SPR MCP MCP MCP SPR MCP SPR SPR MCP 40 1 00 60 60 80 1 00 70 30 20 50 40 20 0 80 SPR MCP MCP SPR SPR SPR MCP MCP MCP SPR MCP MCP MCP 1 00 80 70 40 50 1 00 30 20 60 20 0 60 80 40 SPR SPR MCP SPR MCP MCP MCP MCP MCP SPR SPR MCP MCP d) Wharekohe si l loam NFt 1 30 1 20 1 1 00 1 60 1 70 1 80 1 40 1 0 1 50 1 1 b ff t ' m u er s np 50 0 1 00 70 40 20 60 30 80 1 0 1 1 00 1 40 1 80 1 60 1 70 1 30 1 50 1 20 1 1 0 1 20 1 50 1 30 1 70 1 40 1 1 00 1 60 1 80 1 1 81 1 82 Table 7 .2 Basal ferti l iser appl ied to each plot at each appl ication t ime. Fert i l iser: Rate (ha-' ) Elemental Sulphur 25 kg Calcium sulphate 90.5 kg Muriate of Potash 80 kg Magnesium Sulphate 50 kg Borax 1 5 kg Zinc sulphate 1 0 kg Copper sulphate 5 kg Cobalt 1 00 g Sodium molybdate 50 g dead pasture in the fol lowing harvest. However, the pasture beneath the returned cl ippings grew at a greater rate in the fol lowing period, which may have been due, at least in part, to a possible P response (detected in Autumn 1 992 by herbage P analysis) from P released upon the decay of the returned cl ippings and would probably have resulted in an underestimation of P uptake. Hence, from harvest 5, in late Autumn 1 992 , the cl ippings were returned evenly to the whole plot areas. Shading (mulch effect) and the return of other nutrients may also have influenced greater pasture growth under the returned cl ippings. No d ifferences in pasture N status was detected. 7.3. 1.3 Pasture Sampling and Analysis Pasture samples were col lected from each plot for the two years of the tria l . There was l itt le dead pasture present in the pasture samples taken at each harvest. Pasture dry matter yield samples were col lected with a rotary mower (to 3 .5 cm above ground level ) cutting a single strip 0.46 cm wide down the 3 m length of each plot, at intervals determined by pasture growth. Sub-samples were taken, weighed and dried overnight at 65°C before weighing, to assess dry matter content. 1 83 Herbage N and P analysis samples were col lected at each harvest with hand shears at mower height in a strip paral le l to the pasture yield samples. Al l samples were oven dried at 65°C and stored in a irtight plastic bags. Chemica l analysis was restricted to samples col lected at the rates of appl ied P closest to the rate of P estimated to maintain a steady Olsen or Resin P test value. Herbage N and P concentrations were determined as described in 3 .3 . 5 .3. Botanical composition samples were col lected in a s imi lar manner to herbage N and P analysis samples at five (six for the Wharekohe sandy loam site) of the pasture sampling times. Botanical composition was determined as described in 3 .3 .5 .2 . Statistical Analysis Pasture dry matter yield and botanical composition were subjected to an analysis of variance using Genstat to determine if effects of rate and solubi l ity of appl ied P were significant. The data was transformed (angular transformation) where appropriate. The analysis of variance was adjusted for covariates (position in block) on the Wharekohe si l t loam Ft s ite to take into account the variation i n the site from one side to the other across the slope (one side was noticeably wetter in winter). 7.3. 1.4 Soil Sampling and Analysis Soil Sampling Soi l samples were col lected prior to the appl ication of fert i l iser (July 1 991 ) , i n early February 1 992, late July 1 992, early February 1 993 and late July 1 993. 20 soi l cores (2 . 5 cm diameter by 7 .5 cm depth) were removed from each plot and bulked at each sampl ing. The soi l samples were then air dried and sieved to < 2 mm. Soi ls were stored at room temperature in airtight plastic bags unti l analysis at the end of 1 993. 1 84 Soil Analysis Olsen P was determined for each sample as described in 3. 3 .4.2 . P extractable by a dual resin system (Resin P (Saggar et a I . , 1 990a)) , incorporating both cation (CER) and anion (AER) exchange resin strips, was determined for the samples col lected from plots on the Wharekohe si l t loam Ft site where 0 P and SPR were applied. One gram samples of soi l were shaken in 30 ml of deion ised water contain ing CER and AER strips for 1 6 hours. The AER strips were rinsed in deionised water and the P retained determined colorimetrical ly after shaking them di rectly i n the Murphy and Ri ley ( 1 962) reagent. Statistical Analysis At each site, the relationship between the change in Olsen P with the rate of appl ied MCP and SPR was determined for each sampl ing period using the 8aysian smoothing program, Flexi (Graphs presented in Appendix 7 . 1 ) . At the Wharekohe si lt loam Ft site, the relationsh ip between the change in Resin P with the rate of appl ied SPR was also determined for each sampling period. The dotted l ines represent the 95% confidence intervals for each relationship. Flexi estimated the rate of appl ied P for which there was no change in Olsen P or Resin P, and the standard deviation of this estimate, for each period, at each site. 7.3. 1.5 Calculation of the SLF The soi l loss factor at each site was calculated for each soi l sampl ing period, July 91 -92, Feb 92-93, July 92-93 and July 91 -93 as: SLF = Where: (Appl ied P - Simulated Animal P Loss) = Soil P Loss P Uptake P U ptake Applied P is the annual rate of appl ied P (kg/ha) estimated to maintain a steady Olsen P soi l test for each period, 1 85 P Uptake is the amount of P (kg/ha) taken up by pasture annual ly at the rate of appl ied P closest to that which maintained a steady Olsen P during each period (%P x Dry Matter Yield) , Simulated Animal P Loss was the amount of P (kg/ha) which was lost via cl ipping removal annual ly during each period (0.46 x P Uptake) . Soil P Loss includes P lost from the cycl ing P pool due to the accumulation of non­ labi le P in the soil and the loss of P from the rooting zone in runoff waters (kg/ha) . 7.3.2 Chronosequence Trial In order to determine the effect of pasture age on the component of the SLF which is due to the accumulation of non-labi le P in the rooting zone (SLFsPA) , data presented in the chronosequence study (Chapter 4) was used. The SLFsPA for various periods since pasture development was calculated as: . SLFsPA = Unavailable P Accumulation P Uptake Where: SLFsPA is the component of the SLF which can be attributed to the accumulation of non-labi le P in the rooting zone, Unavailable P Accumulation is the annual accumulation/depletion of P in the top 7 .5 cm into P fractions considered to be unavai lable to plants (kg P/ha) for each period. P Uptake is the annual P uptake (kg P/ha) for each period. Changes in unavai lable P for various periods were calculated from the soi l P fractionation results obtained for the 1 990 orig inal s ites, the 1 993 additional s ites and changes in the three year period from 1 990-1 993 for the orig inal sites. Two versions of unavai lable P accumulation/depletion were calculated: i ) change in NaOH Pi , H2S04 Pi and Residual P i & Po (Residual P i and Po included hot HCI P i & Po where measured separately) , 1 86 and i i ) change in NaOH Pi & Po, H2S04 Pi and Residual P i & Po. P uptake was determined as the mean annual P uptake of the developed sites over the 2 year monitoring period on al l chronosequence s ites for the SLF values calculated from the original 1 990 and additional 1 993 sites. The annual P uptake appropriate to each site was used to ca lculate the SLFsPA over the three year period on the orig inal sites. 7.4 RESU LTS AN D DISCUSSION The primary aim of the present study was to quantify the SLF for the Wharekohe podzols and the effect of various factors l isted in 7 .2 on this SLF. However, in order to evaluate the SLF results, an understanding of pasture P response and the effect of appl ied P on the soi l P test values is required. Therefore the resu lts are presented and d iscussed in 3 main sections investigating: a) the effect of appl ied P on pasture growth response in the smal l p lot fie ld tria l , b) the effect of appl ied P on soi l P test values in the smal l-plot field tria l , c) the SLF values calculated from the smal l-plot field trial and the SLFsPA values calculated from the chronosequence study. 7.4. 1 Effect of Applied P on Pasture G rowth Pasture growth was monitored in the present smal l plot field tria l to determine P uptake at 90% of maximum yield for use in calculat ing the CFAS SLF. L imited pasture response to appl ied P on each site (Figure 7 .2) prevented the accurate determination of P uptake at 90% maximum yield. 7.4. 1. 1 Effect of Applied MCP on Pasture Growth The existing pool of avai lable P was sufficient to maintain maximum pasture production in the first year on a l l sites (Figure 7 .2 , Appendix 7.2a) . An exception was at the Wharekohe si l t loam NFt site where in the autumn there was a significant 1 87 a) Wharekohe silt loam N Ft 1 4000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ii 1 2000 . . . r. . .r:. - � C 1 0000 C) oX: - S 8000 :w u -5 6000 e a.. � 4000 ::J - In � 2000 o o . ·I - - ... .. .. ... ... ... .. .. .. .. .. .. .. .. .. _ - - - - .. .. .. .. ... .. - - - - - - - _ .. .. _ ... .. ... ... - - 1 0 20 -&- Year 1 (1 991 /92) � Year 2 (1 992/93) 30 40 50 60 70 Applied P (kg/ha) 80 90 1 00 1 1 0 b) Wharekohe si lt loam Ft 1 4000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - - - . . . . . . . . . . . . . . . . . . . . Ci 1 2000 .r:. - � C 1 0000 C) oX: 8000 6000 4000 2000 o o I . . . . . . . 1 . . .. . . .. .. .. .. .. ... .. ... ... _ - ... .. .. .. .. .. .. .. .. .. .. .. - _ .. .. .. .. .. .. .. _ - - - - _ .. - 1 0 20 -&- Year 1 (1 991 /92) � Year 2 (1 992/93) 30 40 50 60 70 Applied P (kg/ha) 80 90 1 00 1 1 0 Figure 7.2 Effect of appl ied MCP on annual pasture yield for years 1 and 2 for a) Wharkekohe silt loam N Ft, b) Wharekohe silt loam Ft, c) Wharekohe sandy loam and d) Aponga clay. (vertical bars=S.E.D.) 1 88 c) Wharekohe sandy loam 1 4000 Ii 1 2000 .s::: - � C 1 0000 C) � 8000 6000 4000 2000 o o - - - - - - - - - - - - - - - - - - - - - - - - - . � Year 1 (1 991 /92) � Year 2 (1 992/93) 1 0 20 30 40 50 60 70 Applied P (kg/ha) 8 0 I I 90 1 00 1 1 0 d ) Aponga clay 1 4000 Ii 1 2000 .s::: - � C 1 0000 C) � 8000 6000 4000 2000 o o I .. .. .. ... .. .. .. .. .. .. .. .. ... ... ... .. .. .. .. .. _ .. .. .. .. . . .. .. .. .. .. .. ... .. ... ... .. ... ... .. .. - - 1 0 20 30 � Year 1 (1 99 1 /92) � Year 2 (1 992/93) 40 50 60 70 Appli ed P (kg/ha) 80 90 1 00 1 1 0 1 89 l inear trend for increasing % clover with increasing P (P<0.05) (Appendix 7.2b) and P fert i l i sed p lots grew significantly more pasture than the controls (P< 0 .01 ) (Appendix 7 .2a) . The subsequent l ikely higher N input to the pasture cycle on the NFt site in the fol lowing spring after the return of the autumn cl ippings high in clover with increasing P appl ication, led to a significant trend for increasing % grass and an associated depression in % clover with increasing P appl ication rate (P<0.05) . Pasture production was substant ial ly higher in the second year of the tria l on the two Wharekohe s i lt loam and Aponga clay sites. The higher rainfa l l recorded in the late spring and autumn of the second year (Appendix 7 .3) , in comparison to the first year, was the most l ikely reason for the substantial ly higher growth rates during those periods on the three sites (Appendix 7.2a) . The in it ial sampl ing procedure, where cl ipp ings were only returned to half the area of each plot at each harvest, may also have underestimated pasture production in the first year. In contrast, pasture growth was sl ightly lower in the second year of the trial on the Wharekohe sandy loam. Factors which may have contributed to the lower pasture growth recorded in the second year include h igher losses of nutrients from this flat, poorly drained site when the fert i l iser was appl ied in the second wetter winter and during the extremely wet autumn at the end of the second year and prolonged waterlogging. Total pasture dry matter production (F igure 7 .2a&b)on the unfert i l ised plots on both the Wharekohe si l t loam sites was significantly lower than the fert i l ised plots for the second year (Ft (P<0.05) and NFt (P<0.001 ) ) . However, there was no significant d ifference in dry matter yield between rates of appl ied P. In the second year of the smal l plot field tria l , the unfert i l i sed plots on the Wharekohe s i lt loam NFt site grew consistently less pasture than the fert i l ised plots from January 1 993, but the d ifference was only significant (P<0.01 ) in the two autumn harvests (Appendix 7 .2a) . The fert i l ised plots also grew more pasture than the unfert i l ised plots from October 1 992 on the Wharekohe si l t loam Ft site with differences significant in the October and Apri l harvests (P<0.05) . Reductions in pasture production, due to with-holding fert i l iser P , were also highest i n spring and autumn in a series of North land field trials on a range of soi l types including a podzol ised yel low brown earth (P . Shannon, pers. comm. ) . 1 90 Due to fert i l iser history, pasture had been expected to respond to higher rates of appl ied P (>20 kg P/ha) on the NFt site in comparison to the Ft site. Whi le the Wharekohe s i lt loam NFt site had not received any P fert i l iser for 3 and a half years prior to tria l commencement, its Olsen P level was only sl ightly lower than the Ft site, 1 8 compared to 22, although th is d ifference was significant ( P<0.001 ) . Dry matter yields were much lower on the NFt site for each rate of applied P (Figure 7 .2a&b) . Factors other than Olsen P levels, such as N deficiency and depressed legume growth, are also l ikely to have l imited pasture growth at the NFt site, especial ly in the first year. At 20 kg P/ha P uptake was lower in the fi rst year on the NFt site than on the Ft site wh ich wi l l increase the calculated SLF as described in 7 .4 .3 . 1 . Differences in pasture response to appl ied P may have become more apparent if the trial had continued beyond 2 years as the existing avai lable P pool on the control plots became depleted . Alternatively, differences in pasture response to appl ied P may also have been more apparent at rates of applied P below 20 kg P/ha during the two year trial period. In the long term, both trial sites wou ld be expected to behave simi larly to ferti l iser P application, as the fert i l ity of the NFt s ite is raised under h igher rates of appl ied P and the pool of avai lable P becomes depleted on the Ft site at rates of P insufficient to maintain pasture production. A s imi lar pattern of pasture response to appl ied P, as was encountered in the present tria l , was also recorded in a trial on a South Is land Addison g ley podzol which had been under pasture for 9 years (Morton and Quin , 1 980). No significant response to appl ied P was detected in early stages of the first year. The lack of pasture response to appl ied P was attributed to the residual effect of previous P appl ications, as was probably occurring in the present smal l-plot field tria l . In the second year of the South Island tria l , there was a significant response to appl ied P, although there was no significant d ifference between rates. However, by the third year of the South Island tria l , dry matter yield increased significantly with i ncreasing appl ied P. No response to appl ied P was measured on the Wharekohe sandy loam in either year (Figure 7 .2c, Appendix 7 .2) . The existing pool of avai lable P may have been 1 91 sufficient to maintain pasture production in the sandy loam for the first two years of the tria l . There was a significant response to appl ied P on the Aponga clay site in late Autumn 1 993 (Appendix 7.2a) . The mean dry matter production values in years 1 and 2 were lower on the unfert i l ised plots, with a t-test reveal ing that the d ifference between the unfert i l ised plots and the pooled fert i l ised plots was significantly different (P<0.01 ) in the second year (Figure 7.2d) . The O lsen P levels in a l l treatments were above the minimum of 20 required to maintain 90% of maximum yield on sedimentary soi ls (Edmeades et a I . , 1 991 a) at the end of the tria l . Therefore, avai lable soi l P would most l ikely have buffered against any pasture response to increasing P appl ication and the trial would have had to continue for a longer period to establ ish the effect of increasing P appl ication rates on pasture growth and if the degree of soi l weathering effects d ifferences in the SLF and pasture response to appl ied P. L imited P response has also been noted in the early years of P response trials on a range of other New Zealand soi ls i ncluding podzols and yel low brown earths (S inclair et a I . , 1 994) . Clearly the present smal l-plot field trials would need to have been continued beyond two years to determine P uptake corresponding to 90% of Ymax as required in the calcu lation of the CFAS SLF. Even in trials of longer duration than the present study, large variations in pasture response to appl ied P and soi l P test values at individual sites, both between seasons with in a year, and between years, have been reported (Saunders et a I . , 1 987a; Bol land and Gi lkes, 1 992; S inclair et a I . , 1 994), making it difficult to relate pasture yield to O lsen P or appl ied P. Pool ing data from trials on s imi lar soi ls can help to overcome this variabi l ity so that estimates of yield parameters can be obtained for use in P fert i l iser requirement models (S inclair et a I . , 1 994) . However, pool ing data i s l ikely to be less precise in relating to certain specific sites. 1 92 7.4. 1 .2 Effect of P Fertiliser Solubility on Pasture Growth Except for one harvest in year 1 (harvest 5 ) when the combined MCP rate treatments produced more pasture (P<0.05) growth than the combined SPR treatments (F igure 7 .3 , Appendix 7 .2a) , there was no detectable effect of P fert i l iser form on pasture growth at the Wharekohe s i lt loam Ft site in the two years of measurement. The SPR treated plots had a higher percentage weed content in early spring 1 992 (harvest 7 ) than the MCP plots (Appendix 7.2b) . However, these effects d id not persist. 1 4000 1 2000 - tIS =a, 1 0000 � - 8000 6000 4000 2000 • • �" I . . . . . . . . . . . . . . . . . . . . � . . . . . . . . . . . . . . . . . . . . . . . . . . . . . � . . . . �� .� .I . . -e-Year 1 (1 991 /92) MCP �Year 1 (1 99 1 /92) SPR . • • • • • • . . • . . • . • • • • • � Year 2 (1 992/93) MCP . . • • . . - . - - - - - - Year 2 (1 992/92) SPR o 10 20 30 40 50 60 70 80 90 1 00 1 1 0 Applied P (kg/ha) Figure 7 .3 Effect of fert i l iser solubi l ity on annual pasture yield for years 1 and 2 for Wharekohe si lt loam Ft, adjusted for covariates (OP not included) . (Vertical bars=S.E . D . ) . Although RPR has also been found to be as agronomica l ly effective as soluble P on a range of New Zealand soi ls under pasture (Karlovsky, 1 958; Mackay et a I . , 1 984; Gregg et a I . , 1 988; Mackay and Wewala, 1 990), several other New Zealand studies 1 93 have found the in it ial pasture growth to be slower with RPR appl ication, compared to soluble P , due to the slow dissolution of the former (Grigg and Crouchley, 1 980; S inclair et aI . , 1 990b; Rajan and Watkinson, 1 992). Edmeades et al ( 1 991 b) noted that the lag effect of RPR dissolution on pasture growth "may or may not be observed by the farmer, or measured in an experiment, depending on the in it ial P status of the site . " Ledgard and Jones ( 1 991 ) found that the relative response of pasture and ewe l ive-weights to RPR was h igher at an Olsen of 1 1 than at an Olsen of 8 on a h i l l country yel low brown loam site. The in it ial Olsen P level of 22 on the Wharekohe s i lt loam Ft s ite was with in the optimum range for pasture production and although there was a smal l response to added P in the second year, the high P status of the soi l is most l ikely to have masked any response to P form. It is also possible that d ifferences in dry matter yield response between MCP and SPR may have been detected had rates of appl ied P below 20 kg P/ha been included in the smal l-plot field tria ls . Although the present trial was inconclusive with regard to pasture response to the solubi l ity of P fert i l iser on the Wharekohe si lt loam, it is possible that the use of low solubi l ity P fert i l iser may reduce runoff P losses and hence soi l loss parameter values as d iscussed further in chapters 5 and 8. S ince the pasture response to appl ied P was l im ited at each site, the calculation of P uptake at 90% Ymax was not possible. Pasture P uptake measured at the P appl ication rate closest to that determined to maintain a constant Olsen P level was used to calcu late SLF values. Summary of section 7.4. 1 : • Pasture response to appl ied P was l imited during the 2 year tria l period. The existing pool of avai lable P appeared sufficient to maintain pasture production in the first year of the trial at a l l sites, except on the Wharekohe si lt loam NFt site in autumn where appl ied P stimulated clover growth leading to a significant increase in dry mater yield. The subsequent l ikely higher N input to the pasture cycle under appl ied P stimulated grass growth in the fol lowing spring on the NFt site, depressing 1 94 clover growth. In the second year of the trials, dry matter yield on both the Wharekohe s i lt loam sites and Aponga clay site responded to appl ied P but no d ifference in pasture growth between the rates of appl ied P could be detected. There was no response to appl ied P on the Wharekohe sandy loam site in either of the 2 years . • There was no sustained d ifference in pasture response between appl ied SPR and MCP on the Wharekohe s i lt loam Ft site. The high P status of the Ft s ite and l imited P response may have masked any possible response to P solubi l ity. • As P uptake at 90% Ymax could not be accurately assessed, pasture P uptake measured at the rate of P closest to that determined to maintain a constant Olsen P level was used to calculate the SLF values for each period. 7.4.2 Effect of Applied P on Available P Soil Tests 7.4.2. 1 Effect of MCP Application Rate on Maintaining Olsen P Levels The effect of appl ied MCP on Olsen P levels over the two years in the smal l-plot field trials are presented in Figure 7.4 . A marked d ifference in Olsen P levels were found between years 1 and 2 on the podzols. Compared to year 1 , when increasing P appl ication led to increasing Olsen P levels, Olsen P levels fel l dramatical ly in the second year of the trial at a l l rates of appl ied P. On poorly buffered soi ls , such as the Wharekohe podzols, where the accumulation of P is l im ited and runoff P losses are large, the temporal variation would be expected to be large from year to year, compared to a wel l buffered soi l , l ike the Aponga clay. The amount of P, appl ied as MCP, calculated to maintain a steady Olsen for each time period are presented in Table 7 .3 . (The Flexi graphs used to calculate the rate of P at which the change in Olsen P or Resin P would have been 0 for each site and period are in Appendix 7 . 1 ) . a) Wharekohe si lt loam N Ft 27 24 - - I/J Q) -� 21 (J ::::I a LL 1 8 « :!: - Q. c:::: 1 5 Q) I/J 0 1 2 9 7/91 2/92 7/92 2/93 7/93 Sampling Date b) Wharekohe silt loam Ft 38 - 34 - I/J Q) -� 30 (J ::::I a 26 LL « :!: Q. 22 c:::: Q) 1 8 I/J 0 1 4 1 0 7/91 2/92 7/92 2/93 7/93 Sampl ing Date Figure 7.4 Effect of appl ied MCP on Olsen P levels over time P Rate kg P/ha o 0 .. 20 - - + - · 30 - - <>- - · 40 8 50 - - �- - · 60 - - . - · 70 - - .. - · 80 A 1 00 o 0 .. 20 - - + - · 30 - - <>- - · 40 8 50 - - �- - · 60 - - . - · 70 • 80 A 1 00 for a) Wharekohe silt loam N Ft, b) Wharekohe silt loam Ft, c) Wharekohe sandy loam and d) Aponga clay. Arrows i ndicate P fertil iser application ti mes. 1 95 c) d) Wharekohe sandy loam 27 24 - - In Q) -� 21 (J � a LL 1 8 « � a.. c::::: 1 5 Q) In 0 1 2 9 7/91 Aponga clay 42 38 - - In Q) - 34 � (J � a LL 30 « � - a.. c::::: 26 Q) In 0 22 1 8 7/91 2/92 7/92 Sampl ing Date 2/92 7/92 Sampl ing Date " ' x ' , 0 x 2/93 7/93 7/93 1 96 P Rate kg P/ha o 0 - - Ie- - · 20 - - + - · 30 - - <>- - · 40 o 50 - - �- - · 60 • 70 .. 80 6 1 00 0 0 • 20 30 - - <>- - · 40 0 50 .« 60 - - .. - · 80 6 1 00 1 97 The rates of MCP requi red to maintain a steady Olsen P on the Wharekohe soi ls over the two year trial period (7/91 - 7193) were h igh ( in excess of 80 kg P/ha) (Table 7 .3) . However, as previously mentioned, seasonal variab i l ity in Olsen P l im its the confidence that can be associated with any conclusions, especia l ly as a much lower rate of P (20 kg/ha) maintained Olsen P levels in the first year on both the Wharekohe si lt loam sites. Table 7 .3 Rate of appl ied P as MCP required to maintain a constant Olsen P test at each site for each time period (standard deviations in brackets) . Rate of P Required to Maintain a Constant O lsen P Test (kg P/ha) Site 7/91 - 7/92 2/92 - 2/93 7/92 - 7/93 7/91 - 7/93 Wharekohe sandy loam 72 79.9 NO NO (8) (4. 5) Aponga clay 9.4 Olsen P 64. 7 34 (5.4) Not Oeterm. (4. 3) ( 1 .9) Wharekohe silt loam NFt 21 .4 24 NO NO (9) (8) Wharekohe silt loam Ft 21 .2 NO NO 83 (3 .6) ( 1 .8 ) NO Rate of P required to maintain a constant Olsen P test could not be determined as Olsen P levels fel l at a l l rates of applied MCP. As a l l rates of appl ied MCP were insufficient to maintain the O lsen P test values on the Wharekohe soi ls i n the second year of the smal l plot field tria l , i t was not possible to calculate the rate of P corresponding to a steady Olsen P level for the Wharekohe soi ls for that period. On the Aponga clay, the amount of P required to maintain a steady Olsen P level also rose dramatical ly in the second year from 9.4 kg P/ha to 64.7 kg P/ha. Although there was no d ifference in the rate of P required to maintain Olsen P levels between the two Wharekohe si lt loam sites of differing P fert i l iser history i n the first 1 98 year, it appeared that a lower rate of appl ied P was required to maintain the Olsen P level on the Ft site in the second year, as even 1 00 kg P/ha was insufficient to do 50 on the NFt s ite over the two year period. The rate of P required to maintain a constant Olsen P level in the first year on the Wharekohe sandy loam site (72 kg P/ha) was higher than on the Wharekohe si l t loam sites (2 1 kg P/ha) , implying larger losses of P on the former. Previous studies, presented in this research study, support this finding. Phosphorus is less strongly retained in the Wharekohe sandy loam compared to the s i lt loam (Chapters 4 and 6). C learly the amount of ferti l iser P required to maintain a constant Olsen P value for each period was far h igher on the Wharekohe soi ls than on the Aponga clay, indicating lower P losses from the less weathered yel low brown earth. Large losses of appl ied P have been recorded from the topsoi l of the Wharekohe podzols (Chapters 3 and 5). Losses of P in water runoff are presumed to account for the large d ifference i n the rate of P required to maintain a steady O lsen P test between the Wharekohe and Aponga soi ls in the present study. Assessments of the rate of P required to maintain constant O lsen P levels, on other New Zealand podzols, are l im ited in number and also restricted by the variabi l ity inherent in the Olsen P test and trial conduct (extent of cl ipp ing removal ) . O lsen P values in the Okarito podzol included in the MAF "national series" trial were extremely variable over the 6 year trial period. Olsen P levels were lower after 6 years at a l l rates of annually appl ied TSP up to 40 kg P/ha (twice calculated maintenance from the CFAS model) than the in it ial recorded levels (Smith et a I . , 1 991 b) . However, in contrast to the Wharekohe and Okarito podzols , only 1 6. 5 kg P/ha appeared to be required to maintain Olsen P levels on the podzol ised Hukerenui s i lt loam (Smith et aI . , 1 991 b) . On the two less weathered Northland yel low brown earths, Whangaripo and Warkworth clays, included in the "national series" tria l , 33 kg P/ha appeared to be required to maintain a constant Olsen P level (Smith et a i , 1 991 b) . These resu lts indicate that higher rates of P may not necessarily be 1 99 required to maintain a particular Olsen P level as the degree of soi l weathering increases. Olsen P Variability The large variabi l ity between the 1 99 1 /92 and 1 992/93 years in Olsen P values would appear to l im it the use of Olsen P as an appropriate indicator of soi l P loss. Large temporal variabi l ity in Olsen P data between years has also been observed at many other wel l control led, field trial sites where the effect of P rate on Olsen P has been monitored (Smith et aI . , 1 990; Rowarth et a I . , 1 991 ; P . Shannon et a I . , 1 985). Environmental conditions influence the amount of P in the avai lable soi l pool at any one time by influencing reactions rates of P with the soi l , runoff P losses, plant growth and microbial activity, and therefore contribute to temporal variabi l ity. Factors which may have effected the lower Olsen P levels recorded in the second year of the present smal l-plot field trials include higher P runoff losses of appl ied P in the wetter winter and autumn of the second year (Appendix 7 . 3) . The wetter and warmer autumn of the second year also resulted in increased pasture growth, and hence greater simulated animal P loss on the Wharekohe silt loam and Aponga clay s ites. The wet winter and autumn of the second year, and the extremely dry month prior to sampl ing at the field tria l conclusion may have influenced microbial P contents. Much of the P extractable by NaHC03 is l ikely to be derived from P due to the death of microbes in the Wharekohe soi ls. The proportion of total soi l P found in the microbial biomass in a Wharekohe s i lt loam was by far the largest ( 1 1 .7%) of any of the 21 soi ls surveyed by Perrott and Sarathchandra ( 1 989) . Consequently, Olsen P levels wi l l be more effected by environmental factors influencing the m icrobial population in the Wharekohe soi ls than in most other soi ls. (Rainfal l data is presented in Appendix 7 .3 ) The large year to year variabi l ity in Olsen P levels recorded in the present field study i l l ustrates the necessity of mult iple years of data to establ ish with confidence the rate of appl ied P at wh ich avai lable P wi l l remain constant over t ime for determining the SLF. Calculations of soi l loss parameters for use in fert i l iser P requirement models by AgResearch (and MAF) staff have been restricted to trials which have run for at 200 least 5 years with frequent herbage P and Olsen P analysis (Methere l l et a I . , 1 995). Even so, the determination of soi l loss in th is way from long term trials is sti l l compl icated by the highly variable nature of the O lsen P and is therefore questionable. Rowarth et al ( 1 992b) were unable to determine a rate of annual P addition which maintained a steady Olsen P level over a four year period in New Zealand h i l l country. Data from individual s ites in the MAF 'National Series' trials was variable and considered of l imited value in determining the change in Olsen P as a function of the rate of appl ied P (Roberts et a I . , 1 994) . Poo l ing data from s imi lar sites enables patterns to be more clearly defined (Roberts et aI . , 1 994), but raises doubts about the i ntegrity of soi l loss parameters determined for soi l groups where trial numbers are l im ited . Despite the problems with Olsen P variabi l ity, the method of determin ing soi l loss parameters from average changes in the avai lable P pool over a long period of t ime is sound, provided a 'steady state' situation exists (as in older Wharekohe podzols where soi l P accumUlation has ceased. The 'steady state' concept has been described in 2.6. 1 and 4.4.2) . It may be more appropriate, however, to determine the SLF by monitoring changes in a more comprehensive set of soi l P fractions across a range of P app l ication rates over a longer period. This would enable the identification of whether a 'steady state' exists and the rate of P required to maintain the average 'steady state' over time, and hence the appropriate SLF, so that calculated P requi rements wou ld be more l ikely to reflect the P required for an average year. Variabi l ity in the Olsen P test for a farm block, where a fert i l iser P appl ication rate is recommended, may also lead to large errors in calcu lated P requirements. In order to help compensate for the spatial and temporal variabi l ity inherent in the Olsen P soi l test, the Outlook Manual (Soi l Fert i l ity Service, AgResearch, 1 996) recommends that the start Olsen P be obtained from the average of four or more samples from each block and that resu lts then be averaged over the last two to three sampling dates where fert i l iser has been regularly appl ied at approximately maintenance rates. 201 Variation in the length of t ime that soi ls were stored is not expected to have effected the soi l P test variabi l ity. Bolland and Allen ( 1 996) could detect no change in Olsen P with storage t imes of up to 1 80 days at room temperature in Austral ian soi ls , whi le no change in bicarbonate extractable Colwel l P could be detected in soi ls stored for up to 1 7 years (Bol land et a I . , 1 994) . 7.4.2.2 Effect of P Fertiliser Solubility on Maintaining Available Soil P Levels Olsen P levels were lower where SPR was appl ied than where MCP was appl ied at s imi lar rates in the first year of the present tria l . Olsen P levels only increased over t ime where 80 and 1 00 kg of P/ha as SPR were appl ied on the Wharekohe s i lt loam Ft s ite in the first year (Figure 7 .5) . Consequently, the amount of P required to maintain a constant O lsen P level was greater where SPR was appl ied than where MCP was appl ied (Table 7 .4) . Other studies have also found that Olsen P levels are lower where RPR has been appl ied compared to soluble P ferti l isers at s imi lar P appl ication rates (Mackay et a I . , 1 984; Saggar et a I . , 1 992; Roberts et aI . , 1 994) . Where data from 1 3 sites in the MAF 'National Series' forms of phosphate fert i l iser tria ls was pooled, Olsen P levels remained steady at the maintenance TSP appl ication, decl in ing at a l l rates below maintenance and increasing at twice maintenance over the 6 years of the trial . In contrast, Olsen P values decreased over the 6 years at a l l rates of appl ied SPR including twice maintenance (Roberts et a I . , 1 994) . Such a result i s not surprising given the slow dissolution rates of the SPR in the surveyed soils (Edmeades et aI . , 1 991 b ) i n comparison to the highly soluble TSP. However, at two sites in the 'National Series' which were not i ncluded in the pooled data presented by Roberts et a l ( 1 994) , a South Island gley podzol (Okarito si lt loam) and a recent gley soi l on the West Coast of the South Is land, where runoff P losses are expected to be high, the Olsen P values were s imi lar and at times higher where SPR was appl ied compared to MCP at the same app l ication rates, 1 0 - 40 kg P/ha/yr (Smith et a I . , 1 991 b) . Where soluble P is prone to large losses in runoff, 202 a) MCP 38 P Rate - 34 kg P/ha -II) Q) - 0 0 � 30 CJ ::s .. 20 a 26 - - + - · 30 LL « - - <> - · 40 :!: D 50 - 22 Q.. - - �- - · 60 c:: - - • - · 70 Q) 1 8 II) 0 .. 80 1 4 A 1 00 1 0 7/91 2/92 7/92 2/93 7/93 Sampl ing Date b) SPR 38 - 34 - II) Q) -� 30 CJ ::s 0 0 a 26 - - JC- - · 20 LL « • 40 :!: - - �- - · 60 - 22 Q.. - - ... - · 80 c:: Q) 1 8 A 1 00 II) 0 1 4 1 0 7/91 2/92 7/92 2/93 7/93 Sampling Date Figure 7.5 Effect of P ferti l iser solubil ity on Olsen P levels over time where a) MCP and b) SPR were applied on the Wharekohe silt loam Ft site. Arrows indicate P fertil iser application ti mes. 203 Table 7 .4 Effect of fert i l iser P solubi l ity on the rate of appl ied P required to maintain a constant avai lable P soi l test for each period (standard deviations in brackets). Rate of P Required to Maintain a Constant Soi l P Test (kg/ha) Site 7/91 - 7/92 2/92 - 2/93 7/92 - 7/93 7/91 - 7/93 Wharekohe silt loam Ft, 21 .2 NO NO 83 MCP, Olsen P (3.6) ( 1 . 8) Wharekohe silt loam Ft, 43 55 NO NO SPR, Olsen P ( 1 3) ( 1 1 ) Wharekohe silt loam Ft, NO 25.3 NO 35. 1 SPR, Resin P (4.5) (8) NO Rate of P required to maintain a constant Olsen P or Resin P test could not be determined as soi l P test levels fel l at all rates of appl ied P. RPR may 'drip feed' the avai lable P pool so l im it ing P losses and leading to h igher or more sustained avai lable P levels with time fol lowing P appl ication. This s ituation as appears to have occurred on the above mentioned South Island soi ls . Colwel l bicarbonate P test values have also been found to be lower under ordinary superphosphate use than under the lower solubi l ity coastal superphosphate applied to humic sandy podzols in Western Austral ia from which leach ing of high solubi l ity P fert i l iser occurs (Bol land et aI . , 1 995a) . As Wharekohe soi ls loose substantial quantities of P in runoff (Chapters 3 and 5) , they would be expected to behave in the same way as the two South Is land soi ls. Although Olsen P values where SPR was appl ied were lower than where TSP was appl ied in the first year of the present trial and Olsen P levels fel l at a l l rates of both appl ied MCP and SPR in the second year, the fal l in O lsen P was greater under MCP than under SPR appl ications (F igure 7 .5, Appendix 7 . 1 ) . Hence, runoff losses of P may influence the relative performance of MCP and SPR in maintain ing Olsen P levels on the Wharekohe soi ls also. Relative losses of P from MCP and SPR could depend on the relative roles of surface and subsurface runoff, and d issolved and particulate P in P runoff losses (Chapter 5) . 204 The lower Olsen P values recorded in most soi ls where SPR has been appl ied can part ial ly be attributed to the slow release nature of the RPR in the soi l leading to a reduction in avai lable soi l P relative to the same quantity of soluble P fert i l iser (Perrott et a I . , 1 992a) . However, the Olsen P test also underestimates avai lable soi l P status associated with the use of RPR (Mackay et a I . , 1 984; Saggar et a I . , 1 992; Roberts et a I . , 1 994) . The dual Resin P test, has been found to better predict the relative pasture yield of ryegrass in a g lasshouse study than the O lsen P test, which underestimated the relative yield where RPR was appl ied (Saggar et a I . , 1 992) . A field evaluation of soi ls fert i l ised with SPR and TSP also found that resin P was a better indicator of relative yield than Olsen P (S . Saggar pers. comm. ) . In contrast to the effect of RPR appl ication on Olsen P values in the first year (7. Sb), Resin P increased at al l rates of appl ied SPR in the first year (F igure 7.6). Some of this increase may have been due to seasonal variation, as the increase in Resin P measured on the 20 and 80 kg P/ha plots was simi lar to that recorded on the control plots. As was the case with Olsen P, resin P values fel l at a l l rates of P in the second year of the tria l , indicating that Resin P is also prone to spatial and temporal variabi l ity and the factors which effect these. The amount of P applied as SPR required to maintain the same steady Resin P test was far less than the amount of P appl ied as MCP required to maintain a steady Olsen P on the Wharekohe si lt loam Ft site over the 2 year tria l period (Table 7 .4) . The impl ications of these findings in terms of P fert i l iser strategies for min imising P runoff losses are discussed in chapter 8. 80 75 70 - 65 C) - C) 60 ::. - D. c 55 'in CI) Q:: 50 45 40 35 7/91 2/92 7/92 2/93 7/93 Sampling Date P Rate kg P/ha e a 0 0 .0 0 · 20 • 40 0 0 *" 0 . 60 0 0 " 0 . 80 A 1 00 Figure 7.6 Effect of appl ied SPR on Resin P levels over t ime on the Wharekohe silt loam Ft s ite. Arrows indicate P ferti l iser appl ication times. Summary of section 7.4.2 : 205 • Olsen P and Resin P soi l test values displayed large year to year variabi l ity making it d ifficult to identify a constant avai lable P level . • Both the Wharekohe s i lt loam Ft and NFt sites required 2 1 kg P/ha to maintain a steady Olsen P level i n the first year. Over the two year period 83 kg P/ha was required to maintain a constant Olsen P level on the Wharekohe si lt loam Ft s ite. A higher rate of appl ied P, in excess of 1 00 kg P/ha, appeared to be required to maintain the O lsen P level on the NFt s ite. 206 • A higher rate of appl ied P was required to maintain the Olsen P level on the Wharekohe sandy loam (72 kg P/ha) in the first year, in comparison to the s i lt loam sites (21 kg P/ha) indicating greater P losses in the sandy loam. • The amount of MCP required to maintain Olsen P levels was much higher on the Wharekohe s i lt loam s ites (>83 kg P/ha) than on the Aponga clay site (34 kg P/ha) over the two year period, indicating greater P losses on the former. • Olsen P levels were lower where SPR was appl ied after the first year. However, in the second year, Olsen P levels under SPR did not fal l to the same extent as those under MCP, possibly indicating lower losses of avai lable P in the second year where SPR was appl ied. As d issolution of SPR approaches appl ication rates, the Olsen P levels could become higher than under an equivalent appl ication of MCP on these high P loss podzols. • The rate of appl ied SPR required to maintain a steady Resin P test over the two years (35. 1 kg P/ha) was substantia l ly lower than the rate of MCP required to maintain a steady Olsen P test (83 kg P/ha). It was d ifficult to determine if SPR would offer advantages over Mep, in terms of min imising soi l p losses, i n the long term due to the short duration of the tria l . The impl ications of the results presented in this first section are d iscussed in terms of the SLF values and soi l P losses in 7 .4 .3 . 7.4.3 Quantifying the SLF Calculated SLF values for use in the CFAS model are presented in Table 7 .5. 207 Table 7 .5 Soil Loss Factors determined for each sampl ing period at each site. Period Estimated Pasture P Uptake Simulated Soil P SLF Rate of P (kg/ha) Animal P Loss To maintain (In brackets: Loss (kg Iha) Steady P treatment used to (kg/ha) Olsen P estimate P Uptake) Wharekohe silt loam Ft 7/91 - 7/92 21 42.3 (20 MCP) 1 9.5 1 .7 0.04 7/92 - 7/93 1 00* 64. 1 ( 1 00 MCP) 29.5 70.5 1 . 1 0 7/91 - 7/93 83 50.4 (80 MCP) 23.2 59. 8 1 . 1 9 7/91 - 7/92 43 42.3 (40 SPR) 1 9.4 23.6 0.56 7/91 - 7/93 35. 1 (Resin P) 49. 1 (40 SPR) 22.6 1 2 .5 0.25 Wharekohe silt loam N Ft 7/91 -7/92 2 1 30.2 (20 MCP) 1 3. 9 7 . 5 0.25 2/92 - 2/93 24 32 .3 (20 MCP) 1 4.9 9. 1 0.28 7/91 - 7/93 1 00* 46.8 ( 1 00 MCP) 2 1 . 5 78.5 1 .68 Wharekohe sandy loam 7/91 - 7/92 72 52.7 (70 MCP) 24.2 47. 8 0.91 2/92 - 2/93 80 53.3 (80 MCP) 24.5 55.4 1 .04 7/91 - 7/93 1 00* 59.5 ( 1 00 MCP) 27.4 72.6 1 .22 Aponga clay 7/91 - 7/92 9.4 33.9 (20 MCP) 1 5.6 -6.2 -0. 1 8 7/92 - 7/93 64.7 51 .6 (60 MCP) 23.7 41 0.79 7/91 - 7/93 34 41 .6 (30 MCP) 1 9. 1 1 4. 9 0.36 * No rate of appl ied P sufficient to maintain O lsen P levels, 1 00 kg P/ha used to calculate minimum SLF values. The calculated SLF varied greatly between sampl ing periods on the Wharekohe si lt loam Ft s ite, from 0 .04 in the first year of the tria l to 1 . 1 9 over the two year trial period (Table 7 .5) . This large d ifference in calculated SLF values was due to the large increase in the rate of P required to maintain a steady Olsen P level in the 208 second year of the smal l p lot field trial . Such a large difference in the SLF would create a range of calculated P requirements to maintain 1 5.6 stock units from 8.6 kg P/ha to 47 .5 kg P/ha for a change in the SLF from 0.04 to 1 . 1 9, respectively. This large range of calculated P requirements i l lustrates the importance of rel iably pred ict ing soi l P loss. Because of the large variation in the calculated SLF in the present tria l , it is d ifficult to determine if the SLF of 0 .4 used in the CFAS model (which calculates 21 kg P/ha to maintain 1 5.6 SU) is inappropriate for use in the CFAS model for determining maintenance P requirements for a g iven level of production on the Wharekohe si lt loam. Another approach where part of the SLF factor is calcu lated from a soi l loss derived from the accumulation of P into p lant unavai lable P compounds, as per Rowarth et a l . ( 1 992b), is presented in 7 .4 .3 .7 . 7.4.3. 1 Effect of Soil Fertility Status on the SLF The calculated SLF values for each period were h igher for the Wharekohe s i lt loam NFt site than for the Ft site exposed to the same cl imatic conditions (Table 7 .5) . The much higher SLF, despite simi lar rates of P required to maintain a constant Olsen P level , were a result of the much lower P uptake on the NFt s ite. The low P uptake led to lower simulated animal losses, and consequently h igher soil P loss for a simi lar rate of P required to maintain a constant Olsen P level . Such a large d ifference in calculated SLF values (0.04 to 0 .25 in the first year for the Ft and NFt sites, respectively) serves to i l lustrate how the in it ial fert i l ity of a site can have a large impact on the calculated SLF. The SLF values used in the CFAS model were derived by dividing the estimated soi l P loss by the P uptake corresponding to 90% of maximum yield for that soi l group. In theory, if a l l other l imit ing factors associated with the in it ial lower fert i l ity are overcome, the maximum yield should be simi lar on both Wharekohe s i lt loam sites and eventual ly the rate of P required to maintain a constant Olsen P wi l l be simi lar leading to simi lar calculated 209 SLF values regardless of in it ial P fert i l ity. However, trials rarely continue for long enough for this to occur and in it ial soil fert i l ity is l ikely to impact on the calcu lation of the soi l loss parameters even in the 6 year tria ls used by previous MAF and present AgResearch staff. 7.4.3.2 Effect of Parent Material on the SLF The calculated SLF values were much higher on the Wharekohe sandy loam than either of the Wharekohe s i lt loam sites, 0 .91 compared to 0.25 and 0 .04 in the first year of the tria l for the sandy and two s i lt loam sites, respectively (Table 7 . 5) . This finding is consistent with observations that the Wharekohe sandy loam appears to have a h igher requirement for fert i l iser P than the s i lt loam (P . Shannon pers. Comm. ) at s imi lar stocking rates and that P is less strongly retained in the sandy loam. The h igher SLF is due to the higher rate of P required to maintain a steady Olsen P level and consequently higher calculated soi l losses. However, the large variab i l ity in Olsen P levels, and therefore SLF values, in the present trial reduces the confidence that can be placed in the sandy loam requiring a higher SLF to calculate P requirements than the s i lt loam. 7.4.3.3 Effect of Degree of Soil Weathering on the SLF The calculated SLF values were much lower on the Aponga clay soi l than the Wharekohe si lt loam sites for each sampl ing period (Table 7 . 5) . I n the first year of the tria l , the SLF was negative, implying that the provision of P from existing soi l P was greater than P losses from the cycle. However, in later periods the SLF was much higher. Once again variabi l ity in the SLF between periods makes it d ifficult to decide upon an appropriate SLF for use in the CFAS model for the yel low brown earth. In the first year of the tria l , the calcu lated SLF was wel l below 0.25 which was used in the CFAS model for this soi l group, but in the second year and over the two year period, the calcu lated SLF was higher than 0.25. 21 0 7.4.3.4 Effect of P Fertiliser Solubility on the SLF When the change in Olsen P level was used as an indicator of P loss, the calculated SLF was higher under SPR than MCP in the first year due to the higher rate of SPR required to maintain a steady Olsen P level (Table 7 .5 ) . However, when a change in Resin P was used as an indicator of soi l loss where SPR had been applied, the calculated SLF was substant ial ly lower (0.25) than under MCP ( 1 . 1 0) for the two year period (Table 7 .5 ) . Continuation of the trial beyond 6 years, by which time SPR dissolution should a l low for a 1 99% effective P appl ication rate, wou ld be needed to confirm if the SLF calculated in this way would be lower under applied SPR compared to MCP. 7.4.3.5 Sensitivity of the SLF Rate of P Required to Maintain a Steady Available Soil P Level The large variation in the calculated SLF values between t ime periods was mostly due to the large variation in the rate of P requ i red to maintain a steady avai lable soi l P rather than the much smaller variation in P uptake (and hence also simulated animal loss) between the two periods. The large temporal variab i l ity found in the Olsen and Resin P values led to a large variation in calculated soi l P losses, and hence SLF values. A small error in the rate of P required to maintain a part icular avai lable soil P soil test can lead to large changes in the calculated SLF value, especial ly at lower SLF values (Table 7 .6) . For example, a 1 0% error in the estimation of the rate of P required to maintain a steady Olsen P value leads to a 28% change in the calculated SLF value on the Wharekohe s i lt loam NFt site using the fi rst year results. As previously mentioned in 7 .4 . 1 .2 , variabi l ity in Olsen P has made it impossible to calculate soi l P loss in tria ls of longer duration (4 years on h i l l country at Whatawhata (Rowarth et a I . , 1 992b) and up to 6 years in the range of trial sites included in the "National Series" (Roberts et a I . , 1 994)) . Period Rate of P To P Uptake SLF Effect of changing P Effect of incorrect estimation of The rate of P required to maintain a Maintain (kg/ha) uptake on SLF steady Olsen combined with incorrect estimation of P uptake. Steady Olsen (kg/ha) - 1 0% + 1 0% - 1 0% Rate P + 1 0% Rate P -1 0% P -0% P + 1 0% P - 1 0% P -0% P + 1 0% P Uptake Uptake Uptake Uptake Uptake Uptake Aponga clay 7/91 - 7/92 9.4 33.9 -0. 1 8 -0. 1 5 -0.21 -0. 1 8 -0.21 -0.23 -0 . 1 2 -0. 1 5 -0. 1 8 7/92 - 7/93 64.7 51 .6 0.79 0.93 0.68 0 .79 0.67 0 .57 1 .07 0 .92 0 .79 7/91 - 7/93 34 41 .6 0.36 0.45 0.28 0 .36 0 .28 0 .21 0 .54 0.44 0 .36 Wharekohe sandy loam 7/91 - 7/92 72 52.7 0.91 1 .06 0 .78 0.91 0 .77 0.66 1 .21 1 .04 0 .91 2/92 - 2/93 79.9 53.3 1 .04 1 .21 0 .90 1 .04 0.89 0 .77 1 .37 1 . 1 9 1 .04 7/91 - 7/93* 1 00 59.5 1 .22 1 .41 1 .07 1 .22 1 .05 0 .92 1 .59 1 .39 1 .22 Wharekohe silt loam NFt 7/91 -7/92 21 .4 30.2 0.25 0.33 0 . 1 8 0.25 0 . 1 8 0 . 1 2 0.41 0 .32 0.25 2/92 - 2/93 24 32.3 0.28 0.37 0 .22 0.28 0.21 0 . 1 5 0.45 0 .36 0.28 7/91 - 7/93* 1 00 46.8 1 .68 1 .91 1 .48 1 .68 1 .46 1 .29 2 . 1 5 1 .89 1 .68 Wharekohe silt loam Ft 7/91 - 7/92 21 .2 42 .3 0.04 0 . 1 0 0 0 .04 -0.01 -0.05 0 . 1 5 0.09 0 .04 7/92 - 7/93* 1 00 64 . 1 1 . 1 0 1 .27 0 .96 1 . 1 0 0 .94 0 .82 1 .45 1 .26 1 . 1 0 7/91 - 7/93 83 50.4 1 .1 9 1 .37 1 .04 1 . 1 9 1 .02 0.89 1 .55 1 . 35 1 . 1 9 7/91 - 7/93 43 49 . 1 0.42 0.51 0 .34 0.42 0.33 0 .26 0.61 0 .50 0.42 7/91 - 7/93 35 . 1 (Resin P) 49. 1 0.25 0.33 0 . 1 9 0.25 0 . 1 8 0 . 1 2 0.41 0 .33 0.25 Table 7.6 Sensitivity of the SLF to an incorrect estimation of the rate of P to maintain a steady Olsen P level and P uptake. 2 12 P Uptake It was also not possible to accurately determine P uptake at 90% Ymax, at any of the sites in the present study as they were not part icularly P responsive. Consequently, the actual P uptake measured at the rate of appl ied P closest to that determined to maintain a steady O lsen for each period was used in the calculation of the SLF. A small error in P uptake leads to a s imi lar change in the calculated SLF to the change wh ich is encountered for a simi lar error in the rate of P required to maintain a steady Olsen P value (Table 7.6) . An increase in P uptake wil l result in a decrease in the calcu lated SLF. Other researchers (Rowarth et a I . , 1 992b; S inclair et a I . , 1 994) have also encountered difficulties in determin ing the P uptake which corresponds to 90% of maximum yield. Rowarth et al . ( 1 992b) were unable to determine maximum yield in their study of a range of P treatments encompassing 4 years on hi l l country at Whatawhata. In their calculations of the SLF they used the P uptake for each rate of P , as was done in the present study. Mitscherl ich response curves fitted to pasture dry matter yields on the MAF "National Series" trial s ites fitted the data poorly and the large variabi l ity in the curves' parameters made it impossible to predict the rate of P required for any specific yield at individual s ites (S inclair et a I . , 1 994). Only when data from al l sites were combined, could a mitscherl ich curve be defined. The mowing regime, used in the present study may not be appropriate for determining the SLF to determine pasture P requirements under grazing. Mowing produced s imi lar dry matter yields to yields measured under grazing between July 91 and July 92 on the Wharekohe s i lt loam Ft s ite when pasture growth adjacent to the trial site was mon itored under grazing. However, dry matter yields were 1 1 % lower in the mown SLF trial than i n an adjacent unfert i l ised area under grazing on the Wharekohe si l t loam NFt site. Pasture production was 2 1 -37% less under mowing compared to grazing at three other New Zealand s ites, a lthough the relative response to appl ied P was simi lar under both mowing and grazing (Morton et a I . , 1 995). If pasture yields occurring under grazing were underestimated by mowing, P uptake is also l ikely to be underestimated and mowing wil l result in a lower estimation of the SLF than wou ld be estimated under grazing. 2 1 3 P Uptake and the Rate of P Required to maintain a Steady Olsen P Level Smal l errors in both parameters required to calculate the SLF can lead to very large changes in the calculated SLF values and hence maintenance P requirements. The combined effect of a 1 0% change in both parameters can result in a greater than 50% change in the calcu lated SLF value on the Wharekohe s i lt loam NFt site in the first year (Table 7 .6) . The 64% increase in the SLF factor from 0.25 to 0.41 , when the P rate was increased by 1 0% and the P Uptake decreased by 1 0%, leads to a 34% increase in the amount of P required to maintain a stocking rate of 1 5.6 SU on rol l ing topography, that is 1 5. 7 kg P/ha to 2 1 . 1 kg P/ha. This d ifference serves to i l lustrate why the calcu lation of the SLF from P response trials can lead to large errors in maintenance P rates, even from pooled site data. Simulated Animal Loss Apart from the Wharekohe si l t loam NFt site (where dry matter yield was under 8000 kg/ha in the first year), the simulated animal losses in the present mowing trial were high in comparison to what would be expected under grazed pasture, due to the removal of 46% of the pasture and higher than expected herbage P concentrations and dry matter yields. For example, at a stocking rate of 1 6 SU/ha, only 1 4.4 and 8 kg P/ha would be expected to be removed by animals on an intensive dairy and sheep farm, respectively, on rol l i ng topography. Yet the simulated anima l losses via cl ippings removal in the smal l-plot field tria ls varied from 46% of 42 to 64 kg P/ha ( 1 9. 3 to 29.5 kg P/ha). Grazing animals excrete consumed P which is surp lus to their requ i rements. Phosphorus returned in dung therefore increases with increasing herbage P concentrations (Bromfield, 1 961 ; Barrow and Lambourne, 1 962; Rowarth, et a ! . , 1 988). Hence, although animal transfer losses are higher under high herbage P concentrations (Mackay et a! . , 1 987), losses via animal products wi l l not increase greatly. However, the mowing regime removed 46% of the P taken up by pasture regardless of P concentration or dry matter yield. Consequently, lower amounts of P were returned to the p lots than what would have been returned under grazing 2 14 animals, and hence the mowing reg ime simulates lower rates of P than were actual ly appl ied. For example, where 80 kg P/ha was appl ied in the present tria l , the greater simulated animal loss simulated rates of P closer to 7 1 .2 and 64. 8 kg P/ha for grazed intensive dairy farms and sheep farms, respectively, on the Wharekohe si lt loam Ft site for the two years. Theoretical ly, soi l loss is i ndependent of animal loss and consequently soil losses would have been simi lar if simulated animal losses had been lowered to more real istic levels . Therefore the higher than expected simulated animal losses should not have impacted on the calculated SLF values. 7.4.3.6 Effect of Pasture Age on the SLF When the component of the SLF which can be attributed to the accumulation of non­ labi le soil P (SLFsPA) was calculated from the chronosequence study results, by the equation described in 7 . 3.2 , the SLFsPA decreased with increasing pasture age (Table 7 . 7) . The reduction in the SLFsPA with increasing pasture age was the result of l ittle further accumulation of P in the top soil beyond 1 1 years (Chapters 3 and 4) . The negative SLFsPA values are the result of a decrease in the unavai lable P for those periods due to either measurement error or loss from the unavai lable P pools. The calculated SLFsPA values were increased by the inclusion of organic P up unti l 33 years (Table 7 . 7) . However, Rowarth et al . ( 1 992b) did not consider the accumulation of P into organic compounds as a soi l loss, as their inclusion resulted in far h igher SLFsPA than had been derived from P response trial data for sedimentary soi ls . For the oldest site in the present study, the inclusion of Po accumulation made l itt le d ifference to the calculated SLFsPA, as l itt le Po accumulation had occurred. Although the SLFsPA values a l l decl i ned with increasing pasture age, the higher SLFsPA values calculated from the 1 993 soi l data in comparison to the 1 990 soi l data can probably be attributed to i ncreasing P accumulation associated with the use of RPR during the intervening period. 2 1 5 The interpretation of the SLFsPA values with increasing pasture age at each sampl ing time is compl icated by large d ifferences in the average rates of P appl ied during each time period . The decrease in the SLFsPA with increasing pasture age was also associated with a decrease in P appl ication rates which may have influenced the SLF SPA values. Rowarth et al. ( 1 992b) have found that the SLF SPA general ly decreases with decreasing rate of appl ied P . Table 7 . 7 Effect of pasture age on the SLFsPA for use in the CFAS model Period Average Annual SLFsPA SLFsPA P Application (Unavailable Pi + Hel Po) (Unavailable Pi + Po) ( kg/ha) 1 990 Data 0-8 years 49 0.40 0 .53 0-32 years 32 0. 1 1 0 . 1 9 8-32 years 26 0 .02 0 .08 25-32 years 34 0 .03 0 .09 1 993 Data 0-1 1 years 48 0 .53 0.66 0-35 years 33 0. 1 2 0. 1 9 1 1 -35 years 28 -0.07 -0.03 33-35 years 38 -0. 1 1 -0. 1 8 Difference Between 1 990 and 1 993 Sampling of Original Sites. 8-1 1 years 49 0 .56 0.74 30-33 years 49 -0.05 0. 1 1 32-35 years 49 -0.05 -0.04 However, comparison of the SLFsPA calculated from changes in P fractions on the original s ites between 1 990 and 1 993, where P appl ication was the same on each 2 16 site reveal that the decrease in the SLFsPA does result from an increase i n pasture age rather than decreasing P appl ication (Table 7 .7 ) . Runoff losses from rates of soluble P which would be requi red to maintain pasture production are unavoidable, a lthough they may be min imised by alternative fert i l iser P strategies (discussed in Chapter 5) . Consequently, the SLF must also include a runoff P component. The SLFsPA values presented in Table 7 .7 ind icate that no soi l P loss via accumulation of non-labi le P is occurring in the root zone in the older sites leading to SLFsPA values close to or even lower than O. Consequently, the SLF would consist mostly of the runoff P component in older developed sites on the Wharekohe si l t loam. It is inefficient to continue applying P at the constant rates calculated by using a constant SLF in the CFAS model , if P surplus to production requ i rements and soi l P accumulation is going to be lost from the soi l and runoff losses are increased with pasture age (as demonstrated in Chapter 3) . It is therefore very important that the overal l SLF, and consequently calculated P requirements, are reduced as pasture age increases from development, and that the min imum P runoff losses, which are unavoidable for a given fert i l iser P strategy, are included in the SLF. Unt i l the minimum runoff P losses for various fert i l iser strategies are identified, it is impossible to determine if the CFAS SLF of 0.4 underestimates soi l loss on the older developed Wharekohe sites in the chronosequence study. It appears, however that the SLF appropriate for younger pastures « 1 1 years) on the Wharekohe s i lt loam has been underestimated, as the SLFsPA values were greater than 0.4 and runoff P losses have yet to be added to the SLFsPA to give the overal l SLF. A 'steady state' has yet to be achieved in younger pastures (0-1 1 years) on the Wharekohe si l t loam, as the net accumulation of non-labi le P in the root zone changes with pasture age. Consequently, the use of smal l-plot field trials , where the average rate of P required to maintain a constant avai lable P pool over a long period is estimated, would be inappropriate for determining SLF values on young pastures which are not in a 'steady state' and where the soi l P loss wi l l be changing with time. On such s ites, the monitoring of P uptake and a wider range of P fractions, total P and runoff P losses (by d ifference, that is P runoff = Appl ied P - (An imal P loss + 2 1 7 Accumulated soi l P) ) across a range of P appl ication rates should g ive a more rel iable estimate of soi l loss and SLF values as pasture age increases. Such trials should continue for at least 6 years once a 'steady state' has been identified so that an average SLF can also be determined which would rel iably predict soi l loss from a pasture in a 'steady state' for an 'average' year. Alternative fert i l iser strategies, such as the more frequent appl ication of smal l dressings of soluble fert i l i sers and the use of slowly soluble P fert i l isers such as RPR, to minimise runoff P losses could also be investigated in this way and appropriate SLF values for such strategies be determined. As pasture age and fert i l iser history are often unknown variables when making fert i l iser recommendations, a measure of P saturation (Chapter 6) may be a more appropriate means for determining an appropriate soi l loss factor value. Soi l loss factors would need to be determined for a range of P saturation values on soi ls where P saturation is l ikely to lead to large runoff P losses. Evidence for reducing the magnitude of the soil P loss parameters, with increasing pasture age, are not only restricted to podzols. H igher fert i l iser P inputs and Olsen P levels have been noted to be required to maintain pasture yields at a relative yield of 90% of Ymax in trials on yel low brown pumice soi ls under pastures establ ished for less than 1 5 years, than under pastures which had been establ ished for 30 to 40 years (Saunders et a I . , 1 987b). The change in P requi rements with pasture age were attributed to a decrease in soil P loss as the accumulation of organiC matter and the net organic immobi l isation of P slows with increasing pasture age leading to a greater contribution of P from the organic P pool for plant uptake. L ikewise, it has been noted that lower rates of appl ied P are required to increase O lsen P levels on sedimentary soi ls at 8al lantrae (G. Lambert pers. comm. ) and Winchmore with increasing pasture age (A. Metherel l pers. comm. ) . 7.4.3.7 Implications of Soil Loss Estimation on P Fertiliser Requirements 2 1 8 Each of the factors d iscussed i n sections 7 .4 .3. 1 to 7 .4 .3 .6 wi l l a lso impact on the determination of the soil 1055 parameter used in the calculation of P requirements by the Outlook model . The soi l 1055 parameters currently employed in the CFAS and Outlook models both assume that the quantity of P lost from the cycl ing P pool via soi l 1055 wi l l be a constant proportion of the P avai lable for p lant uptake (that is, a constant proportion of pasture P uptake in the CF AS model and a constant proportion of labi le soi l P estimated by Olsen P in the Outlook model) . This means that increasing P additions above maintenance requirements is expected to lead to an increase in avai lable soil P and therefore an increase in non-labi le soil P and P lost via runoff waters equal to a constant proportion of the appl ied P. However, results in this research project ind icate that an increasing proport ion of the P appl ied in excess to animal 1055 is lost in runoff from the Wharekohe soi ls , as pasture age increases, because net P accumulation in the soi l decl ines. Consequently, the proportion of appl ied P which wi l l be lost from the soil a lso increases with increasing P appl ication rate. A simi lar pattern has been observed on a high P leaching sandy soi l in South Western Austra l ia in contrast to other West Austral ian soi ls (Bol land and Barrow, 1 991 ) . Although podzols have been a l located a h igh soi l 1055 factor (0 .4) in the CFAS model , scientific evidence for using such a high value is lacking. Even higher soi l P losses have been included for the podzols in the Outlook model i n comparison to the older CFAS model (Metherel l et a I . , 1 995) because very h igh soi l P losses were estimated on the associated podzol ised yel low brown earth, Hukerenui s i lt loam site used to calculate soi l 1055 parameters, despite the measurement of very low soi l P 1055 on the true podzol , Okarito si l t loam (Methere l l , 1 994). Whereas the CFAS model previously calculated that 21 kg P/ha was required to maintain an Olsen P level of 20 and a stocking rate of 1 5 .6 on a Wharekohe si l t loam on rol l ing topography (SLF 0.4), the Outlook model now calculates that 44 kg P/ha is required to maintain the same level of production on a Wharekohe podzol (Soi l Loss 0. 1 ) . An appl ication of 44 kg P/ha is very high and is l ikely to lead to large losses of P in 2 1 9 runoff from older Wharekohe sites (Chapter 8) . I n contrast, the maintenance P rates calculated by the Outlook model for the same stocking scenario and in it ial Olsen P level on a sedimentary soi l is 1 9 kg P/ha (Soi l Loss 0 .04) , which is s imi lar to the 2 1 kg P/ha calculated by the CFAS model to maintain the same production levels on a podzol . The use of h igh soi l loss parameters in calculating P requirements for Wharekohe podzols is h ighly questionable if most of the P appl ied surplus to animal losses is going to be lost from the soi l under older and regu larly topdressed pasture. Evidence presented in this research project supports the appl ication of a decl in ing soil P loss parameter in both the CFAS and Outlook models as pasture age increases on Wharekohe podzols. Modified P appl ication strategies, where smal ler quantities of P enter the avai lable P pool more frequently, should also be investigated to reduce runoff P losses. It is l ikely that a s imi lar pattern of P loss is occurring in other NZ podzols and consequently, very low soil P loss parameters and modified P appl ication strategies should also be appl ied on other podzols under older pastures as wel l . A P saturation factor (as described in Chapter 6) could be incorporated into the model to assess the l ikel ihood of P loss via accumulation in the soil and an appropriate soil loss parameter value for each individual farm site on soils where large runoff P losses are l ikely. 7.5 CONCLUSIONS • Calculated SLF values varied enormously from 0 .04 in the first year of the trial to 1 . 1 9 over the two year trial period on the Wharekohe s i lt loam Ft s ite. This variation is a consequence of the large variation in the rate of P required to maintain a steady Olsen P level at each site. It was not possible to determine if the SLF of 0 .4 used for podzols in the CFAS model was appropriate from the SLF trial presented in this study due to this large variation . • The SLF was higher on the Wharekohe si lt loam NFt site (0.25) compared to the Wharekohe si l t loam Ft s ite (0 .04) in the first year, despite s imi lar rates of appl ied P required to maintain a steady Olsen P. This large d ifference was due to the much 220 lower P uptake in the former. Such a large d ifference in calculated SLF values between sites of different fert i l ity i l lustrates how the in it ial fert i l ity of a site can have a large impact on the calculated SLF value. • The calcu lated SLF values were higher on the Wharekohe sandy loam site than on the Wharekohe si l t loam sites reflecting that P is less strongly retained in the sandy loam. • The calcu lated SLF values were higher on the Wharekohe s i lt loam sites than on the Aponga clay site. • SLF values calculated from the change in Resin P under appl ied SPR (0.25) were substantia l ly lower than those calculated from the change in Olsen P under appl ied MCP ( 1 . 1 0) for the two year trial period. A longer trial period would be required to determine if the soi l loss determined from changes in Resin P reflects soi l losses under SPR. • The highly variable nature of the Olsen and Resin P soi l tests has led to large variabi l ity in the calculated SLF values wh ich are very sensitive to incorrect estimation of both the rate of P required to maintain a constant Olsen P level and P uptake. The calculation of a soi l loss parameter whose rel iance is placed on the abi l ity of the highly variable Olsen P test to determine soi l P losses is questionable. A 1 0% increase in the rate of P required to maintain Olsen P levels accompanied by a 1 0% decrease in P uptake led to a 64% increase in the SLF and an associated 34% increase in the amount of P required to maintain 1 5.6 SU on rol l i ng topography on the Wharekohe podzol . • The SLFsPA calculated from the chronosequence data decreased with pasture age. As P appl ied surplus to animal production requirements and P accumulation in the root zone is lost from the root zone in water runoff, the SLF should be reduced with increasing pasture age or else P runoff losses wi l l increase. In order to determine the SLF appropriate for a particular pasture age, the minimum runoff losses which 221 are unavoidable for a given fert i l i ser P strategy must be determined and included in the SLF. • Unt i l the minimum runoff P losses for various fert i l iser strategies are identified, it is impossible to determine if the CFAS SLF of 0 .4 underestimates soi l loss on the older developed Wharekohe sites in the chronosequence study. It appears, however that the SLF appropriate for younger pastures « 1 1 years) on the Wharekohe si l t loam has been underestimated, as the SLFsPA values were greater than 0.4 and runoff P losses were not included to give the overal l SLF. • The use of the current soi l loss parameters in the CFAS, and particularly, the Outlook model is l ikely to lead to very high runoff P losses from older pastures on the Wharekohe silt loam. It is important that soi l loss parameters used in both the CFAS and Outlook models reflect the decrease in non-labi le soi l P accumulation with i ncreasing pasture age in association with improved P appl ication techniques on the podzol soi ls to min imise costs to the farmer and the environment. CHAPTER S MODELLING THE FATE OF P IN A WHAREKOHE SILT LOAM S.1 INTRODUCTION 222 In New Zea land, simple mass balance models, CFAS and Outlook, have been used to predict the P requirements of grazed pastures (Cornforth and Sinclair, 1 982, 1 984; Metherel l et a I . , 1 995). Annual P requirements are calculated to replace animal and soi l P losses from the cycl ing P pool in order to maintain pasture production and stocking rates. These models assume that the fluxes of P between the soi l P pools, within the pasture P cycle, are in a steady state. Once the desired avai lable P and pasture production levels are reached, it is assumed that the amount of soil P loss is a constant proportion of the avai lable P each year, and therefore a constant soi l loss parameter is used in each model . Pasture age is disregarded. However, as a lready described (Chapters 3 and 4) , the rate of accumulation of P into various soi l pools in the Wharekohe s i lt loam decreases with pasture age. If the soi l loss parameters do not reflect this decreasing soi l P accumulation rate, P app l ication rates surplus to soi l P accumulation in the root zone and production requi rements wi l l be calculated by the models and surplus P wi l l be lost in runoff waters from the root zone and u lt imately to the wider environment. In most New Zealand soi ls , soi l P loss is general ly considered to be predominantly non-labi le P accumulation rather than runoff losses (Saggar et a I . , 1 990b; Rowarth et a I . , 1 992b) , as only smal l runoff P losses have been recorded from most New Zealand soi ls (Chapter 5) . A model developed to predict soi l P accumulation/depletion in h i l l country sedimentary soi ls (where predicted P accumulation in the top 1 5 cm = P appl ied - animal gainlloss) was found to account for most of the app l ied P (Saggar et aI . , 1 990b) . However, substantial quantities of P were lost via soi l loss from a yel low grey earth at Winchmore under i rrigation, >53% from the top 7 .5 cm and 30% from the top 30 cm (calculated from data presented by Nguyen and Goh, 1 992) . These losses of P from the soi l at Winchmore exceeded the P losses recorded from a s imi larly aged Wharekohe s i lt loam site (Chapter 3) . In contrast to the Wharekohe soi l , the pasture system at Winchmore appeared to be in a 'steady state' as soi l P accumulation (Quin and Rickard , 1 98 1 ; Nguyen et a I . , 223 1 989), and therefore runoff P losses (by d ifference) , occurred at a constant rate and a constant soi l loss parameter was appropriate for calculating P requirements during the 35 year period from pasture development. Tota l P and organic P accumulation have also been found to accumulate l inearly with both pasture age and the amount of superphosphate appl ied in the top 1 0 cm of sandy soi ls overlying clay in Austral ia over a 25 year period, despite large losses of appl ied P presumably i n lateral runoff (Lewis et a I . , 1 987). These results imply a constant soi l loss and that P saturation had not occurred within 25 years. The sandy texture of the soi ls was the most l ikely reason for the high P loss from the top soi l , rather than the soi ls approach ing P saturation. As would be expected , exchangeable P (Bray 1 ) increased quadratical ly with the amount of superphosphate appl ied. An Austral ian model used to calculate pasture P requirements predicts a decl ine in P i 'fixation' over the first 1 0 years fol lowing the pasture development of P deficient soi ls as avai lable P and pasture production increase (development phase) (B la i r et a I . , 1 977) . However, P fixation was then predicted to remain at a constant level of 20% of appl ied P , rather than approaching O. Soi l P accumulation decl ines with pasture age in Wharekohe soi ls at pasture production levels wh ich reached the maintenance phase. Consequently, it is important that any models developed to predict the fate of appl ied P and fert i l i ser P requirements for the Wharekohe soi ls account for this decl ine i n P accumulation with pasture age. 8.2 OBJ ECTIVES The objective of this study was to model the fate of P in a Wharekohe s i lt loam with pasture development in respect to the fluxes which contribute to soi l P loss, namely: i) accumulation of P i nto non-labi le soi l pools (avai lable P , organic P and total P accumu lation were a lso i nvestigated) , and i i ) losses of P in runoff. 8.3 THE P CYCLE OF A G RAZED PASTURE 224 The fate of P in a Wharekohe podzol can be described by a simple diagram (F igure 8. 1 ) . 8.4 MODEL DEVELOPMENT The tota l P and P fractionation data from the chronosequence study presented in chapters 3 and 4 can be used to model the rate of P accumulation in a Wharekohe si lt loam with increasing pasture age. The work presented earl ier in this research project suggests that P which is appl ied surplus to soi l P accumulation and animal requirements is lost from the soi l via runoff waters. Once P accumulat ion in the Wharekohe podzol has been model led, this could be incorporated into a separate model to predict runoff P losses. 8.4. 1 Soil P Accumulation The soil P pools were estimated from the fol lowing chemical P fractions. The chemical nature of each P pool has already been described in 4 .2 .3 .2 . Soil P Pool : • Read i ly avai lable Pi • Organ ic P • Calcium Pi • Strongly sorbed/precipitated P i and Residual P Estimated by the fraction/s: NH4C I and NaHC03 P i NH4C I , NaHC03 and NaOH Po H2S04 Pi NaOH Pi , Hot HC I Pi & Po and Residual P Data col lected from the original chronosequence sites sampled in spring 1 990 and 1 993 was used to establ ish a relationship between pasture age and the net accumulation of P into each soi l P pool in the root ing zone (0-7 . 5 cm) (where net P accumulation was calculated as the d ifference in each P pool between each Plant P Ferti l iser P 1 Unavai lable --.... Inorganic P .. (CaPi and SsrpP) \ I Litter p I t Soi l P Available Inorganic P .. (AvPi ) Runoff P Loss 225 Animal P Loss t Animal P I Faeces pi t • Organic P ( Po) Figure 8. 1 Fate of fert i l iser P in a grazed Wharekohe podzol (Note: SsrpP includes a smal l quantity of residual organic P extracted by hot Hel ) . 226 developed site and the undeveloped sites) . The mathematical equation wh ich best describes the relationsh ip was derived as fol lows: Rate of P Accumulation = K -a. P(t) or d P(t) -- = K -a. P(t) d (t) Where: (1 ) P(t} is the amount of P which has accumulated in the soi l fol lowing pasture development for a g iven pasture age (kg/ha) . is the pasture age (years) . K is a constant corresponding to the rate of P accumulation at age o. a is the rate constant. Assuming that: i) P is appl ied in excess to or equal to animal loss plus soi l accumulation each year, and i i ) the accumulation of P in the soi l in each year is proportional to the amount already accumulated in the soi l . Equation (2) , wh ich describes the relationship between pasture age and the amount of P wh ich has accumulated in the soi l , can then be derived from equation ( 1 ) (derivation out l ined in Appendix 8. 1 ) . P(t) =G(1 - e-a.t ) (2) Where: G is the maximum amount of P which can accumulate in the soi l fo l lowing pasture development (kg/ha) . The amount of P which can accumulate in the soi l in the year fol lowing each P appl ication can then be determined as the difference between the total amount of P which is pred icted to have accumulated in the soi l at the t ime of P appl ication and 227 the tota l amount of P wh ich is predicted to have accumulated in the soi l one year later (that is pasture age plus 1 ) . that is : Where: P(I + 1) - P(I) = G(1 - e -a(t+1) ) - G(1 - e -a. t ) A nnu al Pace. (I) = Ge -a.t (1 - e -a ) Annual Pace. is the amount of P which accumulates in the soi l in the year fol lowing P appl ication (kg/ha). (3) Equation 2 can be used to determine the relationsh ip between pasture age and each of the soi l P pools, shown in the s imple d iagram of the P cycle presented in F igure 8. 1 , and the total soi l P accumulation. Equation 3 can then be used to predict the amount of P accumulation into each soi l P pool for the year fol lowing each P appl ication. A non-l inear least squares regression procedure using the stat istics package SAS was used to determine values for G and a for the relationsh ip between pasture age and each of the soi l P pools and total P accumulation (Equation 2) . These relationsh ips are presented graphical ly along with their 95% confidence intervals (dotted l ines). Data col lected from the additional s ites in spring 1 993 was then used to val idate each model . The use of pasture age as a parameter in a model pred ict ing the fate of appl ied P presents several problems. information is not always Accurate development h istory, site and soi l type avai lable to consultants making fert i l iser P recommendations for farmers. A P saturation factor, as d iscussed in chapter 6, may be a more appropriate indicator of the soi ls abi l ity to retain , and hence lose, added P. The relationsh ips of P accumulation and runoff loss with pasture age are l ikely to be site and soi l specific and the need for accurate s ite i nformation would be bypassed by the use of a soil test for P saturation rather than pasture age. 228 8.4.2 Runoff P Losses The fol lowing model can be used to pred ict the amount of P which wou ld be lost from the root zone (0-7 .5 cm) via runoff in the year fol lowing P fert i l iser appl ication, that is Phosphorus in Runoff in H igh Loss Soils (PR IHLS) : Annu al P Runoff (t) = Applied P -(A nnu al Ptace. (t) + (.�'R x ALF)) (4) Where: P Runoff is the amount of P lost in runoff water in the year fol lowing P appl ication (kg/ha). Applied P is the amount of fert i l iser P appl ied in that 1 year period (kg P/ha) . Annua/ Place. is the amount of total P wh ich accumulates in the soi l in the year fol lowing P appl ication (kg/ha) . SR is the stocking rate (Stock Units/ha, where 1 SU consumes 550 kg DM/ha/yr) . ALF is the animal loss factor which is the amount of P lost v ia excretal transfer and animal products per stock unit (kg P/SU) . Values for the ALF can be used from either the CFAS or Outlook model . Some of the runoff P lost from the root zone may accumulate in the soi l profi le below 7 .5 cm. However, much of the P which was unaccounted for was l i kely to have been lost from the soi l altogether (chapter 3) . 8.5 P REDICTING P ACCU M U LATION IN EACH SOIL P POOL A relationship could be establ ished between pasture age and avai lable Pi (AvPi) , organic P (Po) , and strongly sorbed/precipitated and residual P (SsprP) accumulation in the top 7 .5 cm, (Figure 8 .2) . The models for predicting avai lable Pi and organic P accounted for 90 and 96% of the variation in accumulated P with pasture age, respectively. However, the model used to predict the sum of strongly 60 so Ii' .c 40 ii: CI 30 .... c: .2 20 � � E 10 � u u c( 0 Q. -10 260 240 220 Ii' 200 .c 180 0, .... 160 c: .2 140 � 120 � E 100 � 0 u 80 c( Q. 60 40 20 0 a) Available PI , , , x x ! A vPi(t) = 44.39(1 - e -0.1 us I) r 2 = 0.90 x o 4 8 12 16 20 24 28 32 36 Pasture Age (years) c) Calcium Pi x x x 0 4 8 12 16 20 24 28 32 36 Pasture Age (years) b) Organic P 240 200 Ii' � � � � - - - - - ! - x .c 160 0, � c: 120 .2 , x � E 80 � x U U c( 40 ' Q. o port) = 195.44(1 _ e -0.0581 t) r 2 = 0. 96 o 4 8 12 16 20 24 28 32 36 Pasture Age (years) d) Strongly Sorbed/Precipitated 100 and Residual P 80 .� 40 � � 20 � o o c( 0 Q. -20 x " SsprP(t) = 60.33(1 - e -0.0581 I) r 2 = O. 72 x " o 4 8 12 16 20 24 28 32 36 Pasture Age (years) Figure 8 .2 Effect of pasture age on pred icted and measured a) Avai lable Pi , b) Organic P, c) Calcium Pi and d) Strong ly sorbed and precipitated and residual P accumu lation in the top 7 .5 cm of a Wharekohe s i lt loam. x -P accumu lation in orig inal s ites sampled i n 1 990 and 1 993 from which each model was derived . ------ P accumu lation in top 7 .5 cm predicted by models. - - - - 95% confidence interva ls for pred icted P accumulation values. • -P accumulation measured in add itional sites sampled in 1 993 . Vertica l l ines = standard errors . 230 sorbed/precipitated and residual P only accounted for 72% of the variation encountered, and the 95% confidence intervals for the fitted curve were much wider than for the other two P pools. The models for both avai lable Pi and strongly sorbed/precipitated and residual P show a rapid in it ial increase in accumulated P , but the rate of increase slows so that the pred icted value for avai lable Pi and strongly sorbed/precipitated and residual P are close to maximum by 1 1 years. Organ ic P , on the hand, a lso increased rapidly with pasture age but continued to accumulate beyond 1 1 years, as the rate of increase in organic P decl ined more slowly compared to the other two P pools . The lack of data for pastures younger than 8 years l imits confidence in the models for predict ing P accumulation in the period up to 8 years. It is possible that avai lable P i and strongly sorbed/precipitated and residual P pools are much closer to their maximum values earl ier than the models ind icate. If this was so, P accumulation wou ld be underestimated by the models during this period. It was not possible to model a relationship between pasture age and calcium P as calcium P accumulation is effected more by l iming history than pasture age (chapter 4) . Calcium-P can be expected to show a dramatic increase where large quantities of l ime are applied during the in it ial stages of pasture development. However, where l ime appl ication is not continued at high rates, l ittle further Ca-P is expected to accumulate apart from unreactive fert i l iser P residues, and P derived from Ca-P may even be leached from the soi l with the Ca over t ime result ing in a possible net depletion in Ca-P in older sites. The data col lected from the additional chronosequence sampl ing to 7 .5 cm in spring 1 993 was used to va l idate the models. The observed values for each of the accumulated soi l P pools measured fel l with in the 95% confidence intervals for the models, despite the use of RPR in the past 3 years on the addit ional s ites. The use of RPR compared to superphosphate can lower avai lable P i as measured by bicarbonate P (Olsen P) on many soi ls (Mackay et a I . , 1 984; Saggar et a I . , 1 992 ; Roberts et a I . , 1 994) . Therefore, the models can be used to predict the accumulation of appl ied P under pasture on a Wharekohe si lt loam into the avai lable 2 3 1 Pi , organic P and strongly sorbed/precipitated and residual P pools with some confidence. The val idation data was measured on sites located on the Kaikohe Research station where the samples used to derive the models were taken. It wou ld have been preferable to val idate the models with data col lected from other areas contain ing Wharekohe soi ls . However, the col lection of further val idation data was restricted by the lack of knowledge of development history at other sites. It is l ikely, due to the very low capacity to retain added P and l ittle natural variabi l ity in the P chemistry of the topsoi l of Wharekohe si l t loams, that P accumulation on other sites would fol low a simi lar pattern under s imi lar cl imatic cond itions. The models to predict the accumulation of P in the root zone into the 3 soil P pools for wh ich relationsh ips with pasture age cou ld be establ ished in the year fol lowing P appl ication for a given pasture age are: A nnual A vPiacc. (t) = 44 e -0.2145 t (1 - e -0.02].15 ) A nnual pOaccJt) = 1 95 e -0.0581 t (1 - e -0.0581 ) A n nual s..,pr Pace. (t) = 60 e -0. 1556 t (1 - e -0. 1556 ) 8.6 P REDICTI NG TOTAL P ACC U M U LATION ( 5 ) ( 6 ) ( 7 ) A relationsh ip between pasture age and total P accumulation in the root zone (0-7 . 5 cm) cou ld be effect ively model led (F igure 8 . 3 ) . The model was able to account for 90% of the variation in total P encountered on the orig inal sites. The total P values for the val idation data measured on the 1 993 addit ional sites fel l within the 95% confidence interva ls for the model , despite the appl ication of P solely as RPR in the preceding three years on the additional sites. The appl ication of RPR is l ikely to have resulted in the greater accumulation of P as Ca-P than would be expected under more soluble P fert i l iser. The model pred icted a maximum accumulation of 380 kg P/ha on this Wharekohe si l t loam which is very low compared to other New 232 Zealand pastoral soi ls (Saunders, 1 959a; Walker et aI . , 1 959; Nguyen and Goh, 1 992) . 600 500 ,,- 400 / I - I n:s .r:. I - 300 C) � I - c:: I 0 :; 200 I n:s '3 I E ,,-:::s / (,) I (,) 1 00 « I D. I I 0 I I I - 100 o 4 8 x i - - ..- ./ Pt(t) = 380(1 - e -0. 1881 1) r 2 = 0. 90 1 2 1 6 20 24 Pasture Age (years) i X - - - - - • X X - - - - - 28 32 36 Figure 8 .3 Effect of pasture age on predicted and measured total P accumulation in the top 7 .5 cm of a Wharekohe si l t loam. X Total P accumulation in orig inal sites sampled in 1 990 and 1 993 from which model was derived . - Total P accumulation in top 7 .5 cm pred icted by the model . - - - - 95% confidence intervals for pred icted values for total P accumulation . • Total P accumulation measured in additional sites sampled in 1 993. Vertical l i nes = standard errors. 233 The fol lowing equation can then be used to model the accumulation of total P in the root zone (0-7 . 5 cm) in the year fol lowing P appl ication for a given pasture age: A nnual PtaccJt) = 380 e -0. 188J t (1 - e -O. 1881 ) ( 8 ) This model can then be incorporated into models for the prediction of runoff P losses (B .7) and fert i l iser P requirements (B. B) . 8.7 PREDICTI NG RUNOFF P LOSS The model presented in equation B pred icts total soil P accumulation for an average year. In drier than average years, less P is l ikely to be lost from the soi l via runoff and hence annual P accumulation wi l l be greater than predicted . However, it is possible that any extra P surplus to predicted P accumulation wi l l be only loosely held in the soi l and therefore may be susceptible to runoff P loss in future wetter than average years. Annual P losses in runoff from the root zone of a Wharekohe si lt loam can be predicted for a g iven pasture age from the PRIHLS model as fol lows: A nnual P Runolf = Applied P - 380 e -0. 1881 age(l _ e -0 . . 1881) - (SR x A LF) ( 9 ) 8.7. 1 Validation of the PRI H LS Model The 95% confidence intervals for pred icted P runoff loss for each of the additional sites were calculated from the PRIHLS model (equation 9 ) using Annual Ptacc. values ca lcu lated for the 95% confidence intervals of equation B and an ALF of 0 . 5 from the CFAS model (F igure B.4) . The estimated cumulative runoff P losses from each site from pasture development are represented in figure B.4 by the sol id circles. Runoff P loss increased with pasture age as P was appl ied at rates surplus to animal loss and soi l P accumulation at each of the additional sites. The 95% confidence intervals for predicted runoff P loss at each site were wide, approximately 250 kg of P . Hence, the PRIHLS model cannot accurately predict runoff P loss but can be 234 used to gain an estimate of runoff P loss for a particular P app l ication rate at a given pasture age and a comparison of runoff losses between d ifferent app l ication rate strategies. 700 600 500 --. 400 ro ..c: - 0> � -- a.. 300 � 0 c: � � 200 1 00 + • + + • • • Measured P Runoff Losses + Predicted P Runoff Losses o 95% confidence intervals for predicted P runoff losses. o 4 8 1 2 1 6 20 24 Pasture Age (years) 28 32 F igure 8 .4 Predicted and measured P runoff losses for add it ional sites sampled in 1 993. 36 Runoff P (Chapter 5) and P retention (Chapter 6) measurements also supported runoff P losses where P is appl ied at rates surplus to animal loss and soi l P accumulation. During the period spring 1 990 to spring 1 993, no P was estimated to have been lost from the youngest site developed in 1 982 ( 1 1 years in 1 993). In contrast, 91 .2 and 94.6 kg P/ha were unaccounted for by either est imated animal loss or accumulation in the top 7 .5 cm of the original sites developed in 1 958 and 1 960, respectively (35 and 33 years old in 1 993). During this period, there was a 235 surplus of 1 660 mm of rain to evapo-transpiration. Therefore, the average P load ing of runoff waters from the top 7 .5 cm was estimated to be 5.5 to 5.7 I-Ig P/ml over the 3 years in the oldest developed sites. A P concentration of 5 .5 I-Ig P/ml is extremely high for an average P load ing of runoff waters from pastoral soi ls . Phosphorus concentrations as high as 1 .94 I-Ig/ml and 1 . 8 I-Ig P/ml have been reported in surface runoff from a Tokomaru s i lt loam (Sharpley and Syers, 1 979a) and in catchment runoff (surface plus subsurface runoff) from a freshly developed catchment in North land which included Wharekohe soi ls (McCol l et aI . , 1 975) , respect ively, where approximately 50 kg P/ha/yr had been applied. Substant ial ly lower average P loadings in runoff waters have been recorded from other New Zealand soi ls (McCol l et a I . , 1 977; Lee et aI . , 1 979; Sharpley and Syers, 1 979a&b; Lambert et aI . , 1 985). However, the measurement of very high D IP concentrations in runoff waters col lected from intact soi l cores in the glasshouse study (Chapter 5), up to 45.6 I-Ig/ml , and suction cups in the field, 1 8 .65 I-Ig/m l , where 1 00 kg P/ha was appl ied in both studies, support high average P loadings in runoff waters from Wharekohe soi ls where P surplus to production requirements and soi l P accumulation is applied. In the P retention study (Chapter 6) the amount of P retained by the soi l sampled from the 1 1 year old pasture was approximately five times greater than the amount of P retained by the soi l sampled from the 35 year pasture, at a final solution P concentrat ion of 5 .5 I-Ig/ml . This serves to support the greater loss of P in runoff from the Wharekohe si l t loam under older in comparison to younger pastures. 8.7.2 Effect of Ferti l iser and Stocking Rates on Runoff P Losses The model can be used to examine the effect of P ferti l iser and stocking rates on the amount of soi l P accumulation and runoff P losses for the Wharekohe si l t loam at d ifferent stages of development. Predicted total P accumulation (from equation 8) and runoff P losses (from the PRIHLS model , equation 9) with increasing pasture age for a Wharekohe si l t loam are presented for two P appl ication rates (40 and 50 kg P/ha) and two stocking regimes ( intensive sheep and dairy) in figure 8 .5 . The P 236 appl ication reg imes included capital appl ications of fert i l iser P for 6 years (as outl ined in F igure 8 .5) fol lowed by the constant appl ication of either 40 or 50 kg P/ha/yr. The capital appl ication rates approximate rates commonly used on developed pastures on the Wharekohe podzols. 70 60 50 - rn 40 .c - Ol � - c.. 30 20 1 0 o X Predicted Annual Soil P Accumulation in top 7.5 em Predicted Annual P Runoff from top 7.5 em for a Sheep Farm Predicted Annual P Runoff from top 7.5 em for a Dairy Farm - - - - - - - - - - - - - '" '" 1 0 20 30 Pasture Age (years) 40 50 kg P/ha 50 kg P/ha 40 kg P/ha 40 kg P/ha F igure 8 .5 Effect of P appl ication rate and stocking type on annual soi l P accumulation and runoff P loss from the top 7 . 5 cm of a Wharekohe s i lt loam. Capital P appl ications = 90 (yr1 ) , 70 (yr2) , 60 (yr 3-6) kg P/ha . SR = 1 5 .6 (Sheep and dairy farm) , ALF for sheep farm = 0.5, ALF for dairy farm = 0.9 . The annual P runoff loss varied in it ial ly whi le chang ing capital P appl ications were made. C learly, where P is appl ied at a constant rate, the amount of P lost from the 237 soi l in runoff increases with pasture age as the amount of P which accumulates in the soi l decreases (F igure 8 . 5 ) . The rate of increase in P runoff decl ines over time approach ing an asymptote as the accumulation of P in the soi l approaches O. Obviously, the more P which is appl ied, the greater the pred icted P runoff losses. Runoff P losses are expected to be lower from a dairy farm than from an intensive sheep farm at a given P appl ication rate, due to higher loss from the pasture cycle via animals on the dairy farm. Hence, less P is appl ied that is surplus to animal requirements, and soi l P accumulation, and therefore susceptible to loss in runoff. The CFAS model ca lculates that 2 1 kg P/ha is required to maintain 1 5 . 6 SU (sheep) on the rol l ing topography of the Kaikohe Research Station with an Olsen P level of 20 (SLF=OAO). Approximately 1 3 kg of this appl ied P is predicted to be lost from an older pasture ( > 30 years) in runoff. The newer Outlook model , currently used for P fert i l iser recommendations in New Zealand, recommends a very much larger appl ication of 44 kg P/ha/yr to maintain the same level of production on the Kaikohe station. Predicted runoff P losses from an older pasture (>30 years) on a Wharekohe si l t loam where 44 kg P/ha is appl ied annual ly are substantial , 36 kg P/ha. The appl ication of such large quantities of P, which result from the high soi l loss parameter assigned to the podzols in the Outlook model , is h ighly questionable as gains in pasture production are unl ikely to outweigh this extremely inefficient use of fert i l iser P and its loss to the wider envi ronment. The PRIHLS model presented in this chapter can be used to predict soil P accumulation and runoff P losses when appl ied P is greater than the sum of the animal losses and soil P accumulation. The appl ication of P at rates below the sum of animal losses and total soi l P accumulation is l ikely to result in both a drop in production and the amount of P wh ich accumulates in the soi l . Runoff P losses are also l i kely to be reduced but are not l ikely to be stopped altogether, part icularly where soluble P is appl ied in one annual dressing. As P accumulation rates are proportional to the amount of P which has already accumulated in the soi l (wh ich is now lower than predicted due to inadequate P appl ication) , the effect of pasture age on P accumulation wi l l be delayed. Once P appl ication rates are returned to required levels, P wi l l accumulate in greater quantities than would be predicted by the model for that pasture age. 8.8 USING THE PRI H LS MODEL TO PREDICT P REQUIREMENTS 238 The PRIHLS model could also be used to pred ict the amount of P required to maintain pasture production, for a given fert i l iser P appl ication strategy, once minimum runoff P losses for that strategy have been determined. Further research would be requi red to investigate minimum runoff P losses under different P fert i l iser strategies such as the more frequent appl ication of soluble P fert i l iser or the use of slowly soluble P fert i l iser (Chapter 5) . It is h ighly un l ikely that runoff P losses can be reduced to negl ig ible levels in the Wharekohe soi l with such a low anion storage capacity. Even if P was appl ied at rates equal l ing animal losses, runoff P losses wou ld sti l l be expected, and hence pasture production would be l ikely to be reduced . Once min imum runoff P losses are determined under alternative P appl ication strategies, they cou ld be incorporated into the PRIHLS model to determine the P appl ication rates required to maintain production as fol lows: P Requirement = (A nnual P Runoff + 380 e -0.1881 age(J _ e _0.1881)) + (SR x ALF) ( 1 0) As runoff P wi " be dependent on pasture age and the amount of P appl ied, Annual P RUfl(�ff would be represented by an equation and P Requirement determined as the amount of appl ied P for which runoff losses are min imised but an adequate level of production maintained. Further research is required to determine the effects of runoff P losses on the environment in areas where Wharekohe soi ls are found so that an acceptable balance can be obtained between losses of P to the environment and agricultural production. In the Netherlands, where runoff P losses have a large negative impact on the environment, P appl ication levels are determined wh ich resu lt in acceptable P losses rather than optimum agricultura l production (Sharpley, 1 995). A P saturation factor could also be incorporated into the model . Phosphorus saturation may be a better indicator of the abi l ity of a soi l to retain P than pasture age as the results obtained from its use are less rel iant on accurate development history, site and soi l type information. 8.9 CONCLUSIONS 239 • Relationships between pasture age and avai lable P i , organic P , strongly sorbed/precipitated and residual P and total P accumulation in the top 7 . 5 cm of a Wharekohe s i lt loam, where P appl ication rates exceed soi l P accumulation rates, were successfu l ly model led. It was not possible to model a relationship between pasture age and calcium P as calcium P accumulation is effected more by l iming history than pasture age. The val idation data fel l with i n the 95% confidence interva ls of the models. • The annual soi l accumulation of total P in the year fol lowing P appl ication for a particular pasture age was described by a model which was then incorporated into a model to predict annual P runoff losses, the PRIHLS model . Phosphorus runoff losses were predicted as the sum of the soi l P accumulation and animal loss subtracted from the amount of appl ied P. Predicted runoff P losses increase with pasture age, where P is appl ied at a constant rate. The rate of increase decl ines over t ime as soi l P accumulation approaches zero. Runoff P losses wi l l be higher at higher P appl ication rates for a simi lar farming reg ime and on farms with lower animal losses per SU , such as sheep farms compared to dairy farms, for a s imi lar P appl ication rate and SR. • Predicted runoff losses from the Wharekohe si lt loam are nearly 3 t imes h igher from older pasture (>30 years) where the Out look model is used to calculate P requirements compared to the CFAS model , due to the higher soi l loss parameter assigned to the Wharekohe soi ls in the former. • Further research is required to quantify runoff P losses under alternative P appl ication strategies, where such losses are min imised, to determine the effects of runoff P losses on the environment and to investigate the use of a P saturation test as an alternative to the pasture age parameter. Once minimum runoff P losses have been determined they could be incorporated into the PRIHLS model to determine appropriate P appl ication rates to maintain production. The P requirement could be calcu lated as the sum of the min imum runoff P , soil P accumulation and animal P loss. Background CHAPTER 9 SUM MARY AND CONCLUSIONS 240 • The accumu lation of P over time under annual ly fert i l i sed permanent pasture can contribute to pasture growth so that eventual ly only mai ntenance appl ications of P are requ i red to bal ance any losses from the pasture P cycle and mainta i n pasture growth at the req u i red relative yield and stocking rate. Various models have been developed to pred ict P req u i rements for pastoral systems in New Zealand. D uring the 1 980's and early 1 990's, the then MAF SFS used the mass balance, Cornforth and S i nclair CFAS model, where P requirements are calcul ated to replace losses from the cycl ing P pool via animals and the soi l , to make ferti l iser P recommendations. • I n the late 1 980's, concerns were raised that h i g her P appl ication rates than those ca lculated by the C FAS model were necessary to maintain req u i red O l sen P levels on many Northland farming properties on Wharekohe podzol s . Consu ltant experience also indicated that to maintain a s i m i l ar Olsen P val ue, the Wharekohe sandy loam may requ i re h ig her P i nputs than the Wharekohe s i lt loam. The podzol soi l group covers 300 000 ha of the 1 . 26 m i l l ion ha in North land, with the Wharekohe s i lt and sandy loams the most widespread, covering approxi mately 60 000 ha, m uch of which is used for i ntensive pastoral farming. Pre l i m i nary i nvestigation on several farms, where the C FAS model was underesti mati ng the amount of P requ i red to maintain the req u i red soil O lsen P val ues on Wharekohe soi l s , i dentified the S L F as the parameter most l i kely leading to the inabil ity of the C FAS model to predict P requirements on these podzols. • Si nce the commencement of this PhD, the C FAS model has been replaced with the new model , Outlook, by the S F S . Outlook also uses a soil l oss parameter to estimate losses of P from the cycl ing P pool through non-labi le P accumulation and runoff P losses. H ence, an i nvestigation of soil P loss in Wharekohe podzols is also relevant to the use of the Outlook for predicting P requirements on these soils. 24 1 • The review of the l iterature revea led that podzol isation results in low sesquioxide contents and consequently low Anion Storage Capacities, and extremely low natural P contents in the A horizons of the more weathered podzols, such as the Wharekohe soi l . The largest proportion of the natural P found i n the A horizon of the most weathered podzols is in the labi le P pool with much smal ler quantities of strongly sorbed and precipitated Fe- and AI-Pi and neg l igible quantities of ca lcium P . Pasture development with P fert i l iser addition on podzols leads to P accumu lation wh ich i n it ial ly accumu lates as Po. Podzo l i c soi l s low in AI are expected to reach an equ i l ibri u m Po content which is lower and reached more rapidly than i n other soi l s with h igher AI content, wh ich have a greater capacity to stabi l i se Po compounds. The abi l ity of podzol s to retai n Pi is also reduced with increased P ferti l i ser app l i cation, as the few ava i l able P retention sites become occupied with the added P. Large losses of P have been recorded from the A horizons of New Zealand podzols under pasture and the potenti a l for subsurface P runoff losses exists. • The main objective of this thesis was to i nvestigate the apparent l i m itation of the CFAS model to predict the maintenance P requirements of the Wharekohe soi ls , and the appropriateness of the soi l loss parameter used in the new Outlook mode l , further by (a) determ i n i ng the fate of appl ied fert i l iser P, (b) exa m i n i ng possible mechanisms for any soil P retention or loss, (c) quantifying the amount of P lost from the pasture P cycle via the soi l ( S L F ) and (d) model l i ng the fate of appl ied fert i l iser P. Determining the Fate of Applied Fertiliser P • A chro nosequence study was conducted to determ i n e the fate of appl ied P i n a Wharekohe s i lt loam u n der pastures of d ifferent age. Pasture development and the associated a p p l i cation of fert i l i ser P on Wharekohe podzo l s resu lted in an i n crease in tota l s o i l P to the top of the E horizon with appl ied P accu m u lating to a h i gher concentration i n the top 3 cm . E v i dence has been presented w h i ch s u p po rts an i n crease i n the movement of appl ied P down the profi l e with i ncreasing pasture age. The Wharekohe s i lt loam top so i l appears to have a fi n ite a b i l ity to ret a i n P ( referred 242 to as the max i m u m P storage capacity) , wh i ch is reached by 8 years in the 0-3 cm depth and by 1 1 years in the 0-7 . 5 cm dept h . • T h e maxi m u m P storage capacity ca n most l y be attri buted to a m ax i m u m P i storage capacity. C a l ci u m-Pi extracted by H 2 S 04 was the l a rg est fract i o n in the top 7 . 5 cm of a l l the devel oped sites. The h i g h proport i o n of P as Ca-P was due to a l i m i t on the accu m u l at i o n of P associ ated with Fe and AI i n the Wharekohe soi l , due to its l ow sesq u i oxide content i n comparison to other N ew Zea land soils. The Wharekohe s i lt loam contained a l a rge quant ity of read i l y ava i l ab l e Pi of wh ich a la rge amount was very wea k l y h e l d i n the s o i l ( extracta b l e by N H4C I ) , and therefore prone to leach i n g i n comparison to P held i n other less weathered s o i l s . I n contrast to P i , P o conti n ued t o accu m u l ate over t i m e t o a t l east 3 2 years a t each depth to levels s i m i l a r to those recorded by other researchers in a less weathered South I s l a n d ye l l ow g rey earth . The rate of Po accu m u lation s l owed with age in the top 3 cm with the most l a b i l e Po fraction extracted by N H4C I reach i n g eq u i l i brium by 2 5 years. • Once the P storage capacity at each depth is reached , there is l itt le further accu m u l at i o n of appl ied P, apart from sma l l quantit ies as fert i l i ser P res i due. Much of the P a p p l ied in su bseq uent appl icat ions moves from the topso i l resu l t i ng in large so i l l osses of P from the pasture cycl e, of u p to 65% from the top 7 . 5 cm of o l der sites ( >30 years ) . As l itt l e of the l ost P cou l d be accou nted for to a depth of 30 cm below the E h orizo n , l atera l movement of P in runoff waters above the E horizon is the most l i ke l y pathway for P loss from the A horizo n . It was not poss i b l e to determ i n e in wh ich form P is m ov i n g through the profi l e of the Wharekohe s i lt loam from the chronosequence study, due to possi b l e changes i n P forms i n situ. Possible Mechanisms for Soil P Retention and Loss • A g l asshouse leach i ng study u s i n g i ntact soi l cores demonstrated that su bstantial quantit ies of P can be transported i n subsurface water movement through Wharekohe podzo ls (:::; 4 5 . 6 I-Ig/m l ) i n contrast to the y e l l ow brown eart h , Aponga 243 clay (� 1 . 07 IJg/m l ) . Movement of d i ssolved P occurs mostly as D I P after the appl ication of fert i l iser P. The levels of P wh ich move through Wharekohe podzo ls are so high, that it i s l i ke ly that they contri bute s i g n ificant q uantit ies of P to waterways creating the potential for eutro p h i cati o n . Add i t i o n a l l y the l osses may represent a l arge eco n o m i c cost to the producer. Fert i l iser a p p l i cation strateg ies a i med at m i n i m i s i n g P l osses were suggested and i ncluded the more frequent app l i cation of s m a l l quantit ies of soluble P fert i l iser or a lternative l y the use of s l owly s o l u b l e P such as react ive phosphate rock ( R P R ) . • N o d ifference i n P movement cou l d b e detected i n re lation t o development h i story in the g l asshouse l each i n g study or in a fi e l d study where P in soi l water was sampled u s i n g porous cera m i c cups under suct i o n . The younger s i te may have been close to P saturation and the rate of P appl ication was most l i ke l y too h i g h to detect the s m a l l d ifferences i n P retent ion recorded between the soi l s in a l a boratory P retention study. The a b i l ity of the Wharekohe s i l t loam to reta i n added fert i l i ser P was fou n d to decl i n e with pasture deve lopment, as the P retention s i tes become fi l l ed with appl ied P . Quantifying the SLF • S o i l loss factors ca lculated from sma l l -p l ot fi e l d tr ials varied enormously as a consequence of the large variation in the rate of P req u i red to m a i nt a i n a steady O l sen P level at each site , from 0 . 04 in the fi rst year of the tri a l to 1 . 68 over the two year tri al period on the Wharekohe s i lt loam sites. F rom the s m a l l -p lot fie l d tr ial data it was not pos s i b l e to determ i n e if the S L F of 0.4 used for podzol s in the C FAS model was ap propriate, due to l a rge vari ation in O l sen P leve l s . The ca l c u l ation of a so i l l oss parameter, whose re l i ance i s p l aced on the a b i l ity of the h i g h ly vari a b l e O l sen P test t o determ i n e soi l P l osses i s quest i o n a b l e . • T h e i n it i a l fert i l i ty o f a Wharekohe trial site had a l arge i m pact on the ca lcul ated S L F values, with l a rger val ues determ i ned for a l ower fert i l ity s i te . The ca lcul ated S L F va l ues were h i gher on the Wharekohe sandy loam site than on the Wharekohe s i l t loam s ites, reflect i n g that P is l ess strongly reta i ned in the sandy l o a m . Th i s 244 concl u s i o n i s supported by the fi n d i ngs of the P fractionation a n d laboratory P retention stud i es, but not by the g l asshouse l each i n g study where no d ifferences were found between the s i l t and sandy loams. S L F val ues ca lculated from the change in R e s i n P u nder appl ied S P R ( 0 . 25 ) were substa n t i a l l y l ower than those calcu l ated from the change in O l sen P under a p p l i e d M C P ( 1 . 1 0 ) for the two year tr ial peri o d . A longer tr ial period wou l d be req u i red to determ i n e if the so i l l oss determ i ned from changes in R e s i n P reflects so i l l osses u nder S P R . • The co mponent of the S L F due to non- l a b i l e P accu m u lation ( S L FsPA ) , ca lcul ated from the chronosequence data , decreased with pasture age. As P a p p l i ed s u rp l u s to a n i m a l production req u i rements and P accu m u l at ion is l ost from the root zone i n water runoff, t h e S L F s h o u l d be reduced w i t h i ncrea s i n g pasture a g e or e l se P runoff l osses wi l l i ncrease. U nti l the m i n i m u m runoff P l osses for various fert i l i ser strateg ies are identifi e d , it i s not poss i b l e to determ i n e if the C FAS S L F of 0.4 underesti mates so i l loss o n the older devel oped Wharekohe s i tes i n the chronosequence study. It appears, however that the SLF appropriate for younger pastures « 1 1 years) on the Wharekohe silt loam has been u nderesti mated , as the S L FsPA val ues were g reater than 0 . 4 and runoff P l osses were n ot even consi dered. In order to d eterm i n e the SLF appropri ate for a particu l a r pasture age, the m i n i mum runoff l osses wh ich are unavoidable for a g iven fert i l i ser P strategy m ust be determi ned and i ncluded in the S L F . Model l ing t h e Fate of Applied Ferti l iser P • Relationsh i ps between pasture age and ava i l a b l e P i , org a n i c P , stro n g l y sorbed/precip itated and res idual P and tota l P accu m u l at ion i n the t o p 7 . 5 c m o f a Wharekohe s i l t l oam were successfu l l y model led, where P app l i cation rates exceed so i l P accu m u l ation rates. It was not poss i b l e to model a re l at i on s h i p between ca l c i u m P and pasture age as ca l c i u m P accu m u l ation is effected more by l i m i ng h i story than pasture age. • The annual tota l soi l P accu m u lation i n the year fo l l owi n g P appl ication 245 for a part i cu l ar pasture age was described by a model wh ich was then i ncorporated i nto the P hosphorus in Runoff in H i g h Loss S o i l s ( P R I H L S ) model d evel oped to ,.,......, pred i ct runoff P loss s. P hosphorus runoff l osses can be pred i cted by the P R I H LS mod e l , for a known rate of appl ied P , as the s u m of the a n n u a l total soi l P accu m u lation and a n i m a l loss, su btracted from the amount of a p p l ied P . P redicted runoff P l osses i n crease with pasture age, where P is a p p l i ed at a constant a n n u a l rate. The rate o f i n crease in runoff P l oss decl i nes o v e r t i me as so i l P accu m u l ation approaches zero. Runoff P l osses wi l l be h i gher at h i gher P appl icati o n rates for a s i m i lar farm i n g reg i m e , and on farms with lower a n i m a l l osses per S U , such as sheep farms com pared to d a i ry fa rms , for a s i m i l a r P a p p l i cation rate and S R . The P R I H L S model cou l d a l so be used to pred i ct the a m o u nt of P req u i red to m a i ntain pasture product i o n , for a g i ven fert i l i ser P app l i cation strategy, once m i n i mu m runoff P l osses for that strategy have been determ i ned. • The use of the current so i l P loss parameters in the C FAS, and part i cu larly the new Outlook mode l , i s l i ke l y to lead to very h i g h runoff P l osses from o l der pastures on the Wharekohe s i lt l o a m . P red icted runoff losses from the Wharekohe s i l t l o a m are n e a r l y 3 t i m es h i gher from older pasture ( > 30 years) where the Outl ook m o d e l i s used t o ca lcu l ate P req u i rements compared t o t h e C FAS mode l , d ue to t h e h i gher so i l l oss parameter assig ned to the Wharekohe so i l s in the former mode l . It is i m portant that soil l oss parameters used the new Outlook model reflect the decrease i n non-l a b i l e soi l P accu m u l at ion with i ncreasing pasture age in association with i mproved P app l i cation tech n i q ues on the podzol so i l s to m i n i m i se costs to the farmer and the e n v i ron ment. Suggestions for Further Research • Losses of P from the root zone of older pastures on Wharekohe podzo l s are h i g h under current fert i l i ser a p p l i cation strate g i es where an a n n u a l pred o m i nantly s o l u b l e P dress i ng i s a p p l i e d . Therefore, fi eld stu d i e s wh ich q u a ntify runoff P l osses from a ltern ative P fert i l i ser strategies ai med at m i n i m i s i ng such losses, wh ich i n vestigate app l i cat i o n frequ ency, appl i cation rate and P fert i l iser form are req u i red. The 246 P R I H LS model cou l d then be further deve l oped to i nclude a fun ct i o n wh ich describes m i n i m u m ru n off P l osses for a range of P fert i l iser strategies in order to pred ict P req u i rements for m a i nta i n i n g pasture prod uct i o n . S u ch runoff P stu d i es s h o u l d a l so i nvestigate the relative roles of subsurface and surface runoff i n P loss, as l osses of s l owly so l u b l e P may st i l l be h i g h if surface eros i o n of part i cu l ate P p l ays a major ro l e in P l oss to the wider environment. • A phosphorus saturation test may be a better i nd i cator of the a b i l ity of a soil to ret a i n P than pasture age, as the resu lts obta i ned from its use are less rel i ant on accurate deve lopment h i story , site and so i l type i nformat i o n . Consequently, further research is req u i red to determ i n e the relat ionsh i p between P satu ration and runoff P l osses i n order to determ i n e appropriate P saturation leve l s to restrict P losses from the podzo l s by red u c i n g appl i cation rates and/or u s i n g a lternative a p p l i cation strateg ies. • The environ m ental i m pacts of runoff P l osses need to be esta b l i shed for areas where Wharekohe soi ls are found to enable more i nformed deci s i o ns to be made about bala nced fert i l iser use. This wou l d a l l ow the P R I H LS model to be further developed to m a i nt a i n an acceptable b a l ance betwee n l osses of P to the environment and the need for m a i nta i n i ng a h i g h level of agricultural prod uction for pastures of d ifferent age. 247 REFERENCES Adams, J .A and Walker, T.W. 1 97 5 . Some properties of a chronosequence of soi ls from granite in New Zealand. 2 . Forms and amounts of phosphorus. Geoderma 1 3 : 4 1 -5 1 . 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United States Department of Agriculture, Soil Conservation Service, Agriculture handbook 436. 754 p . Sparl ing, G. P . , Whale, K. N . and Ramsay, A. J . 1 985. Quantifying the contribution from the soil m icrobial biomass to the extractable P levels of fresh and a ir-dried soi ls. Australian Journal of Soil Research 23: 61 3-621 . Steele, K. W. 1 976. Effect of added phosphorus on the avai labi l ity and forms of phosphorus present in two soi ls of the Manawatu-Rangitikei sand country. New Zealand Journal of Agricultural Research 1 9: 433-439. St-P ierre, N .R and Scobie, G .M. 1 987a. Economics of phosphorus use on pastures. 3. Incorporating animal response. New Zealand Journal of Experimental Agriculture 1 5: 453-462. St-Pierre, N .R and Scobie, G .M. 1 987b. Economics of phosphorus use on pastures. 4. Incorporating risk. New Zealand Journal of Experimental Agriculture 1 5: 463-475. Stoop, W.A. 1 983. 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Effect of pH on phosphate adsorption and isotopic exchange in acid soi ls at low and high additions of soluble P . Journal of Soil Science 2 8 : 48-6 1 . White, RE . , Tiffin , L .O . and Taylor, AW. 1 976. The existence of polymeric complexes in d i lute solutions of a luminium and orthophosphate. Plant and Soil 45 : 52 1 -529. 273 Wilcock, RJ . 1 986. Agricultural run-off: a source of water pol lution in New Zealand. New Zealand Agricultural Science 20: 98-1 03. Wi l l iams, E .G . and Saunders, W.M .H . 1 956. Distribution of phosphorus in profi les and particle-size fractions of some Scottish soi ls . Journal of Soil Science 7: 90-1 08. Wi l l iams, E .G . , Scott, N .M . and McDonald, M .J . 1 958. Soil properties and phosphate sorption. Journal of Science and Food in Agriculture 9: 551 -559. Wi l l iams, J .D .H . and Walker, TW. 1 969a. Fractionation of phosphate in a maturity sequence of New Zealand basaltic soi l profi les: 1 . Soil Science 1 07 : 22-30. Wi l l iams, J .D .H . and Walker, TW. 1 969b. Fractionation of phosphate in a maturity sequence of New Zealand basaltic soil profi les: 2. Soil Science 1 07 : 2 1 3-2 1 9. Wi l l iams, J .D .H . , Syers, J .K. and Walker, TW. 1 967. Fractionation of soi l i norganic phosphate by a modification of Chang and Jackson's procedure. Soil Science Society of America Proceedings 31 : 736-739. Woodruff, J .R and Kamprath, E .J . 1 965. Phosphorus adsorption maximum as measured by the Langmuir isotherm and its relationship to phosphorus avai labi l ity. Soil Science SOCiety of America Proceedings 29: 1 48-1 50. Wright, D .N . 1 975. Calcined Christmas Island C-grade rock phosphate as a fert i l izer. Australian Journal of Experimental Agriculture and Animal Husbandry 1 5: 41 9-423 . Yeates, J . S. 1 993. Soi ls and fert i l izer use in southwestern Austral ia . Fertilizer Research 36: 1 23-1 25. Yeates, J .S . and C larke, M .F . 1 993. Developing alternatives to phosphate fert i l izers of high water solubi l ity. Fertilizer Research 36: 1 4 1 -1 50. Yeates, J .S . , Deeley, D .M. , Clarke, M .F . and Allen, D . 1 984. Modifying fert i l iser practices. Journal of Agriculture Western Australia 3(4th series) : 87-91 . Yeates, J . S . , Deeley, D .M . , Cocketton , G .TB. and Allen, D. 1 986. Influence of l iming on the effectiveness of apatitic and calcined crandal ite rock phosphates for subterranean clover growth and phosphorus leaching losses of an acid sand. In Surface Soil Management pp. 1 1 0-1 1 6. (New Zealand Society of Soi l Science and Austral ian Society of Soil Science: Rotorua, New Zealand . ) 274 GLOSSARY OF TERMS Anion Storage Capacity - percentage of P removed from a solution of NaOAc-HAc contain ing 1 000 �g P/ml buffered at pH 4 . 6 by soi l shaking at a 1 : 5 soi l : solution ratio for 24 hours (Saunders, 1 96 5 ) . Soi l to P ratio is therefore 1 g of soil to 5000 �g P. Previously known as the P Retent ion Test. Maximum P Storage Capacity - maximum amount of added P which can be retained by the soi l in the field by such processes as P sorption , P precipitation and organic P immobi l isat ion. P Sorption - refers to both adsorption (sorption on the surface of soi l particles) and absorption ( incorporat ion into soi l particles). P Retention - refers to the retention of added inorganic P by the soi l through both P sorption and precipitation Runoff P Loss - the loss of P from the topsoi l in both surface and subsurface runoff waters . Subsurface Runoff - the movement of water beneath the soi l surface including both lateral movement (across the top of the pan in the case of the Wharekohe soils) and deeper percolation to the B horizon (considered to be l im ited in the Wharekohe soi ls) . Subsurface waters may reach the surface and be measured as surface runoff and vice­ versa. Surface Runoff - the movement of water across the soi l surface. Surface runoff wi l l often penetrate the soi l to a depth of at least 1 cm as the water moves across the surface in waves. 275 Appen dix 3 . 1 Table of log transformed total P concentration values depth 0-3 cm 3-7.5 cm 7.5 cm-E E E-1 0 -l-E 20-30 ,j,E Age 0 4.294 3 .81 6 3.606 3 . 1 70 2 .074 2 . 584 8 6 .738 5.479 4 .359 3 .276 2 .870 2 .783 23 6 .660 6 .297 5.640 3 .837 4 . 1 1 9 4 .368 25 6 .772 6 .263 5.539 3 .487 2 .855 3. 585 30 6 .533 6 .090 5.437 3 .552 3 .981 4 . 1 68 32 6 .898 6.432 4.744 3 . 1 90 2.445 2 . 1 75 S .E .D.s for comparison of log transformed total P concentration values at each depth of the undeveloped site with other depths at a l l sites. Age Depth 0-3 cm 3-7 .5 cm 7.5 cm-E E E-1 0 ,j,E 20-30 -l-E 0 0-3 cm * * * * * * 0 3-7 .5 cm 0 . 1 226 * * * * * 0 7 .5 cm-E 0 . 1 226 0 . 1 226 * * * * 0 E 0 . 1 548 0 . 1 548 0 . 1 548 * * * 0 E-1 0 ..i-E 0. 1 723 0 . 1 723 0 . 1 723 0 . 1 892 * * 0 20-30 ..i-E 0.2028 0.2028 0.2028 0 .21 74 0.2335 * 8 0-3 cm 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0 .21 58 0.2409 8 3-7 .5 cm 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0 .21 58 0.2409 8 7 .5 cm-E 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0.21 58 0.2409 8 E 0.2231 0.2231 0.2231 0.2422 0.2537 0 .2754 8 E- 1 0 ..i-E 0.2231 0 .2231 0.2231 0.2422 0 .2537 0 .2754 8 20-30 ..i-E 0.2231 0 .2231 0.2231 0.2422 0.2537 0.2754 23 0-3 cm 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0.21 58 0 .2409 23 3-7 .5 em 0 . 1 787 0. 1 787 0 . 1 787 0.2021 0 .21 58 0.2409 23 7.5 cm-E 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0 .21 58 0.2409 23 E 0.2231 0.2231 0.2231 0.2422 0.2537 0.2754 23 E-1 0 ..i-E 0.2231 0.2231 0.2231 0.2422 0.2537 0.2754 23 20-30 ..i-E 0.2231 0 .2231 0.2231 0.2422 0.2537 0.2754 25 0-3 cm 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0 .21 58 0.2409 25 3-7 .5 cm 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0.2 1 58 0.2409 25 7.5 cm-E 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0.21 58 0.2409 25 E 0.2231 0 .2231 0.2231 0.2422 0.2537 0.2754 25 E- 1 0 ..i-E 0.2231 0 .2231 0 .2231 0.2422 0.2537 0.2754 25 20-30 ,j,E 0.2896 0.2896 0.2896 0 .3046 0 .31 39 0 .33 1 6 30 0-3 cm 0 . 1 787 0 . 1 787 0 . 1 787 0.2021 0 .21 58 0.2409 30 3-7 . 5 em 0 . 1 787 0 . 1 787 0 . 1 787 0 .2021 0 .21 58 0 .2409 30 7 .5 cm-E 0 . 1 787 0 . 1 787 0 . 1 787 0 .2021 0 .21 58 0.2409 30 E 0.2231 0.2231 0.2231 0.2422 0 .2537 0.2754 30 E-1 0 ..i-E 0.2231 0.2231 0.2231 0.2422 0 .2537 0.2754 30 20-30 ..i-E 0.2231 0.2231 0 .2231 0.2422 0 .2537 0.2754 32 0-3 cm 0 . 1 787 0 . 1 787 0 . 1 787 0 .2021 0 .21 58 0.2409 32 3-7 .5 cm 0 . 1 787 0 . 1 787 0 . 1 787 0 .2021 0 .21 58 0.2409 32 7 .5 cm-E 0 . 1 787 0 . 1 787 0. 1 787 0.2021 0.21 58 0 .2409 32 E 0.2231 0.2231 0 .2231 0 .2422 0.2537 0.2754 32 E-1 0 ..i-E 0 .2231 0.2231 0.2231 0.2422 0.2537 0.2754 32 20-30 ,j,E 0.2231 0.2231 0.2231 0.2422 0.2537 0.2754 276 S.E.D.s for comparison of log transformed total P concentration values at each depth of the 8 year old site with other depths at all developed sites. Age Depth 0-3 cm 3-7 .5 cm 7 .5 cm-E E E-1 0 ,i.E 20-30 ,i.E 8 0-3 cm * * * * * * 8 3-7 .5 cm 0 . 1 734 * * * * * 8 7 .5 cm-E 0 . 1 734 0 . 1 734 * * * * 8 E 0.21 89 0 .21 89 0 .21 89 * * * 8 E-1 0 ,i.E 0.21 89 0.21 89 0.2 1 89 0.2453 * * 8 20-30 ,i.E 0.21 89 0.21 89 0.21 89 0 .2453 0.2453 * 23 0-3 cm 0.2063 0.2063 0.2063 0 .2458 0 .2458 0.2458 23 3-7 . 5 cm 0.2063 0 .2063 0.2063 0.2458 0 .2458 0.2458 23 7 .5 cm-E 0.2063 0 .2063 0.2063 0.2458 0.2458 0.2458 23 E 0.2458 0.2458 0.2458 0.2797 0.2797 0.2797 23 E-1 0 ,i.E 0.2458 0.2458 0.2458 0.2797 0.2797 0 .2797 23 20-30 ,j,E 0.2458 0.2458 0.2458 0.2797 0.2797 0 .2797 25 0-3 cm 0.2063 0.2063 0 .2063 0.2458 0 .2458 0.2458 25 3-7 .5 cm 0.2063 0.2063 0.2063 0.2458 0 .2458 0.2458 25 7.5 cm-E 0 .2063 0.2063 0.2063 0.2458 0.2458 0.2458 25 E 0.2458 0.2458 0.2458 0 .2797 0 .2797 0.2797 25 E-1 0 ,i.E 0.2458 0.2458 0.2458 0 .2797 0 .2797 0.2797 25 20-30 ,i.E 0 .3075 0 .3075 0 .3075 0 .3352 0 .3352 0.3352 30 0-3 cm 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 30 3-7 .5 cm 0.2063 0.2063 0.2063 0.2458 0 .2458 0.2458 30 7.5 cm-E 0 .2063 0.2063 0.2063 0.2458 0.2458 0 .2458 30 E 0.2458 0.2458 0.2458 0.2797 0.2797 0.2797 30 E-1 0 ,i.E 0.2458 0.2458 0.2458 0.2797 0 .2797 0.2797 30 20-30 ,j,E 0.2458 0.2458 0.2458 0 .2797 0 .2797 0 .2797 32 0-3 cm 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 32 3-7 .5 cm 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 32 7.5 cm-E 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 32 E 0.2458 0 .2458 0.2458 0.2797 0.2797 0 .2797 32 E-1 0 ,i.E 0.2458 0.2458 0.2458 0.2797 0.2797 0 .2797 32 20-30 ,i.E 0.2458 0 .2458 0.2458 0.2797 0.2797 0.2797 277 S.E.D.s for comparison of log transformed total P concentration values at each depth of the 23 year old site with other depths at all developed sites of 23 years and older. Age Depth 0-3 cm 3-7 .5 cm 7 .5 cm-E E E-1 0 ,J..E 20-30 ,J..E 23 0-3 cm • • • • • • 23 3-7 .5 cm 0 . 1 734 • • • • • 23 7 .5 cm-E 0 . 1 734 0 . 1 734 • • • • 23 E 0 .21 89 0 .21 89 0.21 89 • • • 23 E-1 0 ,J..E 0.21 89 0 .21 89 0.21 89 0 .2453 • • 23 20-30 ,J..E 0.21 89 0.21 89 0.21 89 0.2453 0.2453 • 25 0-3 cm 0.2063 0 .2063 0.2063 0.2458 0 .2458 0 .2458 25 3-7 .5 cm 0.2063 0.2063 0.2063 0.2458 0 .2458 0 .2458 25 7.5 cm-E 0.2063 0.2063 0.2063 0 .2458 0.2458 0.2458 25 E 0 .2458 0.2458 0.2458 0 .2797 0 .2797 0 .2797 25 E-1 0 ,J..E 0 .2458 0 .2458 0.2458 0.2797 0 .2797 0.2797 25 20-30 ,J..E 0. 3075 0 .3075 0 .3075 0. 3352 0. 3352 0.3352 30 0-3 cm 0.2063 0.2063 0.2063 0.2458 0 .2458 0 .2458 30 3-7.5 cm 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 30 7.5 cm-E 0.2063 0 .2063 0.2063 0.2458 0.2458 0 .2458 30 E 0.2458 0 .2458 0.2458 0.2797 0.2797 0 .2797 30 E- 1 0 ,J..E 0.2458 0.2458 0.2458 0.2797 0.2797 0 .2797 30 20-30 ,J..E 0.2458 0.2458 0.2458 0 .2797 0 .2797 0.2797 32 0-3 cm 0.2063 0.2063 0.2063 0.2458 0 .2458 0 .2458 32 3-7 .5 cm 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 32 7 .5 cm-E 0.2063 0.2063 0.2063 0 .2458 0 .2458 0.2458 32 E 0.2458 0.2458 0 .2458 0 .2797 0 .2797 0.2797 32 E-1 0 ,J..E 0.2458 0.2458 0.2458 0.2797 0 .2797 0.2797 32 20-30 ,J..E 0 .2458 0.2458 0.2458 0.2797 0 .2797 0 .2797 S .E .D .s for comparison of log transformed total P concentration values at each depth of the 25 year old site with other depths at all developed sites of 25 years and older. Age Depth 0-3 cm 3-7 .5 cm 7 .5 cm-E E E-1 0 ,J..E 20-30 ,J..E 25 0-3 cm • • • • • • 25 3-7 . 5 cm 0 . 1 734 • • • • • 25 7.5 cm-E 0 . 1 734 0 . 1 734 • • • • 25 E 0.21 89 0 .21 89 0.21 89 • • • 25 E-1 0 ,J..E 0 .21 89 0.21 89 0.21 89 0.2453 • • 25 20-30 ,J..E 0.2864 0 .2864 0.2864 0. 3071 0 .3071 • 30 0-3 cm 0.2063 0.2063 0.2063 0 .2458 0.2458 0.2458 30 3-7 .5 cm 0.2063 0.2063 0.2063 0 .2458 0 .2458 0.2458 30 7 .5 cm-E 0.2063 0.2063 0.2063 0 .2458 0.2458 0.2458 30 E 0.2458 0 .2458 0.2458 0.2797 0.2797 0 .2797 30 E- 1 0 ,J..E 0.2458 0.2458 0.2458 0.2797 0.2797 0.2797 30 20-30 -!.E 0.2458 0 .2458 0.2458 0.2797 0 .2797 0.2797 32 0-3 cm 0.2063 0.2063 0.2063 0 .2458 0 .2458 0.2458 32 3-7 .5 cm 0 .2063 0.2063 0.2063 0 .2458 0 .2458 0.2458 32 7 .5 cm-E 0.2063 0.2063 0.2063 0 .2458 0 .2458 0.2458 32 E 0 .2458 0.2458 0.2458 0 .2797 0 .2797 0.2797 32 E-1 0 ,J..E 0 .2458 0.2458 0 .2458 0 .2797 0 .2797 0.2797 32 20-30 ,J..E 0.2458 0.2458 0.2458 0 .2797 0 .2797 0.2797 278 S.E.D .s for comparison of log transformed total P concentration values at each depth of the 30 year old site with other depths at a l l developed sites of 30 years and older. Age Depth 0-3 cm 3-7.5 cm 7 .5 cm-E E E-1 0 ,j,E 20-30 ,j,E 30 0-3 cm • • • • • * 30 3-7 .5 cm 0 . 1 734 • * • • • 30 7 .5 cm-E 0 . 1 734 0 . 1 734 * • • • 30 E 0.21 89 0.21 89 0.21 89 • • • 30 E-1 0 ,j,E 0.21 89 0.2 1 89 0.21 89 0.2453 • * 30 20-30 ,j,E 0.21 89 0.21 89 0.21 89 0.2453 0 .2453 • 32 0-3 cm 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 32 3-7 .5 cm 0.2063 0.2063 0.2063 0.2458 0 .2458 0.2458 32 7.5 cm-E 0.2063 0.2063 0.2063 0.2458 0.2458 0.2458 32 E 0.2458 0.2458 0.2458 0.2797 0.2797 0 .2797 32 E-1 0 ,j,E 0.2458 0.2458 0.2458 0.2797 0.2797 0.2797 32 20-30 ..l.E 0.2458 0.2458 0.2458 0.2797 0.2797 0.2797 S .E .D .s for comparison of log transformed total P concentration values at each depth of the 32 year old site with other depths at that site . Age Depth 0-3 em 3-7 .5 em 7 .5 cm-E E E-1 0 ,j,E 20-30 ,j,E 32 0-3 em • * • * * • 32 3-7 .5 cm 0 . 1 734 • • • * • 32 7 .5 cm-E 0 . 1 734 0 . 1 734 • • * • 32 E 0.21 89 0.21 89 0.21 89 • * * 32 E- 1 0 ,j,E 0.21 89 0.21 89 0.21 89 0.2453 * • 32 20-30 ,j,E 0.21 89 0.2 1 89 0.21 89 0.2453 0.2453 * Appendix 3 . 2 20 Q. C CD .! 1 0 o • Spring 1993 ., ., ., ., ., ., • • • ., • ., ., -., ., • ., ., ., ., 279 • • • ., . . . . . . .. . . . . . . . • • • • • • • • • • . . - . • • • . � o���--����--�--�--� o 1 0 20 30 Years of Development Appendix 3.3a 1 0000 . � 6000 C 4000 a 1 6000 .- co � 1 4000 � '-" � c 1 2000 , , , 0 a) Ye a r 1 " , , . " , .. - - - - - - - - - - . - -- \ , \ 1 0 20 30 Years of Development b) Yea r 2 .. ... ... ... ... -- • , - - .- .- � • - - - - - - - - - - - - - - - - -.- � -- � ... ... ... 1 0 20 30 Years of Development 280 , -1 1 /2/91 4/4/91 20/5/91 » BOO - - - -0 .. .. . .. ... - - - -0 .. .. .. .. .. ... as 1 500 as 600 • • as 1000 ... - - - - - w � 400 � oE � 1ooo w • C- D> --- � 2oo :E 500 :E 500 • Q • Q ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. ... ... - - -0 0 0 10 20 30 0 1 0 20 30 0 10 20 30 Years of Development Years of Development Years of Development 5/8/91 23/9/91 21 /1 0/91 3000 , as - - - as 2000 as 1500 - - - �20oo - -. - - - - - - - - � � 1ooo .. ... .. .. c- .:t! � 1000 • i 1000 :E 500 ... - - - - .. .. .. ... .. .. .. .. .. .. Q Q 0 0 0 10 20 30 10 20 30 0 10 20 30 Years of Development Years of Development Years of Development 1 8/1 1 /91 1 0/2/92 1 1 /5/92 .. .. .. .. 3500 � 2ooo - - - .. 2000 . - - - - - III .. ... ... .. .. ... .. as III � 1500 • �3000 � 1 5oo • i 1000 • C- • • • :E :E Q 500 .. .. ... .. .. .. .. Q 2500 Q 1000 .. ... .. .. .. - - -0 0 10 20 30 0 10 20 30 0 10 20 30 Years of Development Years of Development Years of Development 1 8/8/92 3/1 1 /92 1 4/12/92 6000 3200 ' - as3000 � 5ooo as 3000 � . - - - � 2800 - 2500 C- 4000 :E � 3ooo :E 2600 . Q .. ... .. ... ... - - - - - .. ... .. _ - .. - .. .. ... ... Q · · - - - - - - • · · - - 2400 · · 2000 2000 · 0 10 20 30 0 1 0 20 30 0 10 20 30 N Years of Development Years of Development Years of Development (l) � Append ix 3 . 3 b CD � 30 ctS ... C. ::J a.. 20 70 ..-... ctS ..c: ........ C) � I • --- o a) P Uptake ' Y e a r 1 , , , - - - - - - - - .. .. .. .. .. .. .. .. .. .. - .. - - -.. .. .. - 1 0 20 Years of Development b) P Uptake Y e a r 2 .. .. .. .. ... ... 30 - - - - - • , .. � 60,-------�----------__________ _!� ... • C. :J a.. 50 o • . . . .. .. .. .. .. .. - - . . .. 1 0 20 Years of Development • .. ... ... .. .. 30 282 1 1 /2191 as t 4 � 2.0 . . . . .. ... .. .. .. .. .. .. .. .. .. .. .. .. • � 1 .5 • .!II:: -; 3 CD .!II:: .!II:: .s 1 .0 co a. g. 2 :J 0. 0.5 0. 0 10 20 30 0 Years of Development 5/8191 1 10 t 10 . . . . . . . • . . . . . . . . . � � CD 5 .!II:: co 5 - _ ... - - - - - - - - - - - � a :J :J 0. 0 0. 0 0 10 20 30 0 Years of Development 1 8/1 1 /91 as s as 14 � 6 .. ... .. .. .. .. .. ... l 12 • • ....... � 4 CD � 10 .s • • §- 2 Ci. :J S 0. 0 0. 0 10 20 30 0 Years of Development 1 8/8/92 i 16 i 25 . � 14 � 20 CD CD t 12 .!II:: .s 15 a. :J 10 :J 0. 0. 10 0 10 20 30 0 Years of Development 414/91 t s " . .. .. .. .. ... .. .. .. ... ... .. . . ' � 6 • CD .!II:: 4 .s §- 2 . . . . . . . . . . . . . . . .. . . . . . . . . 0. 0 10 20 30 0 Years of Development 2319/91 as � 6 � � 4 co a � 2 10 20 30 0 Years of Development 1 012192 as � 10 � CD .!II:: .s 5 a. :J 0. 10 20 30 Years of Development 311 1 192 _ 14 " . � 1 3 .!II:: -; 12 i l l .. .. .. - . . . • :J 10 0. 10 20 30 Years of Development 0 0 2015/91 . . . . • . . . . . • • .. - - - .. .. .. .. .. .. ... .. 10 20 30 Years of Development 21 /1 0/91 .. .. .. -• .. - - - 10 20 Years of Development 1 1 /5/92 ' . . . . . . . . . .. . . . • . - - - .. - - .. . . ' 10 20 Years of Development 1 4/1 2192 30 • " . 30 . . . .. .. .. .. .. . . • • . . . . . . - .. .. - .. .. .. - - _ .. ... .. - .. 10 20 30 Years of Development .......... » "'0 "'0 w w 0- ......... • • I\.) OJ W ...-.. 1 1 /2/91 4/4/91 20/5/91 l> "0 0.60 "0 0.5 . • 0.55 0.35 .. .. .. .. .. .. .. .. .. W Q. Q. Q. 0.50 • w 40 1 1 /02/9 1 � 0 20 0 0 , , , 15 , , , 4 . - - - . . • ... .. .. - -UJ "C 10 ::J Q) - Q) 0 � � � 0 0 2 5 • • . - - • 0 0 0 1 0 20 30 0 Years of Development - .- - - - . . - - - - - - - - - ... , , 10 20 30 Years of Development - . - - , , , , , - , , - ' , , , • • - . 10 20 30 Years of Development 40 � 30 Q) > 0 U � 20 0 10 0 0 40 � 30 Q) -0 � 0 20 1 0 0 , , 1 0 20 .. - .. - . • • Years of Development • • 30 , , '. 1 0 20 30 Years of Development » -0 -0 CD :::J a. x' c..u c..u (') N CD CD --.. » "0 90 "0 , , 30 w w () - - - '-' 80 - - UJ .... . - - - - - - -s a m p l i n g date UJ � 20 tU .... 0 � 70 "0 2 0105/9 1 � 0 0 - - - 1 0 60 , • • , , - - - - - - . , 50 0 0 10 20 30 0 1 0 20 30 Years of Development Years of Development , , , • , 20 • , , 2 • • 1 0 - - - - - - - - "C 1 5 UJ "C :l Q) .. .. - .. - tU 15 Q) Q) � "C , � ffl. 10 � , 0 0 1 5 - .. - -- - - - - 5 0 0 0 0 1 0 20 30 0 1 0 20 30 0 10 20 30 Years of Development Years of Development Years of Development J\.) ex> <.D 90 � 80 sa m p l i n g d ate ctS .... C) 1 8/ 1 1 /9 1 "#- 70 60 0 30 4 , \ , 3 , , 20 U) , "C � • Q) (5 , Q) � 2 , ;: � 0 0 , • 10 - . - - 1 , , , , • , . • 0 0 0 1 0 20 30 0 Years ot' Development ... - - - - - - - - . .. ... .. .. - - - • • • • ... .. ... ... , , 10 20 30 Years of Development • 10 20 30 Years of Development 15 .... Q) > o 10 () � 0 5 0 1 .2 1 .0 "C ctS Q) "C 0.8 � 0 0.6 0.4 - » "0 "0 - - - w w n • ........ • .. .. ... ... , , 10 20 30 Years of Development - - - - - - • • • • .. .. .. ... 0 10 20 30 Years of Development I'\) CD ...... s a m p l i n g d ate 1 0102/92 1 5 . • . . . .. - - ... .. en 1 0 ::l (5 � 0 5 0 • 0 1 0 20 Years of Development I I • 30 50 en 40 � Ol � o 30 20 20 "0 Q) Q) == eft 1 0 0 o 0 I , I . I . I . ' . I 1 0 20 30 Years of Development 1 0 20 30 Years of Development 40 30 � Q) > .Q 20 u � 0 1 0 O;------�-----�-----�-----r-�._�� o 30 "0 as Q) "0 � 0 20 0 1 0 20 30 Years of Development . . - .. - - - - - • • , .. ' - . . . , , 10 20 30 Years of Development - » 'U 'U W W 0 ........ I\) CD I\) (/) ::l s a m p l i n g date 1 8/0 8/92 3 .. _ - - - - - - - _ . '" • (5 2 � 0 1 0 0 10 20 Years of Development , , • 30 (/) (/) 90 85 � 80 0) � o 75 70 4 "0 Q) Q) ;: � o 2 0 o 0 ... - - - .. ., ... - - ... • • • , .. .. ... , , , , . . 10 20 30 Years of Development . . • , • • • 10 20 30 Years of Development , 20 ... � o C3 '#. 10 - - - - - - - - - - . . O�--,_--T_--�--r_--��� o 4 , , "0 3 as Q) "0 '#. 2 1 0 0 . 10 20 30 Years of Development , , . . , , 10 20 30 Years of Development I\.) (0 w UJ :l -0 � 0 s a m p l i n g date 3/ 1 1 /92 1 0 5 , , 0 0 1 0 20 Years of Development 80 70 1 5 -0 10 CD CD ;: � 0 5 0 30 0 0 1 0 , , , , 20 , , Years of Development , , .. .. ... ... • 1 0 20 Years of Development 30 • 30 15 � CD � 13 � 10 o 5 0 8 , , 6 -0 as � 4 � 0 • 2 0 0 ... ... ... ... ... ... .. - - - 1 0 20 - - - , , • . - - . , , . 30 Years of Development ... .. - .. ... • • • • ... - - - ... 10 20 30 Years of Development I\J (() .s::. Appendix 4 . 1 40 Q. - 30 o :i 20 z 10 Spring 1 993 Inorganic P , ' " , . . . . . . . , ' , ' . " " ' " , O+-�--���--�-r--� 60 Q. .., 0 40 0 i 20 z 0 80 Q. 60 � 0 40 ca Z 20 o 0 10 20 30 Years of Development 10 .. .. ... ... .. .. .. 20 .. .. .. .. . 30 Years of Development .. .. .. .. .. ... .. .. .. . .. . . . . O+--+--�---------T--- o 400 0:, 300 , 0 , , "1. 200 X 100 0 0 . 10 20 30 Years of Development .. .. .. .. .. .. ... , , , , , , , • . • . . . . . . , , , , 10 20 30 Years of Development 30 80 60 40 20 0 150 100 50 0 , , Organic P " . . . .. . . 20 30 Years of Development 10 20 30 Years of Development 10 20 30 Years of Development 295 Appendix 4 .2 Spring 1 990 a) Exchangeable Ca _ 20 Cl o 1 8 o :!: 1 6 C" Q) 1 4 .§. 1 2 tU () 1 0 8 .!! ,Q tU Q) Cl c: tU .c (J )( 6 ""m' _ __ "'._. "._."."._,. ,·· .. ·_· .. ·_·_ .. ,·· _ .-&_',·..gsc-...... -...... . . � •.•••••• -.. 4 , //'-" '� w 2 ./�//' ,.>'r"'.-if" o t/ I o 5 1 0 1 5 20 25 30 Years of Development b) Exchangeable Ca (concentration in oven dried soi l) _ 4000 Cl g 3500 ..... CT 3000 Q) .§. 2500 tU () 2000 .!! � 1 500 Q) Cl c: tU .c (J )( 1 000 500 .. -.-... -.� 4 ............... .. _ . ... "m."'"w'''' ' .. .. _ ._ .. _ •. f,-,._t'..:-........ " ...... 7.t •. , ••• -. . """""".: � I 35 w ;-.///"",..,. //� o ��·/------�-----+I------�----�I-------r------+-----�I o 5 1 0 1 5 20 25 30 35 Years of Development c) Total Ca (concentration in oven dried soil) - 6000 Cl 0 0 5000 ..... -C" Q) 4000 .§. tU () 3000 .!! ,Q tU 2000 Q) Cl c: tU 1 000 .c (J )( w 0 . ,.�,�,,.,. .... o _ .. _· .. ··(?<···"tz-·····_ . . . .. -J.i " " ./', . . " f.f ............. , ... _ .• _ .... . .... . ... ... . , ... _..... .' .. ' ...• � 5 1 0 1 5 20 25 30 Years of Development 35 296 �O-3 em -B-3-7.5 em .... f.,- 7.5-E em �O-3 em -B-3-7.5 em .. ·� .... 7.5-E em �O-3 em -B-3-7.5 em ��· 7.5-E em (App. 4.2 ) Spring 1 993 6.0 5.5 � Q. 5.0 . . . 4.5 . . . . 4.0 0 10 20 30 Years of Development .. - . .. . . . . . .. 10 i u 5 0 0 10 20 30 Years of Development . 4000 . . . . B 3000 - � 2000 0 1000 0 0 10 20 30 Years of Development 15 B 10 - 5 0 0 0.4 � 0.3 0.2 0.1 0 . . ' 0.20 as oS 0,15 . . . 0 1 .0 . . . . . . .. .. .. .. . . . . . . . . . . . . . ' . , • .. .. .. .. .. .. .. .. .. 10 20 30 Years of Development · . . . . . . . . . . . . • 10 20 30 Years of Development . . . . . . . . " . . . . . .. . ' .. .. " . 10 20 30 Years of Development : . . . .. . . . . . . . . . . . . . . . . . . . • . · · . � 0.8 - 0.6 . . . . . . . ' . . . . . . . . . . o 10 20 30 Years of Development 297 .. . . . Appendix 4 .3 Historic Olsen P levels (MAF Quicktest) for 3 Wharekohe silt loam sites on the Kaikohe Research Station (pH , i n water, included at youngest site) . Site Developed in 1 982 Site Developed in 1 960 Site Developed in 1 958 Olsen P pH Fertiliser Aplied Olsen P Fertiliser Aplied Olsen P Fertiliser Aplied Sampling Olsen P Date Amount & Sampling Olsen P Date Amount & Sampling Olsen P Date Amount & date Form date Form date Form 1 979 1 1 / 1979 21 .5 1 1 / 1979 21 .5 (SSP) (SSP) 3/1 980 1 1 / 1980 1 5.5 1 1 /1 980 1 5.5 (SSP) (SSP) 6/1 981 6 4.7 1 211981 73 6/1 981 1 5 1 981 0 1 981 0 (SSP) 6/1 982 9 5.1 9/1 982 48.5 (SSP) 6/1 982 7 1 1 /1 982 36 1 1 /1 982 36 (SSP) (SSP) 611 983 1 211 983 36 6/1 983 1 3 211 984 28 211 984 28 lSSPJ_ (SSP) (SSP) 5/1 984 1 1 5.7 211 985 32 5/1 984 20 211 985 32 5/1 984 1 5 1 /1 985 26 (SSP) (SSP) (SSP) 6/1 985 14 5.8 211 986 38 6/1 985 1 5 411 986 30 (SSP) 6/1 985 1 5 1 1 /1 985 39 (SSP) (SSP) 611 986 20 5.5 1 1 /1 986 50 611 986 15 1 211 986 33 (SSP) (SSP) 5/1 987 1 2 5.8 1 211 987 34 5/1 987 1 2 1 /1 988 27 _Oonm (long) 6/1 988 1 0 6.0 5/1 989 31 6/1 988 10 5/1 989 31 (long) (long) 8/1 989 9 6.2 1 1 /1989 1 6.5 8/1 989 1 1 1 1 /1 989 1 6.5 (long) (long) 1 990 3/1 990 35 1 990 3/1 990 35 (long) (long) 3/1991 1 7 6.2 5/1 991 56 3/1991 22 5/1 991 56 (RPR) (RPR) 411 1 992 1 0 5.8 5/1992 37 4/1 1 992 18 5/1 992 37 (RPR) (RPRJ 3/1993 22 5.8 5/1993 31 3/1 993 1 5 5/1 993 31 (RPR) (RPR) 1 /1 994 1 3 5.9 20 -en 0) - == 1 0 0 en c 0) 5 en 0 c 0) 0 C> C ctS ..c u -en <1> :: 15 ·0 en 1 0 c 0) !!2 5 o c 0 0) C> c -5 ctS ..c u 0 o Aponga clay J u ly 9 1 -92 • • • • • . - . -e - - - - - - _ _ 50 rate of P applied Aponga clay J uly 9 1 -93 50 rate of P applied 100 100 Aponga clay » -0 - J u ly 92-93 -0 en CD 0) ::l a. - x' 0 --..j en 5 -->. c . 0) !!2 0 0 c 0) C> c -5 ctS - - - - - ..c u 0 50 1 00 rate of P applied - en Q) - ·0 5 en c Q) en 0 0 c Q) 0) -5 C res .c 0 -en Q) - 0 ·0 -5 en c Q) en o -1 0 c Wharekohe sandy loam J u ly 9 1 -92 # # # -. - - - # . • ... - - .. ... ... • --.- - - - # • # 50 rate of P applied Wharekohe sandy loam J u ly 9 1 -93 Q) • .. - ... - 100 0) _� - - ' � -1 5�-�-�- -�-�-�-�-��- -� ---.-�����-,- o 0 50 1 00 rate of P applied . - CJ) Q) - ·0 CJ) c Q) -10 CJ) 0 c . � - 12 0) C (tj .c 0 0 Wharekohe sandy loam July 92-93 • - - - - - - - '- - - - tIr - - - - ... - ... • • • - - - - - - - - .- - - - - ... - . ... ... ... ... 50 rate of P applied Wharekohe sandy loam 1ii Feb 92-93 Q) - ·0 5 CJ) c Q) CJ) o 0 _ _ _ _ _ _ _ _ • _ _ _ _ _ _ _ - # # # C Q) 0) _ # • • - - - - - - - - - - -. - . - • 1 00 � -5 .c �--�--���--���--�.- o 0 50 100 rate of P applied - » "'0 "'0 ........, � ........ w a a Wharekohe si lt loam NFt J u ly 91 -92 .... en Q) .... 0 Cl) 5 c Q) El 0 .s o Q) C> C tll .!: 0 0 50 1 00 rate of P applied Wharekohe si lt loam N Ft .... J u ly 91 -93 � 0 .... � -2 c � -4 0 c -6 Q) C> -8 c m .!: 0 0 50 1 00 rate of P applied ...-en Q) � -5 0 00 -6 c Q) El -7 0 c .- - 8 Q) C> c -9 ro .!: 0 0 ...-00 Q) ...- '0 00 5 c Q) El 0 .S 0 Q) C> c ro .!: 0 0 Wharekohe si lt loam NFt J uly 92-93 - - - - - - - - - - - -. - - - - - - - . - - - - - - - - - - - - - - - • • 50 rate of P applied Wharekohe silt loam NFt Feb 92-93 50 rate of P applied 1 00 1 00 ..-. » "'0 "'0 -....J � - (.t.) 0 ' � Wharekohe si lt loam Ft ..- J u ly 9 1 -92 ..-en CJ) Q) Q) ..- .- � 1 5 -6 0 '0 CJ) CJ) c 1 0 c Q) Q) -8 J!l .!!1 0 5 - - - 0 - -c c Q) a .- -1 0 Q) C> C> C C co -5 co .c .c u 0 50 1 00 u rate of MCP applied Wharekohe si lt loam Ft ..- July 9 1 -93 .-en m a Q) .- .- '0 5 0 CJ) CJ) -2 c 0 c Q) Q) J!l CJ) 0 o -4 -5 c c Q) Q) � -1 0 C> -6 c ccs co .c .c u a 50 1 00 u rate of MCP applied Wharekohe silt loam Ft July 92-93 � . � � � - - - - - - - - -. - - - - - a - - - - - - - - � - - - - - • • � . e - - - - - _ 50 rate of MCP applied Wharekohe si lt loam Ft Feb 92-93 � - - - - - - - - - - - - - -.- - .. � -. � � - � • - - - - - - - - - • • a 50 rate of MCP applied • 1 00 - - - - - 1 00 ..-... » "'0 "'0 --.I � w a i'l ...... C/) Q) ...... 0 5 C/) c: Q) .J!1 0 0 c: Q) 0) -5 c: en ..c () 0 ...... � 0 ...... o C/) c: -5 Q) C/) o .£: -1 0 Q) 0) c: Wharekohe si lt loam . Ft J u ly 91 -92 - - - - - - - - • . - - - - - - ... - - .. - .. - - - - - 50 rate of SPA applied Wharekohe si lt loam Ft J uly 9 1 �93 - - - - - .- • ... - - - - - .. 100 � -1 5 �' ��--��������--� () 0 50 rate of SPA applied 1 00 (;j Q) . ...... o en -5 c: Q) en 0 -6 c: Q) 0> -7 c: CO .c o 0 (;j Q) ...... ·0 en c: � 0 o c: Q) � -5 co .c o 0 Wharekohe si lt loam Ft J u ly 92-93 - - - - - - - -- - -- - - - ... - ... ... • • ... - - - - - - - - - - - - • 50 rate of SPA applied Wharekohe si lt loam Ft Feb 92-93 . . ' . 50 rate of SPA applied 1 00 1 00 Wharekohe si lt loam Ft , resin P � J u ly 9 1 -92 ..... == 20 o en a... 1 5 - - - - - - - - - - - - - • c:: • � 1 0 L----.=---- _ -�- - - - - - _ _ - - - c:: • - - - 50 1 00 rate of SPA -applied Wharekohe si lt loam Ft , resin P ..... en 2 1 0 o en a... 0 c: en � -1 0 c:: Q) 0> -20 c: co £ 0 J u ly 9 1 -93 - - - - - - - - - - - - - " • ... - ... - - - - - ... -- - 50 rate of SPR applied 1 00 Wharekohe si lt loam Ft , resin P ..... en J u_ly 92-93 � 0 - - - - - ... g - ' • a... -1 0 c:: '00 a.> -20 L... c:: a.> -30 C) c:: co .r:. u 0 ... ... ... ... ... ... ... ... 50 rate of SPR applied Wharekohe si lt loam Ft , resin P ..... en a.> ..... 1 0 '0 en a... .� 0 � c: � -10 c: m £ 0 Feb 92-93 ... ... ... ... ... - .. - - - 50 rate of SPR applied 1 00 1 00 Appendix 7.3a Pasture Dry Matter Production for SLF Trial. Aponga clay DM 1 DM 2 DM 3 DM 4 DM 5 DM 6 DM 7 DM 8 DM 9 DM 10 DM 1 1 DM 1 2 Year 1 Year2 Cut 27.9.91 23.10.91 22 . 1 1 .91 1 6.2.92 3.6.92 4.8.92 5.1 0.92 25. 1 1 .92 5.1 .93 1 3.4.93 9.6.93 25.7.93 0 1 375 1465 1 464 2244 1 040 1 1 41 3065 3462 3556 861 1 1 52 668 8730 1 2764 20 1 563 1512 1694 2448 934 1461 3414 3432 3758 915 1 273 81 1 961 1 13603 30 1 489 1643 1532 2 178 925 1 227 3229 3439 3738 702 1 399 690 8995 13197 40 1 765 1656 1678 2369 1 350 1458 3229 3530 3725 721 1 388 797 1 0276 1 3390 50 1590 1592 1628 2251 1 332 1 255 3121 3564 3465 884 1 331 722 9648 13086 60 1 771 1696 1693 1 943 1 362 1 1 92 2920 3452 3563 1 039 1 51 8 704 9656 13196 80 1869 1828 1807 2297 1 473 1 537 3188 3577 3736 780 1 459 816 1 081 1 1 3557 100 1 354 1577 1 649 2340 1 039 1 306 2954 3573 3824 1 1 34 1 269 739 9265 1 3493 F pro 0.604 0 . 142 0.405 0.633 0.254 0.29 0.686 0.91 9 0.63 0.067 0.01 6 0.645 0.357 0.795 S.E.D. 302.7 1 19.3 1 42.8 248.6 261 . 1 1 78.6 276.6 1 51 .3 203.9 1 40.7 91 .4 95.2 876.2 542.5 LSD ns ns ns ns ns ns ns ns ns ns 1 90. 1 1 2 ns ns ns Harvest 1 1 , significant linear and quadratic trend with lowest Dm at 0 and highest at 60 kg P/ha, effect did not last. Wharekohe sandy loam DM 1 DM 2 DM 3 DM 4 DM 5 DM 6 DM 7 DM 8 DM 9 DM 1 0 Dm 1 1 Dm 1 2 DM 1 3 Dm 1 4 Year 1 Year 2 Cut 24.9.91 22.10.91 23. 1 1 .91 1 2 .2.92 1 3. 5.92 20.6.92 1 .8.92 1 6. 10.92 7 . 1 2.92 1 1 . 1 .93 2 .3.93 14.4.93 8.6.93 27.7.93 0 1 332 2046 1 936 3528 1 908 918 723 2786 3331 1 350 7 16 657 1 642 944 1 2390 1 1426 20 1 258 2090 1998 3764 2097 783 841 2854 3086 1 307 818 518 1 406 967 1 2833 10958 30 1 163 1975 2029 3924 1 981 669 772 2737 3071 1032 753 637 1566 1 021 1 2514 10817 40 1 304 1832 2023 4020 2092 683 702 261 3 3531 1 299 919 573 1529 1 009 1 2657 1 1 473 50 1 1 1 2 1758 2003 4204 2 1 80 829 802 2679 3086 1 419 9 15 579 1 530 1 022 1 2887 1 1 230 60 1 199 1916 2046 4224 1 620 805 797 2678 3399 1084 935 666 1 675 926 1 2608 1 1 364 70 1 176 1 891 1895 3775 1 955 758 708 2772 3379 1221 838 560 1 580 1028 1 2 158 1 1 378 80 1025 1 765 2022 4060 2 1 35 731 808 2474 3249 1 084 781 631 1 568 1021 1 2545 1 0808 100 1 1 25 1 778 1831 3949 1 870 685 709 2622 3123 1447 919 596 1 708 1 138 1 1948 1 1 553 F pr. 0.746 0.023 0.61 6 0.075 0.649 0. 1 56 0.58 0.671 0.217 0. 182 0.209 0.925 0.681 0.795 0.813 0.868 S .E .D. 1 74.6 102.3 1 1 5.6 2 15.9 281 .8 88.2 81 .2 1 88.5 1 94.9 1 70.8 94.6 1 1 4.7 1 51 .2 1 15.6 581 .8 598 L.S.D. ns 2 12 . 17 ns ns ns ns ns ns ns ns ns ns ns ns ns Harvest 2, significant linear trend for reducing DM with increasing P, trend did not persist. Wharekohe silt loam NFt OM 1 DM 2 OM 3 OM 4 OM 5 OM 6 OM 7 OM 8 OM 9 OM 1 0 O M 1 1 O M 12 Year 1 Year 2 Cut 30/9/91 25/10/91 23/1 1/91 1 6/2/92 27/5/92 31 17/92 29/9/92 9/1 2/92 1/3/93 2 1/4/93 1 7/6/93 26/7/93 Year 1 Year 2 0 1 450 936 1 1 72 2 100 575 860 1444 4924 631 513 1 387 633 7093 9532 20 1 483 1 008 1232 2393 903 750 1865 5180 785 823 1 899 905 7769 1 1457 30 1 405 946 1203 2428 764 696 1776 5420 918 906 2043 895 7442 1 1 958 40 1 1 42 905 1 273 231 8 673 824 1728 4895 841 815 2048 920 7136 1 1 246 50 1 310 983 1 209 2506 999 1 026 1779 5308 921 1 084 2 120 933 8032 1 2 146 60 1 234 864 1 1 97 2576 884 822 1550 5157 753 770 1 983 861 7576 1 1 073 70 1 388 915 1 073 2507 1031 862 1790 5760 780 1 047 2148 981 7777 1 2505 80 1 291 873 1 172 241 1 926 830 1430 5164 774 812 1992 892 7503 1 1066 100 1 1 15 943 1 239 2524 1 161 1 080 1698 5461 854 1067 2233 909 8062 1 2223 F pr. 0.794 0.91 7 0.463 0.289 0.002 0.788 0.076 0. 144 0 . 13 0.003 <0.001 0.061 0.589 <0.001 S .E .D. 245.8 1 07.5 80 1 77.4 1 24.1 227.2 155.2 292.7 95.2 1 23.9 1 53.2 93.2 541 .8 536.9 l.S.D. ns ns ns ns 256.14 ns ns ns ns 255.73 316.2 9 ns 1 108 . 16 Harvest 5, significant linear trend of increasing OM with increasing P. Harvest 1 0, significant linear trend of increasing OM with increasing P. Wharekohe silt loam Ft (MCP) (Means from analysis of variance adjusted for covariates ie plot effect) DM 1 DM 2 OM 3 DM 4 DM 5 DM 6 OM 7 DM 8 DM 9 OM 1 0 Om 1 1 Om 1 2 OM 1 3 YEAR 1 YEAR 2 Cut 26/9/91 24/ 10/91 23/1 1/91 1 3/2/92 2/6/92 29/7/92 25/9192 201 1 1 192 6/1/93 813193 2714193 1 8/6/93 2417193 0 2 1 73 1 498 1444 2780 624 929 1679 3868 2298 375 589 1 430 855 9448 1 1095 20 2094 1689 1595 3001 716 1 039 1951 4060 2685 500 854 1 761 942 10 134 1 2752 30 2060 1496 1528 3 189 536 871 1770 4765 2621 525 778 1 586 972 9681 1 3017 40 2282 1 623 1 584 3133 844 1 1 26 1833 4485 2873 460 741 1 577 928 1 0592 1 2896 50 2369 1 609 1 71 9 2874 704 1 1 80 1975 4140 2952 500 844 1 901 1 1 07 1 0456 1 3419 60 2085 1 702 1690 3152 851 1 204 1877 3899 2861 500 995 1 689 886 1 0685 1 2705 70 21 1 1 1648 1652 331 7 585 1 002 1991 4289 2886 515 1 007 1 674 1018 1 03 15 1 3381 80 1 977 1568 1498 3153 626 896 1627 4216 271 1 443 769 1 698 958 971 8 1 2421 100 2281 1560 1537 3290 593 1 087 1799 4639 2695 534 1 064 1 589 1078 1 0347 1 3398 F pr. 0.689 0.391 0.327 0 . 129 0.367 0.358 0.293 0.D15 0.062 0.379 0.03 0.618 0.305 0.36 0.01 3 S .E.D. 2 19.9 1 03.5 1 1 9.4 1 95.6 144.7 1 64.8 16 1 .6 255.7 192.8 65.3 1 28.1 2 1 7.7 100.4 594. 1 569.4 L.S.D. ns ns ns ns ns ns ns 530.32 ns ns 265.68 ns ns ns 1 1 80.94 Harvest 1 1 and Year 2, significant linear trend for increasing Om with P, basically due to 0 lower than all others. When SPR included in analysis, ie all together treatment effects were significant at Harvests 8, 13 and Year2. At Year 2, 0 significantly smaller than all other treatments except S PR60. Wharekohe silt loam Ft (MCP) (Means adjusted for covariates) OM 1 OM 2 OM 3 OM 4 OM 5 OM 6 OM 7 OM 8 OM 9 OM 1 0 Om 1 1 Om 1 2 OM 1 3 YEAR 1 YEAR 2 Cut 26/9/91 24/ 10/91 23/ 1 1 /91 1 3/2192 216192 2917/92 25/9/92 20/1 1 /92 611 193 8/3/93 2714/93 1816193 2417/93 0 20 2094 1 692 1600 2983 7 15 1036 1947 4038 2672 493 868 1 765 939 10121 1 2722 40 2300 1 654 1629 3080 835 1 109 1827 4431 2814 443 825 1573 912 10606 1 2824 60 2083 1 706 1695 3138 852 1 203 1875 3885 2851 494 1006 1694 883 10677 1 2690 80 1987 1 576 1508 3121 6 18 885 1615 4167 2679 428 802 1697 952 9695 1 2340 1 00 2259 1 509 1460 3325 595 1 1 01 1784 4638 2749 532 955 1603 1099 1 0249 13360 Wharekohe silt loam Ft (SPR) (Means adjusted for covariates) OM 1 OM 2 OM 3 OM 4 OM 5 OM 6 OM 7 OM 8 OM 9 OM 1 0 Om 1 1 Om 1 2 OM 1 3 YEAR 1 YEAR 2 Cut 2619/91 2411 0/91 23/1 1/91 1 3/2192 216192 29/7/92 25/9/92 20/1 1 /92 611/93 813/93 27/4193 1816193 2417/93 0 20 2138 1 685 1 753 3338 594 1081 1889 4241 2582 527 975 1689 1067 1 0589 1 2970 40 2418 1 595 1685 3160 569 1078 1755 4590 2788 534 706 1550 945 1 0505 1 2869 60 2065 1584 1569 3085 624 931 1657 41 53 2763 487 715 1371 733 9857 1 1880 80 2147 1 651 1519 3094 504 991 1772 4372 2923 499 769 1 707 1 059 9905 13100 1 00 2051 1 537 1693 3235 635 1008 1894 4301 281 1 482 898 1890 980 1 0158 13257 F pr. FxR 0.52 0.39 0.561 0.463 0.524 0.449 0.629 0.556 0.778 0.404 0.325 0.43 0. 143 0.35 0.532 S.E.O. FxR 155.2 76.3 158.4 1 92.6 1 29.8 1 54.5 194.2 286.3 215.7 54.6 1 33.2 226.7 96.2 462.1 655.6 l.S.O. FxR ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns Harvest 5, pooled MCP grew more OM than pooled SPR, F Pr was 0.027, effect did not last. Harvest 1 3, significant rate effect ie 60 lower than 20, 80 and 100. 308 Appendix 7 .3b Botan ic composition for SLF tria l . i ) Aponga clay Grass Harv1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 27/09/91 3/06/92 5/1 0/92 5/01 /93 25/07/93 0 74. 1 69.88 55.3 22 .7 57.2 20 77.5 72 60 20.9 55.4 30 69.3 69.76 54.3 1 4.4 42.4 40 70.5 72.57 60.4 1 8 49.2 50 64.2 73. 1 60.9 1 8 .5 55 .2 60 69.1 70.28 56.3 1 6 4 1 .6 80 69.4 71 .83 68.2 23 51 .9 1 00 69.8 73.96 59.7 22 . 1 47.7 F pr o trt 0.8 1 1 0 .939 0 .461 0 .586 0.729 S .E .D . 7.67 3 .928 6 . 1 9 5 1 0.6 L .S .D. ns ns ns ns ns Clover Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 27/09/91 3/06/92 5/1 0/92 5/01 /93 25/07/93 0 21 .7 1 1 .96 35.3 64.6 35.5 20 1 7 .5 1 8.48 32.6 71 .5 41 .5 30 21 .7 1 3. 97 39.9 74.6 52.2 40 23.3 1 3 .25 33.3 71 .4 45.6 50 30.6 1 2 .26 33.8 72.9 41 .9 60 27.4 1 2 .05 34 .5 74.3 52.5 80 27.9 1 1 .27 27.4 69 .2 45. 1 1 00 25.3 1 0 .03 29.6 64.5 4 1 .2 F pro trt 0.661 0.428 0.64 0.493 0.692 S .E .D . 7 .05 3 .51 2 6 . 1 5 5.79 9 .91 L .S .D. ns ns ns ns ns Lotus Harv1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 27/09/91 3/06/92 5/1 0/92 5/01 /93 25/07/93 a a 0 a 0 0 20 0 a a a 0 30 0 0 0 0 0 40 0 . 1 1 0 0 0 0 50 0.84 0 a a 0 60 0 0 0.0434 0 0 80 a 0.259 0 a a 1 00 a a 0 a 0 Statistical analysis inappropriate. - - ----- 309 Weeds Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Harv 1 2 Rate P 27/09/91 3/06/92 5/1 0/92 5/0 1 /93 25/07/93 sformation 0 2 .87 7 .93 7 .9 8 .7 7 .26 1 4 .58 20 3.48 0 .94 5.6 2.8 2.71 9 . 1 8 30 7 .41 7 .99 3 .7 7 5 . 1 5 1 2.77 40 4 .36 3.89 5 5 .8 4 .9 1 1 . 54 50 3 .24 3 .87 3 .7 4 .7 2 .34 8.46 60 1 .83 7.54 7.4 4 .9 5 .84 1 3. 54 80 1 .24 3.29 2 .5 1 2 .59 9.07 1 00 3.02 5 .05 9.3 9 .4 1 0.69 1 8.72 F pr. trt 0.469 0 . 1 74 0 .809 0 .469 0 .059 0 .047 S.E.D. 2.671 2.809 4.63 4.08 2 .596 3 .069 L .S.D. ns ns ns ns ns 6 .38352 Harv 1 2 , Angular transformation , significant quadratic trend, but a bit nonsensica l . Dead Harv1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 27/09/91 3/06/92 5/1 0/92 5/01 /93 25/07/93 0 1 .4 1 0.23 1 .5 1 3 .97 0 .092 20 1 .53 8.58 1 .81 4 .88 0.428 30 1 .59 8.29 2 . 1 5 3 .89 0.292 40 1 .72 1 0.29 1 .33 4 .79 0.29 50 1 . 1 1 1 0.78 1 .56 3 .83 0 .51 1 60 1 .73 1 0. 1 3 1 .8 4 .73 0 . 1 49 80 1 .49 1 3 .35 1 .85 6 .83 0 .32 1 00 1 .85 1 0.95 1 .52 3 .98 0.433 F pro trt 0.941 0 .529 0.807 0.442 NA S.E .D . 0 .584 2.33 0 .505 1 . 394 NA L.S.D. ns ns ns NA - - - 31 0 i i) Wharekohe sandy loam Grass Harv 1 Harv 5 Harv 8 Harv 9 Harv 1 0 Harv 1 4 Rate P 24/09/91 1 3/05/92 1 6/1 0/92 7/1 2/92 1 1 /01 /93 27/07/93 0 50.5 39.9 61 .6 71 .9 45.8 87.4 20 52.7 33.6 70.3 73.2 48.4 81 .3 30 46.6 34.6 65.4 67.7 49 75.7 40 55 .5 24 .5 64.7 65.6 42 77.9 50 63 . 1 29.8 61 68.8 43 81 .6 60 63 31 .8 71 . 1 74 .9 56.3 87.2 70 53.9 38.2 64.2 62.4 37.8 79.3 80 57.9 40 .3 72 .9 70 .5 50.3 83.3 1 00 60 . 1 31 .5 65.3 63.3 37.9 75.5 F pr. trt 0. 1 75 0 .469 0.231 0.671 0.25 0.699 S.E .D . 6 .25 7.29 4 .93 7 .26 7.26 7 .54 L .S .D . ns ns ns ns ns ns Clover Harv 1 Harv 5 Harv 8 Harv 9 Harv 1 0 Harv 1 4 Rate P 24/09/91 1 3/05/92 1 6/1 0/92 7/1 2/92 1 1 /01 /93 27/07/93 0 47 45.2 33.6 1 8. 1 42.2 1 1 .5 20 41 .9 48 .7 24.3 1 7.6 35.2 1 5. 9 30 46. 5 51 .2 29.8 21 31 . 1 1 9.9 40 40.5 55.9 27.5 23.1 45.7 1 7 .6 50 31 .7 51 .9 30.2 1 8 .4 33 . 1 1 4. 8 60 33.8 56 .9 26 1 1 .8 32 .3 1 1 . 1 70 36. 5 43.8 22 . 1 1 7. 5 36 .9 1 8 .2 80 36.4 40.5 21 .3 1 9.5 34 .9 1 2 .8 1 00 31 .3 47.2 22.9 22 .8 43 1 9. 5 F pro trt 0 .292 0.086 0 .287 0 .72 0 . 1 05 0 .793 S .E .D . 7.34 5 .39 5 . 16 5 .92 5 .3 6 .3 L .S .D. ns ns ns ns ns ns Lotus Harv 1 Harv 5 Harv 8 Harv 9 Harv 1 0 Harv 1 4 Rate P 24/09/91 1 3/05/92 1 6/1 0/92 7/1 2/92 1 1 /0 1 /93 27/07/93 0 -0.07 -0.037 -0.07 0 .04 0.003 -0.041 20 0 . 1 6 0 0 0 0 0 30 0 .7 0 0 0 0 .055 0.226 40 0 . 1 2 0 .528 0. 1 7 1 . 52 0 0 .437 50 0.25 0 . 1 69 0.26 0.35 0 .301 0 . 1 5 60 0.78 0 0 1 .28 0 . 1 23 0 70 0.48 0. 1 01 0 .27 0 0.05 0 80 0.49 0 0 0 0 0.048 1 00 2.46 0.088 1 .98 1 . 1 5 0.34 0.281 Statistical analysis inappropriate. 31 1 Weeds Harv 1 Harv 5 Harv 8 Harv 9 Harv 1 0 Harv 1 4 Rate P 24/09/91 1 3/05/92 1 6/1 0/92 7/1 2/92 1 1 /01 /93 27/07/93 0 0.62 2 . 1 2 1 .42 4.4 8.4 1 . 1 1 20 3 .96 3 .96 2.79 4 .5 1 2 2 .81 30 4 .59 2 .6 1 .95 5.9 1 5 .5 4. 1 7 40 1 .88 8 .51 4 .61 3 .8 8 .2 4.01 50 1 . 58 6 .36 6 .45 8.6 1 9. 7 3.47 60 0.93 1 .53 0.59 5.8 7.5 1 .74 70 7 .86 6.77 1 0.58 1 4 .8 20.4 2 .45 80 3 .37 5 .86 3 .31 4 .9 1 0 .4 3 .82 1 00 4.05 1 0.06 7 .22 9 1 5 .4 4 .78 F pr. trt 0.582 0.425 0.272 0.444 0 .873 0 .543 S.E .D . 3 .5 1 4 4.062 3 .938 4 .88 1 0.29 1 .82 L .S .D . ns ns ns ns ns ns % Weeds needed to be transformed, angular transformation used but sti l l no significant differences, so table of means above are untransformed. Dead Harv 1 Harv 5 Harv 8 Harv 9 Harv 1 0 Harv 1 4 Rate P 24/09/91 1 3/05/92 1 6/1 0/92 7/1 2/92 1 1 /0 1 /93 27/07/93 0 1 .87 1 2 .8 3.49 5 .58 3 .59 0 20 1 .28 1 3.73 2 .64 4 .77 4 .42 0 30 1 .59 1 1 .58 2.9 5 .46 4 .38 0 40 2 .05 1 0.58 3 .01 5 .96 4 .06 0 50 3.45 1 1 .74 2.08 3.82 3 .8 0 60 1 .44 9 .74 2 .31 6 . 14 3.77 0 70 1 .24 1 1 .09 2.85 5 .31 4 .93 0 80 1 .8 1 3 .3 1 2.48 5.24 4.44 0 1 00 2 . 1 1 1 . 1 3 2 .56 3.68 3 .39 0 F pro trt 0 .009 0 .67 0.701 0 .596 0 .892 NA S .E .D . 0 . 503 2 . 1 79 0 .71 1 . 355 1 .063 NA L.S.D. 1 .043 ns ns ns ns NA Harv 1 sign ificance nonsensical . 3 1 2 i i i) Wharekohe silt loam NFt Grass Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 30/09/91 27/05/92 29/09/92 1 /03/93 26/07/93 0 79.2 20.09 41 .6 24.4 69.9 20 73.2 1 8.78 38.4 26 .5 73 .4 30 76 .5 1 3 .58 36.7 29.5 74.2 40 76.7 1 2.25 31 24.8 73.7 50 67.5 1 4 .84 40.6 29 71 .8 60 72.6 1 6 .38 40 27.8 73.8 70 76. 1 1 5 .32 41 . 1 32.6 74 . 1 80 70.7 1 7 .45 46.8 29. 1 69 1 00 77 1 4 .57 52.9 36 68.2 F pro trt 0.81 7 0 .346 0 .014 0 .71 0 .91 5 S .E .D. 7 . 1 3.251 4.87 6.35 5 .39 L .S.D. ns ns 1 0.076 ns ns Harv 7 , F test Sign ificant Lin and Quad Trend for increasing %grass with increasing P. Clover Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 30/09/91 27/05/92 29/09/92 1 /03/93 26/07/93 0 1 1 60.76 45.7 22.4 24.5 20 1 5 .9 54.02 48.9 23. 1 24.6 30 1 3 66.24 47.6 1 7.3 23 . 1 40 1 3.5 67. 1 9 53.4 23.2 24.2 50 23.2 65. 1 4 50.9 26.4 24 .5 60 1 5 .6 65.6 46.9 21 .2 23.3 70 14 .3 62.43 45.5 21 .6 23.6 80 20.2 62.83 40 1 7 . 1 25.5 1 00 9.2 70.26 36 22 29.9 F pr. trt 0 .51 7 0.039 0 .038 0 .804 0 .938 S .E .D. 6 .4 4 . 1 29 4 .7 5 .51 4 .95 L.S.D. ns 8.5429 9.724 ns ns Harv 5, Significant l in . trend for increasing clover with increasing P . Harv 7, Significant l in . and quad. trend for decreasing clover with increasing P . Lotus Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 30/09/91 27/05/92 29/09/92 1 /03/93 26/07/93 0 1 .6 0 0 . 1 7 0 .205 0 20 0.76 0 . 12 0 .22 0 0 30 0.22 0 . 1 1 .32 0 0 40 0.89 1 .33 1 .94 0 0 50 0.44 0 0 0.264 0 60 5.25 0 .52 0.68 -0.02 0 70 3.75 0.81 0.92 0 0 80 0.49 0.77 1 . 1 2 0 0 1 00 2.24 0. 1 4 0 a a Statistical analysis inappropriate. 31 3 Weeds Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 30/09/91 27/05/92 29/09/92 1 /03/93 26/07/93 0 4.75 1 3 .96 1 2 .03 36.3 5.44 20 7.21 23.82 1 1 .97 36 1 .45 30 5 .62 1 7.75 1 3.97 37.2 2 .52 40 5.25 1 6 .58 1 3 .04 34.3 1 .58 50 5.67 1 7.77 7.81 30.6 3 .37 60 4 . 1 9 1 5 .54 1 1 .8 34.6 2.67 70 2 .58 1 8 .77 1 2 .09 25.3 2 . 1 4 80 5.87 1 6.86 1 1 .48 35.2 5 .37 1 00 7 .02 1 2.42 1 0 .28 23.3 1 .58 F pr. trt 0.598 0.23 0.509 0.474 0.028 S.E.D. 2.225 3.793 2.559 7 . 1 8 1 . 325 L .S.D. ns ns ns ns 2 .74 Harv 1 2 , Although F sign ificant, a look at the data reveals that 0=80 and significance is nonsensical bought about by small percentages involved and nature of weed pieces ie one big heavy piece can double % weeds. Dead Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 2 Rate P 30/09/91 27/05/92 29/09/92 1 /03/93 26/07/93 0 3.41 5 . 1 9 0 .516 1 6 .76 0.204 20 2.94 3 .26 0.502 1 4 .4 0.484 30 4 .73 2.32 0.328 1 6 .04 0.21 5 40 3.66 2 .64 0.547 1 7 .67 0.478 50 3 . 1 7 2.25 0.691 1 3 .67 0 .384 60 2.4 1 . 97 0.668 1 6.42 0 .224 70 3 .26 2.66 0.464 20.56 0. 1 73 80 2 .78 2.09 0.603 1 8 .57 0 . 1 5 1 00 4.52 2.6 0.738 1 8.7 0.29 F pr. trt 0 . 1 99 0.052 0.729 0 .454 0.669 S.E.D. 0.88 0.905 0.2245 3.052 0.21 38 L.S.D. ns ns ns ns ns iv) Wharekohe silt loam Ft MCP Treatments Only Adjusted for covariates Grass Harv 1 Harv 5 Rate P 26/09/91 2/06/92 0 70.8 35.7 20 67.6 34.7 30 67.4 25.8 40 70 .3 34.8 50 64 .8 30.7 60 65.9 32.7 70 71 .3 33.9 80 65.5 32 .5 1 00 72.4 34.2 F pr. trt 0.906 0 .602 S.E.D. 6 .44 4 .8 L .S.D. ns ns Clover Harv 1 Harv 5 Rate P 26/09/91 2/06/92 0 1 7 .5 46. 1 2 20 22.8 49.97 30 1 9.9 54.33 40 1 8 .2 52.36 50 24.9 53.48 60 21 .6 48.04 70 1 9.4 47.32 80 21 .6 46.76 1 00 1 7.9 52.32 F pro trt 0.941 0.401 S.E.D. 6 .23 4 .293 L.S.D. ns ns Lotus Harv 1 Harv 5 Rate P 26/09/91 2/06/92 0 0.67 0.09 20 1 .76 0 .48 30 0 .53 0.33 40 -0.69 -0.47 50 0.68 0.28 60 2 .75 0.29 70 0.66 0.41 80 1 .35 0 .73 1 00 0 .98 0.28 Statistical analysis inappropriate. Harv 7 Harv 9 25/09/92 6/01 /93 60.5 35 53.3 38.5 53.3 36.6 60.5 38.9 50.7 40. 1 54 33.8 53 37.5 59.6 24.8 55 43. 1 0 .746 0.421 6 .23 7.01 ns ns Harv 7 Harv 9 25/09/92 6/01 /93 26.21 23.8 33.5 33.2 31 .43 1 7 .3 29.48 27 34.91 25 33. 1 4 31 .3 30.64 25.6 26.55 1 5 .3 32.94 20.8 0 .399 0.262 4 .084 7 .34 ns ns Harv 7 Harv 9 25/09/92 6/01 /93 0.01 -0.02 1 .73 1 .32 0.37 0.3 -0.44 0. 1 7 0.48 0.36 0 .7 -0.29 0 .94 0.58 1 .56 0 .5 0 .71 0 .46 -- --� Harv 1 3 24/07/93 80.8 83.2 83.3 79.8 84.6 89 84.4 73.9 80.7 0.231 4 .91 ns Harv 1 3 24/07/93 1 5 1 3.33 1 4.36 1 6.69 1 4. 1 2 9 . 1 4 1 3.78 23.05 16 . 1 0.2 1 6 4 .31 9 ns Harv 1 3 24/07/93 0.071 0.055 0.01 -0.008 0.01 8 -0.004 0.21 4 0 . 1 5 1 0 . 1 02 3 1 4 ---- 3 1 5 Weeds Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 3 Rate P 26/09/91 2/06/92 25/09/92 6/01 /93 24/07/93 0 8 . 1 9 16 . 94 1 2 . 1 9 39.2 3 .97 20 5 .01 1 2 .8 1 0 .49 25.9 3 . 1 7 30 8.93 1 8 .07 1 3.41 44.7 2.21 40 1 0 .02 1 1 .77 9.46 32.2 3.28 50 7 .26 1 4 .01 1 2 .8 33.2 1 .2 60 6.8 1 7 . 1 4 1 0.89 33.8 1 .67 70 5.49 1 6.41 1 4.49 34.4 1 .37 80 9 .59 1 8 . 1 4 1 1 .66 58. 1 2 .88 1 00 5.88 1 1 . 1 1 1 0 .57 33.1 2 .72 F pr. trt 0.867 0 .309 0.971 0. 1 92 0.676 S .E .D . 3 .71 2 3.539 4.238 1 0.91 1 .563 L .S .D. ns ns ns ns ns Dead Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 3 Rate P 26/09/91 2/06/92 25/09/92 6/01 /93 24/07/93 0 2.83 1 . 1 7 1 . 1 1 1 .97 0. 1 65 20 2.86 2 .01 1 .0 1 5 1 . 1 7 0.25 30 3.25 1 .47 1 .49 1 .04 0.082 40 2.09 1 .53 1 .0 1 9 1 .76 0.238 50 2.32 1 .55 1 . 1 47 1 .48 0 .053 60 2.98 1 .78 1 .281 1 . 1 7 0 . 1 49 70 3 . 1 9 1 .97 0 .923 1 .97 0. 1 94 80 1 .96 1 .9 1 0.668 1 .29 0.043 1 00 2 .78 2 . 1 1 0 .773 2.59 0 .427 F pr. trt 0 .802 0.903 0.663 0 . 1 06 0.404 S .E .D . 0.861 0.695 0.4201 0 .524 0 . 1 633 L .S .D. ns ns ns ns ns --- v) Wharekohe silt loam Ft, MCP vs SPR Analysis of variance adjusted for covariates Grass Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 3 Rate P 26/09/91 2106192 25/09/92 6101 193 24/07/93 MCP SPR MCP SPR MCP SPR MCP SPR MCP 20 67 68.8 34.91 36.79 53.3 60.2 38.7 37.7 83.3 40 69 62. 1 36.06 32.2 61 48.7 39.9 28.3 80 60 65.4 68.3 32.9 1 29.84 54. 1 54 34 32.7 89.2 80 64.5 66.5 32.79 26.28 59.5 48.9 25.2 33 74. 1 1 00 72.9 69.6 32. 1 8 33.87 53.7 58.6 4 1 .7 38. 1 80.9 F pro RxF 0.607 0.433 0.083 0.384 0.595 SED RxF 5 .21 3 .471 5.3 6 .87 4.95 L.S .D . ns ns ns ns ns RxF is Rate X Fertiliser Type Clover Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 3 Rate P 26/09/91 2106192 25/09/92 6101 193 24/07/93 MCP SPR MCP SPR MCP SPR MCP SPR MCP 20 23.3 20. 1 49.83 50.09 33.56 27.31 33.3 24.5 1 3.2 40 20.4 2 1 . 8 52.07 43.2 29.82 3 1 .36 26.4 23.2 1 6.59 60 22 23.6 47.96 49.42 33.2 27.8 3 1 .5 26.3 9.02 80 22.7 22.9 46.44 52.64 26.67 3 1 . 37 1 5.2 27.2 22.87 1 00 1 5 .7 2 1 .4 52. 1 5 50.68 32.46 28.03 22. 1 25.5 1 5. 8 1 F pro RxF 0.764 0. 1 1 5 0 .383 0.238 0.77 SED RxF 4.92 3.929 4.404 6 .91 4 .353 L.S .D . ns ns ns ns ns RxF is Rate X Fertiliser Type Harv 1 3 rate was significant ie F pr 0.045 and <0.001 for quadratic component but this dif not picked up in MCP analysis. Lotus Harv 1 Rate P 26/09/91 MCP 20 1 .74 40 -0.67 60 2.73 80 1 . 33 1 00 0.85 F pro RxF NA SED RxF NA L.S.D. NA RxF is Rate X Fertiliser Type -- - - ---- Harv 5 2106192 SPR MCP 1 .76 0.49 0. 1 8 -0 .45 0 .52 0 .3 0.76 0.74 0 .71 0.24 NA NA NA Harv 7 Harv 9 Harv 1 3 25/09/92 6101 193 24/07/93 SPR MCP SPR MCP SPR MCP 0.27 1 .7 1 .77 1 . 32 0 .86 0.055 0 -0.46 0.08 0 .32 -0.09 0 .014 0.38 0.68 0.25 -0. 1 4 0. 1 7 -0.004 0.49 1 .49 0.56 0 .52 1 .02 0. 1 56 0.78 0 .57 0.65 0 . 1 4 0.38 0.058 NA NA NA NA NA NA NA NA NA - - - 3 16 SPR 87. 1 78.8 82.8 70.8 83.3 SPR 1 0.67 1 7. 57 1 3.36 27.08 1 5.71 SPR -0. 0 18 0 .01 1 0.005 0.01 8 0.062 Weed Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 3 Rate P 26/09/91 2106192 25/09/92 6101 193 24/07/93 MCP SPR MCP SPR MCP SPR MCP SPR MCP 20 5.04 5.83 1 2.76 1 1 .96 1 0.42 9.82 25.5 35.2 3. 1 4 40 8 .96 1 4.09 1 0.8 22.67 8.49 1 9.34 3 1 . 3 46.7 3 . 1 4 60 6 .84 5 .54 1 7.06 1 8 .57 1 0.76 1 7. 1 9 33.4 39 1 .65 80 9.41 7 .2 1 8.07 1 7.88 1 1 .6 1 7.22 57.7 37.6 2.8 1 00 8 .04 5 .58 1 3 . 1 6 1 3.72 1 2.63 1 1 .67 33.8 34.8 2.85 F pr. RxF 0.657 0 .2 1 9 0.391 0. 1 43 0.4 1 1 SED RxF 4.063 4. 1 3 4.49 1 0 .23 1 .574 L.SD. ns ns ns ns ns RxF is Rate X Fertiliser Type For harvest 7, F value for Fert type was significant ie SPR significant higher weed content than MCP overall. ie F pr for fert was 0.046 for SPR higher than MCP. Dead Harv 1 Harv 5 Harv 7 Harv 9 Harv 1 3 Rate P 26/09/91 2106192 25/09/92 6101 193 24/07/93 MCP SPR MCP SPR MCP SPR MCP SPR MCP 20 2.91 3 .5 2 .02 0.9 1 . 03 0.88 1 .23 1 . 83 0.261 40 2 .32 1 .87 1 . 5 1 1 .92 1 . 1 2 0 .51 2 .06 1 .93 0.259 60 3.03 2 .02 1 .78 1 .79 1 .29 0.75 1 .21 1 .88 0. 1 58 80 2.06 2.62 1 .96 2 .71 0 .71 1 . 97 1 .44 1 .28 0 .06 1 00 2.51 2.66 2.27 0 .95 0.64 1 .04 2.29 1 .27 0.424 F pro RxF 0.709 0.454 0.046 0.205 0.367 SED RxF 0.958 0.883 0.479 0.566 0 . 1 894 L.SD. ns ns 0.98674 ns RxF is Rate X Fertiliser Type Harv 7 significant interaction probably not a real effect. -------- 3 1 7 SPR 1 .8 3 .44 3.62 1 .95 0.78 SPR 0.41 1 0. 1 44 0.232 0. 1 8 0. 1 1 6 3 1 8 Appendix 7 .3 Monthly rainfa l l (mm) at the Kaikohe Research Station for the two year trial period (3 months prior to trial start in brackets). Month 1 991 1 992 1 993 January - 80 24 February - 45 61 March - 53 25 Apri l - 54 292 May 1261 1 61 1 24 June (1 411 1 49 1 26 July 11 791 295 26 August 1 83 251 - September 224 1 98 - October 93 1 28 - November 55 1 1 0 - December 69 1 1 6 - Appendix 8 . 1 Derivation of Equation 2 Rate of P Accumulation = K -a.P(t) or so then and and and and d P(t) = K -aP(t) d (t) d P(t) = d(t) K - a P (t) 1 -- In(K - a P(t)) = t + c a In(K - a P(t)) = -at - ac K - a pet) = exp -at-ac -a. P(t) = -K + exp -at-ac K exp-at-ac P(t) = - - --=--- a a where c is an arbitrary constant which is in the form P(t) = F - Ge-at K where F = - , a G = � a -ac At t ime 0, P(t) = F-G and if th is is 0 (which it is) , G=F and so P (t) =G -Ge-a.t Therefore P (t) = G(1 - e -a.t ) Where: (1) (2) P(t} is the amount of P wh ich has accumulated in the soi l fol lowing pasture development for a given pasture age (kg/ha). I is the pasture age (years). K is a constant corresponding to the rate of P accumulation at age O. a is the rate constant. G is the maximum amount of P wh ich can accumulate in the soi l fol lowing pasture development (kg/ha). 3 1 9