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Item An investigation of composting poultry manure in relation to nitrogen conservation and phosphate rock dissolution : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University(Massey University, 1993) Mahimairaja, SantiagoGaseous losses of nitrogen (N) through ammonia (NH3) volatilization and denitrification diminish the fertilizer value of animal and poultry manures and form atmospheric pollutants. Appropriate methods of composting can improve the fertilizer value by conserving manure N and minimise the risk of environmental pollution. Additionally, acids produced during composting and nitrification of conserved N in manure, provide a source of protons (H+) which have the potential to dissolve phosphate rock (PR) when composted with manure. The objective of this research was to examine the methods of composting poultry manure with different amendments in relation to N conservation and PR dissolution. Firstly, the methods for preserving and measuring N in manure samples were evaluated. The results suggested that freeze drying and inclusion of a strong oxidizing agent (KMnO4) prior to routine Kjeldahl digestion were required to achieve complete recovery and accurate measurement of N in manure samples. Methods for reducing the loss of N through NH3 volatilization and denitrification during aerobic and anaerobic composting of poultry manure with different amendments were investigated in controlled 'flow through' incubation experiments. The amendments included carbonaceous bedding materials (woodchip, paper waste, wheat straw and peat), acidifier (elemental sulphur-S°) and adsorbents (zeolite and soil). The loss of N through NH3 volatilization from aerobic composting was about 17% of the manure N which was reduced by 90-95% under anaerobic composting. Under aerobic composting the addition of various amendments reduced the volatilization loss by 33 to 60%. Although the rate of denitrification was negligible (<1µg kg-1 day-1) in fresh manure, it increased enormously (3.7mg kg-1 day-1) during composting. The presence of nitrate (NO3-) was found to be a rate determinant for denitrification in manure. Amongst the treatments, the addition of S° was very effective in reducing NH3 volatilization and denitrification. The dissolution of PR during composting with poultry manure was examined using radioactively (32P) labelled synthetic francolite and North Carolina phosphate rock (NCPR). The use of 32P labelled francolite indicated that PR dissolution in poultry manure/PR composts could be measured more accurately from the increases in NaOH extractable phosphorus (∆NaOH-P) than from the decreases in HCl extractable P (∆HCl-P). Low levels of francolite and NCPR dissolution (<16%) occurred when PR is mixed with poultry manure. This was attributed to the high concentrations (4.8 * 10-2mol L-1) of calcium (Ca2+) in manure solution which inhibited the dissolution of PR through the Ca common-ion effect. Addition of S° to poultry manure/PR compost reduced the pH and thereby enhanced PR dissolution. A system for the dissolution of PR, using the acid (H+) produced during the nitrification of NH3 released from poultry manure, was developed. The inhibitory effect of manure Ca on PR dissolution is avoided if the NH3 released from decomposing manure is absorbed in bark and soil materials containing PR, which are kept either as manure covers or in separate columns. Although both methods were found effective in the absorption of NH3, the result demonstrated that bark absorbs more NH3 than does soil. Extensive PR dissolution occurred in the bark (82.3%) and the soil (33.2%) even in the absence of NH3 absorption from poultry manure. Higher levels of PR dissolution in bark is attributed to its high exchangeable acidity (80.5cmol (+) kg-1) and large Ca sink size (82.7cmol (+) kg-1). However, when the bark and soil materials were kept as manure covers, accumulation of Ca in the covers due to the diffusion of manure Ca, reduced the PR dissolution. When the bark and soil materials were kept separately in columns, nitrification of absorbed NH3 resulted in small increases in PR dissolution in bark (15%) and soil (5%). However, most of the protons (50-95%) released during nitrification are involved in the buffering of the bark and soil materials. The laboratory studies showed that the addition of S° to poultry manure during aerobic composting not only reduced the loss of N, but also enhanced PR dissolution. Based on this observation, sulphocompost (a blend of poultry manure, woodchip with S° and PR) and phosphocompost (a blend of poultry manure, woodchip with PR) were prepared and their agronomic effectiveness were compared with fresh manure mixtures and urea using field grown winter cabbage and summer maize crops. The crop yield, N use efficiency and N recovery were greater for sulphocompost than for phosphocompost. The sulphocompost and phosphocompost were approximately 60% and 12%, respectively, as effective as urea treatments for winter cabbage. Both composts were equally effective as urea for the second season's maize crop. The study has shown that poultry manure enriched with PR and S° can be used as a source of N, P and S.Item Storage potential of kiwifruit from alternative production systems : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, New Zealand(Massey University, 1999) Benge, Jason Ronald; Benge, Jason RonaldThe effects of nine soil treatments on the storage potential of 'Hayward' kiwifruit were examined over three consecutive seasons at sites in Palmerston North and Te Puke, New Zealand (NZ). The treatments comprised three ground covers (viz. bare, grass and mulch), in factorial combination with three fertiliser regimes (viz. conventional, organic and organic plus (= organic + gypsum)). Each season, several fruit and vine attributes were measured at harvest and the subsequent softening behaviour of fruit was evaluated during storage. In the second and third seasons, several soil, fruit and vine attributes were also monitored before harvest. At both sites, significant and consistent differences were observed in many of the soil attributes that were measured. In particular, conventional plots often contained more inorganic nitrogen (N) and potassium (K) than organic and organic plus plots while organic plus plots nearly always contained more calcium (Ca) than conventional plots. Bare soil consistently contained less moisture, and experienced higher 2.00 pm and lower 6.00 am temperatures, than covered soil, while the mulch increased the surface rooting of vines. The soil amendments also had some consistent, though not statistically significant, effects on the mineral composition of vines, especially in the third season. In particular, fruit and leaves from conventional plots tended to contain more N and K but less Ca than those from organic and organic plus plots while fruit and leaves from grass plots consistently contained less N than those from bare and mulch plots. Of all the soil amendments, grass had the largest effect on fruit storage behaviour i.e. fruit associated with that amendment were consistently firmer throughout storage and developed significantly less [i.e. fewer] soft patches than fruit from bare and mulch plots. Although fruit from conventional plots tended to soften slightly more rapidly and develop more soft patches than fruit from organic and organic plus plots, the differences were never significant. Generally, soil, vine and fruit attributes did not differ significantly with the interaction of ground cover and fertiliser regime. In addition to the above work, in 1996 only, fruit were sampled from ten pairs of organic and conventional (i.e. Kiwigreen) orchards throughout the Bay of Plenty in NZ, to compare the responses of those fruit to typical postharvest handling and storage regimes and their compositional attributes. Generally, fruit from conventional orchards were harvested more mature, as indicated by soluble solids concentrations (SSC), although the average firmness of fruit from the two systems did not differ significantly. The average concentrations of N, K, magnesium (Mg) and phosphorous (P) in fruit did not differ significantly with production system. However, organic fruit often contained more Ca with the average difference being on the borderline of significance. Despite differences in maturity, whole fruit softening did not differ significantly with production system. On the other hand, fruit from organic orchards nearly always developed less soft patches than fruit from conventional orchards with the average difference being significant. This difference may have been partly due to the difference in the Ca concentration of fruit. Typical postharvest handling practices, compared to harvesting directly into trays, did not significantly affect whole fruit softening but did significantly decrease the incidence of soft patches, for reasons that are not clear. Across all the grower lines, the incidence of soft patches was significantly and negatively associated with the average concentrations of Ca in fruit. Combinations of other fruit attributes (i.e. SSC, initial firmness and the concentrations of N and Mg) with Ca concentration, produced indicators that were very strongly associated with the incidence of soft patches. These attributes would appear to be important in the development of soft patches. If these relationships are subsequently shown to be consistent, then they could form the basis for a predictive tool that would allow at-harvest segregation of fruit lines with different storage potentials. In all of the current work, fruit that developed soft patches consistently contained less Ca than healthy fruit. It therefore seems that enhancing the Ca content of fruit could be beneficial to fruit storage life. However, it appears that under some conditions at least, the uptake of minerals, particularly Ca, may be constrained at the root level and so manipulating the soil environment may not always guarantee an improvement in the storage potential of kiwifruit.Item Developing sustainable soil fertility in southern Shan State of Myanmar : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand(Massey University, 2001) Tin Maung AyeLiterature review and a farm survey results indicated that the productive capacity of the Red Earths and Yellow Earths (Acrisols in FAO/UNESCO system) in Southern Shan State, Myanmar, continues to decline as poor, near-subsistence, farmers usually fallow-crop these soils and are unable, for economic reasons, to provide the necessary fertilisers and crop management strategies that could improve and maintain soil fertility. This thesis reports on investigations undertaken to determine ways in which the fertility of these soils can be improved in an economical and sustainable manner. A preliminary soil survey confirmed that soils of the Southern Shan State study area had low soil P status, low pH, low SOM (low reserves of N, P and S) and low base saturation, which are likely constraints to crop production on these marginal soils. A farmers' survey indicated farmers were aware of the P availability and soil acidity problems, however, they use low levels of N, P, K fertilisers, green manure and liming material because their economic margins were insufficient to buy expensive P fertilisers and liming materials. Preliminary glasshouse trials were conducted to establish the key soil fertility constraints. These trials included testing the P responsiveness of crops (pigeon pea (Cajanus cajan), rice bean (Vigna umbellata) and upland rice (Oryza sativa L)), the influence of liming materials on legume growth (black gram (Vigna mungo), cowpea (vigna unguriculata), kidney bean (Phaseolus vulgaris), lentil bean (Culinaris lentil), pigeon pea, soybean (Glycine max), stylosanthes (Stylosanthes quyanensis)), and the effectiveness of green manures for building SOM and P reserves in the Red Earths and Yellow Earths. The glasshouse trials confirmed that low P status is the factor most limiting to crops particularly legumes grown in the Yellow Earths. Upland rice, however, did not respond to added P suggesting that upland rice may not be a suitable test plant for evaluating the relative P effectiveness of fertilisers in the Red Earths and Yellow Earths. This pot experiment with the upland rice trial also demonstrated that indigenous Myanmar phosphate rock (MPR) and Farmyard manure (FYM) were ineffective P sources for raising soil plant-available P in the short-term. Local dolomite, however, was an effective liming material and when added with P enhanced the nodule formation of pigeon pea. Laboratory incubation studies showed that incorporating organic amendments (FYM, pigeon pea and upland rice residues) into the Red Earths and Yellow Earths significantly increased soil organic C, improved soil pH buffering, increased the total P content and increased labile-P fractions that could be used for plant growth. The Walkley and Black organic C determination can be used as a suitable 'low tech', portable method for the determination of organic C to monitor soil organic matter levels. Based on the results from the preliminary glasshouse trials, field trials were designed to establish relationships between P fertiliser form, application rate and soil P test levels for maize (Zea mays) and legumes (rice bean and black gram) grown in a Yellow Earth. In soils with initial Olsen-P values of approximately 8 kg P kg-1 soil, two maize field trials consistently indicated that near maximum yield (90%) of maize can be obtained by application of water-soluble P fertiliser (Triple superphosphate, TSP) at 40 to 50 kg P ha-1 in year one and reapplication of TSP at 25 kg P ha-1 in year two. Phosphate supplied as FYM, Tithonia diversifolia, Chinese partially acidulated phosphate rock (CPAPR) and Sechura reactive phosphate rock (SPR) were agronomically less effective as short-term P sources for maize than TSP, with TSP substitution ratio of 0.1, 0.17, 0.61 and 0.07 respectively at an application of 40 kg P ha-1 rate. A legume-wheat rotation field trial showed that rice bean (the legume) yield was significantly increased by the application of TSP at 40 kg P ha-1 plus local dolomite. Subsequently, rice bean plots fertilised with 40 kg P ha-1 as TSP produced the largest wheat grain yields in the Yellow Earth. In all trials the Olsen-P test was a suitable soil P test for providing an index of plant-available P. Olsen-P values at which optimum crop growth occurred (90% maximum yield) ranged between 30 to 40 mg P kg-1 in the Red Earths and Yellow Earths, depending on the season, plant age when harvested and crop grown. Use of the sulphuric acid acidulated CPAPR and elemental S (S°) with SPR confirmed that plant growth in the Yellow Earths was very responsive to sulphur application. Chinese PAPR can be used as a cost-effective P and S fertiliser to increase the crop yield in these Red Earths and Yellow Earths, particularly where S is also deficient. Use of local dolomite (2.5 t ha-1) in the field trials caused significant increases in soil pH in the first season and in the legume-wheat trials resulted in increases in legume yield. The research findings on soil P testing, alternative P sources and liming materials presented in this thesis are discussed in terms of the role they might have in the establishment of sustainable agriculture practice in Southern Shan State, Myanmar. Future research directions that should be taken to realise the productivity of these soils and farming systems are also discussed.Item Magnesium fertiliser effects on forest soils under Pinus radiata : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand(Massey University, 2000) Mitchell, Andrew DavidMagnesium deficiency is of concern in a number of forest regions in New Zealand and has been linked in recent years to a condition in Pinus radiata called upper mid crown yellowing (UMCY). Magnesium deficiency is also acknowledged as a common nutrient disorder linked to 'new type forest decline' in Europe and the USA. With increases in the number of rotations and increased growth rates through tree breeding, the incidence of Mg deficiency and UMCY is expected to increase. This study investigated the Mg fertility of a range of forest soils, their responses to the application of Mg fertilisers and Mg uptake by P. radiata. New Zealand Forest Research (Institute Ltd) has established a series of Mg fertiliser trials (FR190 series) in a range of forest soils. Five of these trial sites, located in the North Island of New Zealand, where calcined magnesite (calmag) was applied at 150 kg Mg ha-1, were sampled to investigate the effectiveness of calmag in increasing plant available Mg and to determine the fate of the fertiliser. At all of the sites, within two and three years following calmag application, both soil exchangeable Mg and solution Mg concentrations were increased in the top 5 cm. However, the increases were significant only at three of the sites, where the initial soil exchangeable Mg was low. The other two soils had medium to high concentrations of soil exchangeable Mg prior to Mg fertiliser application. Between 70-90% of the applied fertiliser Mg had dissolved and estimated losses due to leaching ranged from 0-20%. Foliar concentrations of Mg were generally improved in the fertilised trees, but the increases were not significant. Symptoms of Mg deficiency and UMCY are worse in trees that have high foliar K:Mg ratios. High foliar K:Mg ratios reflect changes in the pools of soil exchangeable Mg and K. Therefore, the effects of Mg fertiliser application on soil K:Mg molar ratios at the FR190 series trials were examined. In the trials where Mg fertiliser application significantly increased soil exchangeable Mg the soil K:Mg molar ratio was significantly reduced. This could see a reversal of the trend of the soil K:Mg ratios increasing with time and a reduction in the severity of Mg and UMCY symptoms. In the trials that had medium to high concentrations of Mg, the K:Mg molar ratio was not affected by increases in exchangeable Mg from Mg fertiliser application. As there are several Mg fertilisers with varied solubilities available to foresters a study was conducted to determine the rates of dissolution of a range of Mg fertilisers applied at 200 kg Mg ha-1 to a pumice soil under P. radiata in Kaingaroa Forest near Rotorua. Twenty seven months after fertiliser application the mean percentage of Mg dissolved were 100% for Epsom salts, 92% for calcined magnesite 1-2 mm, 91% for Granmag 20 (granulated product from 20% acidulation of calcined magnesite, 2-4 mm), 83% for calcined magnesite 2-4 mm and 70% for forestry grade dolomite. The specific dissolution rate constants (µg fertiliser cm-2 day-1) for the slowly soluble Mg fertilisers were 279 for calcined magnesite 1-2 mm, 220 for calcined magnesite 2-4 mm, 212 for Granmag 20 and 13 for forestry grade dolomite. A computer program based on an elemental sulfur (Sº) oxidation model, where the rate of Sº oxidation depends on surface area of the particles, explained the rate of dissolution of Mg fertilisers within a narrow fertiliser particle size range. Application of Mg fertiliser has been shown to increase plant-available Mg. However, there has been no significant increase in foliar Mg concentrations in the fertilised trees. It was thought that though the bulk soil had sufficient plant-available Mg, some factors in the rhizosphere might be inhibiting Mg uptake by P. radiata. Therefore, trials were conducted to increase the understanding of Mg availability in the soil immediately surrounding the tree roots. Two glasshouse experiments were conducted investigating the tree-induced changes in Mg availability in the rhizosphere of P. radiata seedlings. The first used pumice topsoil fertilised with various forms of Mg fertilisers. The second used pumice sub-soil that had lower exchangeable Mg concentrations and pH buffering capacity. The subsoil was fertilised with different rates of Mg and K fertilisers. There was a significant accumulation of exchangeable Mg in the soil layers near the rhizosplane, compared to the bulk soil for the Epsom salts and granmag fertiliser treatments in the first experiment. A similar accumulation occurred for treatments where Mg fertiliser was applied in the second experiment. Magnesium accumulation at the root surface is probably due to a higher rate of Mg movement by mass-flow compared to Mg uptake by the seedlings. The higher rate of Mg movement was probably caused by high seedling transpiration rates. Magnesium accumulation in the rhizosphere could have also been influenced by ectomycorrhizal fungi growth. Soil pH in the rhizosphere soil of the first experiment was generally unaffected by nutrient uptake of the seedling compared to the bulk soil, probably due to the high buffering capacity of this soil. Whereas, in the second experiment the soil pH, because of the low pH buffering capacity of the soil, was significantly reduced in the rhizosphere compared to the bulk in all treatments. Cation-anion balance without considering N uptake, showed that the seedlings took up an excess of cations compared to anions. Because the ionic form of N taken up by the seedlings was not determined, it was not possible to explain the rhizosphere acidification from the cation-anion balance in the seedlings. Magnesium concentrations in the fertilised seedling in the first experiment increased for all fertiliser types used, but only the increases in root Mg concentrations were significant. In the second experiment Mg fertiliser application significantly increased Mg concentrations in both the shoots and roots. Recently, Forest Research installed a fertiliser trial that manipulated the soil K:Mg ratio through the application of Mg and K fertiliser. This trial was used to study the losses of Mg due to leaching under P. radiata after the application of Mg and K fertiliser. Suction cup lysimeters were installed at 2 depths (10 cm and 45 cm) to monitor changes in soil solution Mg concentrations in the top-soil where the active roots are and the leaching losses of Mg down below 45 cm over an 18 month period after fertiliser application. Magnesium and K fertiliser application resulted in significant increases in soil solution Mg and K concentrations in the 0-10 cm soil layer soon after fertiliser application. However, by 90 to 180 days after application concentrations have returned to levels not significantly different from those of the control treatment. The soil solution K:Mg molar ratio in the 0-10 cm soil layer was significantly increased by both K fertiliser treatments at all sampling times. Magnesium fertiliser application generally decreased the soil solution K:Mg molar ratio, although none of the decreases were significant. Magnesium fertiliser application significantly decreased the soil exchangeable K:Mg molar ratio and K fertiliser application significantly increased the soil exchangeable K:Mg molar ratio. Between 180 to 240 days following application, concentrations of Mg and K in the sub soil lysimeters peaked. Concentrations of solution Mg in the sub soil lysimeters of the fertilised and unfertilised plots were generally greater than solution K concentrations. Estimated leaching losses of Mg were 39.4 kg Mg ha-1 in the Mg fertilised plots and 11.2 to 26.9 kg Mg ha-1 in the K fertilised plots. Estimated leaching losses of K were 8.9 kg K ha-1 for the 200 kg K ha-1 treated plots and 17.4 kg K ha-1 for the 400 kg K ha-1 treated plots. Magnesium fertiliser application did not cause any increase in the leaching losses of K. This thesis has increased the knowledge base of the Mg fertility of a range of forest soils and their response to application of Mg fertiliser. More research is required to determine the reasons for the slow tree response to increases in soil Mg from Mg fertiliser application and the role of ectomycorrhizal in the Mg uptake by P. radiata.Item Pasture growth constraints on dry steep East Coast hill country : a thesis presented in partial fulfillment of the requirements for a Doctor of Philosophy in Soil Science, Massey University(Massey University, 2002) Blennerhassett, Jamie DavidDry hill country provides a diverse environment in which variations in aspect and slope affect pasture growth and soil processes in a number of different ways. The AgResearch fertiliser and pasture growth trial at Waipawa in the east coast of the North Island of New Zealand, provided an ideal location, with contrasting topography and fertiliser treatments at which to measure pasture growth constraints, with particular reference to nitrogen (N), in dry east coast hill country over the period of a year. Initial soil testing at the trial site revealed large differences in soil total N and phosphorus (P) concentrations between aspects, slopes and P fertility regimes in the top 75 mm of the Waipawa silt loam soil. The measured differences in soil N concentrations (ranging from 2.60-5.80 mg N/g) are a result of contrasting energy and moisture regimes between the aspects and slopes, affecting soil forming processes and subsequent soil total N concentrations. The differences in soil total P concentrations (ranging from 0.42-0.88 mg P/g) between aspects and slopes can be largely attributed to stock transfer, while the variation between P fertility regimes can he attributed to a large application of P fertiliser to the high P treatment before commencement of the trial. The existing trial provided three main contrasts over which the major restrictions on pasture growth could be measured. These were phosphate status (high phosphate (HP) and low phosphate (LP)), aspect (North and South), and slope (steep (25° +), easy (15-20°) and flat (0-12°)). This gave a total of ten combination of P status and topography Each of these sites was replicated three times to give a total of 30 locations. At each location two 0.5 m2 cages were used to measure pasture growth. One of the cages was used to measure pasture growth under the current soil fertility regime, while the other cage had 120 kg N/ha as urea applied to it, to allow growth under non-limiting N conditions. The cage locations were pretrimmed before cage placement and pasture was cut monthly (or when growth conditions permitted) and analysed. Soil samples were taken from adjacent to the cage under which pasture growth was measured in the absence of added N fertiliser. Each sample comprised six cores that were taken to a depth of 75 mm at the start of each growth period and analysed for a range of soil measurements, which were related to pasture growth. There was a wide range of total pasture yield and pasture growth rates between sites and seasons. Measured total annual pasture production ranged from 2394 - 9067 kg DM/ha/yr in the absence of added N on HP northerly steep (HPNS) and HP northerly easy (HPNE) sites respectively. Total annual pasture production in the presence of non-limiting N (potential yield) ranged from 8001 - 18532 kg DM/ha/yr on LP northerly steep (LPNS) and HPNE sites respectively. Daily pasture growth rates in the absence of added N ranged from 1.13 kg DM/ha/day on the LPNS sites in mid spring to 34.51 kg DM/ha/day on HP southerly easy (HPSE) sites in the same period. Daily pasture growth rates in the presence of non-limiting N ranged from 3.78 kg DM/ha/day on LPNS sites in mid spring to 77.95 kg DM/ha/day on the HPNE sites in early spring. The major constraints on pasture growth within each site fluctuated throughout the year, between climatic and nutrient inputs. However, the most consistent and major constraint on all sites was N deficiency. Total annual pasture yield depression from potential due to N deficiency ranged from 5757 - 9465 kg DM/ha/yr on LPNS and HPNE sites. Daily pasture yield depression from potential growth rate ranged from 2.65 kg DM/ha/day on LPNS sites in mid spring to 45.36 kg DM/ha/day on HPNE sites in early spring. Increased P levels provided increased total annual yields, with HP sites yielding on average 1038 and 2209 kg DM/ha/yr more pasture in the absence and presence of non-limiting N respectively than LP sites. NE and flat sites produced the largest and most consistent P responses throughout the year, whilst the P response was generally largest in dry periods when soil moisture was limiting. This was suggested to result from an increased rooting depth on HP sites allowing them to source more water. Sunlight and temperature were major constraints on pasture growth on southerly and flat sites throughout late autumn and winter, whilst soil moisture was the major constraint on northerly sites from mid spring through to the end of summer as well as on southerly and flat sites from mid to late summer. Clover yields were small and scattered throughout the trial in all seasons. The HPNE and HPSE sites generally yielded the most clover throughout the year, with the highest yield of 1497 kg DM/ha/yr measured on HPNE sites in the presence of non-limiting N. The Mineralisable N soil test showed some potential for use as an indicator of potential pasture yield, however the varying temperature regimes between aspects and slopes limited its applicability to all sites due to the heavy dependence of the mineralisation rate on temperature. The Olsen P and ammonium-N and nitrate-N soil tests provided no significant relationship with pasture yield. A pasture growth model relating pasture growth (G) to evapotranspiration (Et) according to the equation G = kEt (where the proportionality constant (k), with units of kg D.M/ha/mm, is a site-specific factor which could be an index of soil fertility status), showed potential to be used in hill country. The model was able to account for much of the greater than two fold difference in total annual pasture yields between the lowest producing and highest producing sites in the presence of non-limiting N (R2 = 0.66), using a single growth constant (k) for LP and HP sites. When separate k values were used for LP and HP sites the relationship between measured and predicted total annual yield improved (R2 = 0.89 & 0.73 for LP and HP sites respectively). When the seasonal accuracy of the model was tested, prediction on some sites in some periods was found to be inaccurate. Some simple adjustments such as altering the depth of available water and winter growth rates on southerly steep (SS) sites was able to increase the accuracy of the model on some of the sites. Major factors decreasing the accuracy of the model appear to be depth of available water, soil hydrophobicity in summer, recharging of soil water through capillary action in spring and warm convection currents increasing pasture growth on SS sites in winter to above predicted levels. Further development of the model will require the effect of these factors to be quantified and accounted for. The model was also able to reasonably accurately predict the soil gravimetric water contents (top 75 mm) of sites of contrasting aspect and slope over a four year period. Overall, dryland hill country provides an extremely variable pasture production system which is affected by a great number of factors. The greatest factor affecting pasture growth appears to be plant available N, which in dryland hill country is inherently severely deficient. The other major factors are mostly climate-related and vary between sites depending on aspect and slope location. The intensification of farming systems demands that these variations can be accurately accounted for so that more accurate whole farm system models can be used to develop farming policy and economic models. Calculating E, and the subsequent depression from maximum yield by soil fertility, appears to be a method by which future models may more accurately aid farmers in analysing the efficiency of their system.Item A study of the combined effects of irrigation frequency and phosphorus fertility on summer pasture production : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Soil Science, Institute of Natural Resources, Massey University, Palmerston North, New Zealand(Massey University, 2003) Hewana Arachchige SumanasenaDuring the last five years, there has been an increase in both the area of irrigated pasture in New Zealand and the intensity of this irrigation. Research has failed to keep pace with this change: the benefits of irrigation to pasture production have not been studied in a sustained manner since the 1980s. Since then a number of factors have changed including; a change in the type of irrigation system commonly employed, the productive potential of new pasture cultivars, an appreciation of the importance of relationships between water and nutrient uptakes by plants, and a heightened awareness of the environmental implication of irrigation. It is claimed that the ability of irrigation systems such as centre pivot and long lateral systems to increase irrigation frequency affords a major advantage to pasture production. As yet, these claims are largely unsubstantiated in New Zealand. In addition, there has been no research of the mechanisms or processes that might account for this phenomenon. The study described here set out to quantify the benefits of more frequent irrigation (in the readily available water range) to ryegrass and white clover production, including the relationship with increased nutrient status, and to elucidate the mechanism(s) that might explain this response. The responses of ryegrass and white clover to irrigation frequency (within the readily available water range) and nutrient addition, particularly phosphorus (P) were investigated with a pot experiment using Ramiha silt loam. The rate of fertiliser addition to the pots had a significant and consistent effect on a number of indicators of ryegrass and clover performance including total yield. In contrast, irrigation frequency did not significantly or consistently affect total pasture production. It was concluded that when soil, nutrients and plants roots constitute a relatively homogenous mix (i.e. the pot environment), more frequent watering is not significantly advantageous to plant growth and, therefore, all of the readily available water is equally available. Although there was no response in pasture production to irrigation frequency in the pot experiment, it was hypothesised that irrigation frequency (in the readily available range) in the field, where P values vary with depth in the soil profile, would affect pasture production. The response of swards of ryegrass and white clover growing in Manawatu fine sandy loam to irrigation frequency and P status was measured in a field experiment during the summer of 2000/01. Three irrigation frequencies within the readily available water range (irrigation triggered at soil water deficits of 20 mm (I-20), 40 mm (I-40) or 60 mm (I-60)) were combined with two P fertility treatments (no P fertiliser added or 40 kg P ha-1 applied). For comparative purposes, there were also 4 non-irrigated, non-P fertilised plots outside the main trial block. Plant production, nutrient content of plant material, soil moisture content, soil N and P contents, and nitrate-N, ammonium-N and phosphorus concentrations of soil water samples were measured. The herbage on the plots was cut and removed i.e. there was no grazing. In the field, irrigation frequency had a significant effect on ryegrass and clover production. Irrigation of ryegrass and white clover at I-20, over the summer period resulted in the greatest pasture production and was associated with the most efficient water use (defined as k with units kg DM ha-1) of the irrigated treatments, I-60 gave the smallest production and water use efficiency. Application of the recommended quantity of P fertiliser (40 kg ha-1) significantly enhanced total pasture production and hence water use efficiency. Soil P and N was most concentrated in the surface soil. The results of the field trial support the hypothesis that ryegrass and white clover production is greatest when the plant is taking most of the water it requires from the surface soil where nutrients are most concentrated i.e. the frequent (I-20) irrigation case. Production is smaller when the plant is extracting large quantities of water from depth where nutrient concentrations are smaller i.e. the less frequent irrigation (I-60) case. The effects of irrigation frequency and P fertility on root re-growth activity of ryegrass and white clover swards were evaluated using a modified refilled core method. Root growth of both species decreased with depth. Fertiliser P application significantly increased root growth of both species in two of the three sampling depths at the December and February harvests. In only one root harvest did irrigation frequency significantly affect root activity. At the April harvest, the greatest root growth in the surface soil was observed for I-60, P-0 plots. It is suggested that in addition to encouraging more moisture uptake from nutrient-rich surface soil, an additional benefit of frequent irrigation is that in soils that are consistently moist, plants need to produce fewer roots. A simple water balance model was developed to simulate volumetric soil water contents in the three depths of the Manawatu fine sandy loam that are most closely related to the three irrigation frequencies i.e. 0-150 mm, 0-300 mm, and 0-450 mm. The model illustrates how initially the plants extract most of their water requirement from the surface soil and then as the profile dries they remove more water from lower depths. Accordingly, it highlighted differences in soil water contents between the irrigation frequencies particularly in the surface soil (0 - 150 mm). Soil water sampling was conducted using ceramic suction cups. Estimated total nitrate-N losses until 31 July, 2001 indicated that irrigation frequency of ryegrass during the previous summer did not have a major effect on the overall nitrate-N leaching losses during the late autumn/early winter period but nitrate-N losses under clover tended to be lower under less frequent irrigation. P and ammonium-N leaching losses were negligible. Using the understanding developed in the pot and field trials, a model was constructed to predict ryegrass and clover production on Manawatu fine sandy loam under the different irrigation frequency and P fertility regimes. The model relates pasture production (G) to evaporation from a series of soil water deficit ranges (Ei) according to G = k1E1 + k2E2 +...+ knEn (where kiEi is the pasture production when soil water is in the ith soil water deficit range, and ki is the water use efficiency when soil water is in the ith soil moisture deficit). The ki values were derived using the production data from the field trial. The model was used to simulate the effect of irrigation frequency and P fertility on seasonal (1 November to 30 April) pasture production for a range of climate conditions using the past 26 years weather data. The simulation illustrates how pasture production under irrigation varies markedly with climate, irrigation frequency, P fertility status and the ryegrass:clover composition of the sward. Increasing irrigation frequency from I-60 to I-20 increased pasture production, on average, by 1473 kg DM ha-1 (23%) and 1105 kg DM ha-1 (19%) for P-0 and P-40, respectively. For the farmer contemplating the adoption of irrigation, the purchase of a system that allows more frequent irrigation is as significant a consideration as the decision to adopt irrigation itself. On a cautionary note, the model suggests that I-20 irrigation typically increases drainage losses by about 40 mm (42%) compared to I-60 irrigation.