Browsing by Author "Moir, James Laing"

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    Influences of climate and fertiliser application history on various measures of soil fertility and productivity in Wairarapa hill country : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Soil Science at Massey University, New Zealand
    (Massey University, 1994) Moir, James Laing
    During the agricultural downturn of the mid 1980's, it became uneconomic for many Wairarapa farmers to apply fertiliser. Those farmers who could afford to apply maintenance fertiliser, often chose to apply diammonium phosphate (DAP). The increasing popularity and apparent effectiveness of fertilisers such as DAP, which has a nitrogen component, prompted further questions of whether traditional P and S fertiliser application (as single superphosphate), applied to stimulate pasture legume growth and biological nitrogen fixation, was the most cost effective fertiliser strategy. A review of literature revealed that soil fertility and pasture production data for different rainfall regimes in the Wairarapa was scarce. No published data was available on how effective single superphosphate (SSP) applications to Wairarapa hill country farms had been in increasing annual N fixation rates and plant­ available soil N, and hence increasing pasture production. It was concluded that it was necessary to conduct a series of pasture field trials on sites varying in soil type, fertiliser history and climate (annual rainfall) in order to provide data applicable to the Wairarapa region. Sites falling within three rainfall regimes were selected, those being Mauriceville (high rainfall), Gladstone (summer dry) and Whareama (low rainfall) sites. Using total soil phosphorus, sulphur and nitrogen analyses, a total of 14 sites were selected for study, with sites ranging in fertility status within each climate zone. The objective has been to characterise the soil types, soil nutrient status, climate and current pasture production of Wairarapa hill country farms, with a view to completing further studies examining in more detail the complex interactions of soil, climate and pasture. Soil chemical analyses revealed a wide range of soil fertility status across all sites. Soil total phosphorus (P) contents ranged from 430 µgP/g soil (site 4) to 1470 µgP/g (site 1), while total soil sulphur (S) showed less variation, ranging from 345 µgS/g (site 11) to 860 µgS/g (site 9). Soil total nitrogen (N) contents followed a similar pattern to that seen for total S, and ranged from 4280 µgN/g (site 11) to 7950 µgN/g (site 9). Fertiliser history had a large influence on the accumulation of P, S and N in these hill country soils, where higher rates of accumulation were associated with greater levels of fertiliser input. However, P accumulated at a far greater rate than S and Nin these soils, which is possibly the result of high S and N leaching losses. Measurements of plant-available nutrient levels followed a similar trend to that seen for soil total elemental analyses, where higher levels of nutrient were correlated with higher fertiliser inputs. Estimates of the efficiency of past fertiliser applications were made for these sites, using the results of various soil analyses. Traditional P and S fertiliser applications and pasture management on Wairarapa hill country appear to have been particularly inefficient in causing soil N to accumulate. The calculations used to derive these estimates have limitations, but do indicate that either product N and leaching losses are high or N fixation rates are low (or both) in these soils. July 1993 to March 1994 was an average growing season, and total herbage yields harvested from August 1993 to March 1994 ranged from 4 tDM/ha (site 4) to 15.5 tDM/ha (site 1). Soil fertility status (and hence historical SSP applications) was the main factor influencing total herbage yield, where high yields were recorded at high fertility sites, and the reverse for low fertility sites. Climate (soil moisture levels) also influenced total yield but to a lesser extent than soil fertility status. Pasture growth at Gladstone and Whareama sites stopped when soil volumetric water content in the top 7.5 cm fell below 0.2. At the wetter Mauriceville sites, soil moisture was not limiting until mid-February 1994. Legume growth was particularly sensitive to soil moisture stress. By converting pasture production to stock units, gross margin analyses were performed. The most profitable options in all three rainfall regimes were sites which had received frequent fertiliser applications. This suggests that historical fertiliser applications are economically effective, which is an important factor in sustainable agricultural enterprises. Herbage N and P uptake results supported this finding, and showed that pasture N uptake varied widely between high and low fertility sites. Pasture N uptake ranged from 70 kgN/ha at low fertility site 4, to 250 kgN/ha at high fertility site 1 for the period of early August 1993 to early January 1994. This implies that historical superphosphate applications have been effective in providing large increases in annual amounts of plant-available soil N at high fertility sites when compared to unfertilised sites, despite the fact that soil N accumulation was less than expected. Acetylene reduction activity (ARA) measured at each harvest showed that annual N fixation levels are limited by extended summer dry periods which stop legume growth. The wetter Mauriceville sites fixed more N on an annual basis than Gladstone and Whareama sites. ARA was linearly related to yield. Variations in the data indicated that species and other short-term soil condition changes have a large effect on the relationship between ARA and N fixation rates. Although soil N accumulation is slow in these pasture systems, annual pasture N uptake is dramatically increased where fertiliser inputs have been high. The results indicate that rapid cycling of soil/plant N is occurring, and that annual leaching and product losses of N may nearly equal N fixation rates. This was exemplified in a simple budget of the nitrogen cycle, taking account of N losses and gains in a low fertility and high fertility system. There was insufficient information to conclude why soil N is not accumulating in these grazing systems. Further research is required to fully explain this N cycle, including the relative quantities of N inputs and losses from the system.
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    Investigations into the influence of fertiliser history and climate regime on the soil fertility, soil quality, and pasture production of Wairarapa hill soils :|ba thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1994) Moir, James Laing
    The effects of long-term application of single superphosphate (SSP) on soil plant-available nutrient supply and indicators of soil biological quality was investigated on Wairarapa hill soils ranging widely in previous fertiliser history (from 0 to 250 kg SSP ha-1 yr-1) and climate regime (annual and seasonal rainfall distribution). At 12 field sites spring pasture response to strategic N fertiliser application was measured, while the plant-available nutrient (P, N and S) supplying capacity of the soils was assessed in glasshouse studies. Based on the pasture growth patterns in field and glasshouse studies, a new climate-driven, soil fertility dependent pasture growth model was developed and tested. In addition, the suitability of the Biolog™ GN microtitre plating system was assessed as an indicator of soil 'quality', using these Wairarapa hill country soils. Results of soil analyses indicated that small increases in mineralisable N, in the order of 280 kg mineralisable Nha-1, with increased rates of fertiliser (P and S) may represent inefficient use of P and S fertiliser. Soil mineralisable N increased by approximately 8.6 kg mineralisable Nha-1 for every 1 unit increase in Olsen P. The ratio of accumulated plant-available N:P:S of these soils, resulting from long-term SSP applications, is approximately 17:2:1. Olsen P status was shown to be strongly correlated with measures of plant-available N and S. Pasture growth response in the field to strategic N fertiliser (30 kgNha-1) applied in spring was highly variable across sites, and within the range of 0:1 to 31:1 kgDM kgN-1. Simple single factors representing soil fertility indices, or climatic regime, could not explain the variation in site-to-site pasture growth response to applied N. Factors constraining N response are discussed. In glasshouse studies, on samples of the same soils, ryegrass and white clover showed large yield differences (clover, 0.27-2.29 gDM pot-1; ryegrass, 0.22-2.25 gDM pot-1) on low P status and high P status soils respectively. Glasshouse DM yields did not correlate with those measured in the field, confirming that at field sites yield responses to nutrient availability are strongly modified by (site-specific) climate. The relationship between Olsen P and clover yield in the glasshouse (curvilinear, R2 = 0.80) was similar to that previously seen in (spring) field conditions. The S:P and N:P ratios of clover in the glasshouse trials confirm that P availability in these soils is the major growth-limiting factor, probably followed by S or N, which becomes limiting when P availability is adequate to high. A modified Stanford and DeMent bioassay technique was used to estimate the amount of plant available N, P and S in each soil. Using an exhaustive cropping regime, these soils exhibited a large variation (range) in ryegrass yields when soils were the sole source of P and N. Yields for each soil were strongly correlated with various soil tests for N, S and P availability. S availability to plants was less variable across soils, but the smaller variation in S limited yield was still strongly correlated with the variation in a newly developed soil hydrogen peroxide-extractable S test. Results from both glasshouse experiments provide strong evidence that the Olsen P soil test is a valuable soil fertility indicator of plant-available P, N and S on legume-based pasture soils with a history of superphosphate use. The amount of dry matter production, when considered with the quantity of soil used for each treatment (-N, 100g; -P, 50g; -S, 25g), suggest that these soils have large pools of plant-available or mineralisable P and S, and, relative to plant demand, small pools of soil mineralisable N. A four-fold increase in field DM production resulted from a 3-fold increase in soil mineralisable N at these sites. This suggests that the rate of N cycling probably also increases with yield increase, and that the size of the soil mineralisable N pool is not directly related to pasture N supply. A new climate-driven, soil fertility dependent pasture production model has been developed and tested using actual DM yields from the field trial sites. The model assumes that pasture growth is proportional to evapotranspiration, and that the proportionality constant (k) depends on soil fertility Pasture growth per mm of evapotranspiration was strongly related to soil available P status at these sites. From results of the glasshouse study, it was concluded that Olsen P was a strong indicator of 'general' (plant-available P, N and S) across these sites, and therefore suitable for use as the soil fertility proportionality constant in the pasture production model. Soil-limited evapotranspiration is calculated from a simple daily soil water balance model. Values for k varied from 11 to 19 kg DM ha-1 mm-1 of evaporation. With the exception of growth after severe drought conditions, the model shows potential to closely predict actual pasture yield. It is hoped that discrepancies between the modelled and measured production may lead to useful speculation and further research on the interacting effects of weather and fertility on pasture growth. The Biolog™ GN microtitre plate system, for comparing substrate use patterns of 95 single C compounds was assessed as an indicator of soil microbial functional diversity across the 12 test hill soils. Preliminary studies showed that saline extracts of different fertility status pasture soils used for Biolog™ microtitre plate assay inoculation contain significant amounts of readily available C. It was concluded that in order to interpret the substrate use patterns correctly, this effect must be corrected for. The Biolog™ microtitre plate system, for use as an indicator of soil quality and health, was shown to have limited application to this range of pasture soils with differing pasture histories. Adaptive factors, such as constitutive and inducible enzyme activities, were shown to complicate the interpretation of microbial growth on the C substrates. Substrate use patterns also changed when soils were rewetted and incubated. Possible 'indicator' substrates were identified, but it was concluded that these were low-energy decomposition products, and as such, are not useful as indicators of microbial functional diversity across these soils. Further research would be required to establish how stable the substrate use patterns are, or the relevance of these indicators to field soil processes. However, as a research tool, the Biolog™ assay showed potential to separate these soils on the basis of microbial functional diversity. The direction of future research, and limitations of current techniques used in this field are discussed.