A. G. Sinclair

Relationship between pasture dry matter yield and soil Olsen P from a series of long‐term field trials

Abstract Forty‐six data‐sets from a series of 17 long‐term field trials measuring the effects of rates and forms of phosphate (P) fertiliser on pasture production and soil fertility in New Zealand were used to characterise the relationship between pasture dry matter (DM) yield in any year (expressed as relative yield, RY) and soil Olsen P test at the end of the annual DM measurement period. Average coefficients of variation in the measurement of these parameters were 7% and 17% for DM yield and Olsen P, respectively. The results failed to demonstrate that the RY at any site in any year could be reliably assessed from a common relationship between RY and the Olsen P test. However, the mean relationship did take the general “diminishing returns” form as expressed by the Mitscherlich equation, and as Olsen P levels increased the probability of low RY decreased. There was no evidence that RY increased with increase in Olsen P test values above 20, with the exception of the one yellow‐brown pumice soil in the ...

Soil acidification: A provisional model for New Zealand pastoral systems

Abstract Soil acidification is a major problem internationally and occurs in pastoral systems as a result of nitrate leaching, nutrient transfer/removal, and soil organic matter (SOM) accumulation. Existing mechanistic models on soil acidification calculate acidification rates retrospectively, based on quantitative information on these acidifying processes. This paper presents an outline of a predictive soil acidification model that includes sub‐models for estimating the extent of nitrate leaching, nutrient transfer/removal, and soil organic matter accumulation. The model was used to predict acidification rates in a fertiliser trial on sheep‐grazed pastures, receiving superphosphate fertiliser for 35 years. The predicted rates ranged from 1.46 to 3.83 kmolc/ha.year, which was in moderate agreement with acid additions rates of 3.09 to 3.43 kmolc/ha.year as calculated from measured changes in soil pH and the pH buffer capacity (pHBC). Further development of the model is needed to include a SOM accumulation ...

Effects and interactions of phosphorus and sulphur on a mown white clover/ryegrass sward: 1. Herbage dry matter production and balanced nutrition

Abstract Phosphorus (P), sulphur (S), and nitrogen (N) concentrations were measured on white clover herbage from a fertiliser field trial on a mown white clover/ryegrass sward involving 5 rates of P by 5 rates of S in a factorial design. Samples were analysed from two harvests in Year 1 and three harvests in Year 2 of the trial. P fertiliser markedly increased % P and slightly reduced % S in white clover, whereas S fertiliser had corresponding effects on the percentages of S and P in white clover. % N in white clover was increased by P fertiliser in Year 1 and by S fertiliser in both years, with S fertiliser having much the larger effect. By relating herbage chemical composition to white clover DM yield response patterns, S/P ratio in white clover herbage corresponding to balanced nutrition was calculated to be 0.63 in Year 1 and 0.72 (at high fertiliser rates) in Year 2. When calculations were based on white clover N uptake instead of white clover DM, S/P ratio for balanced nutrition was 0.04–0.06 higher...

Agronomy, modelling and economics of reactive phosphate rocks as slow-release phosphate fertilizers for grasslands

Reactive phosphate rocks (RPRs) from Sechura, Peru (SPR) and North Carolina, USA (NCPR) were compared with triple superphosphate (TSP) as phosphate (P) fertilizers for permanent grass/clover pastures in four field trials in New Zealand. Trial sites ranged in initial pH (in water) from 5.7 to 6.3 and in rainfall from 712 to 1338 mm yr−1. SPR and NCPR were used in the unground ‘as-received’ state. Fertilizers were applied annually for six years. Pasture was harvested by frequent mowing, and herbage dry matter (DM) yields were measured at each cut. Herbage P concentrations were measured at each cut in two trials and on most cuts in the other two.For all sites combined, DM production from RPRs was initially significantly less than from TSP but it improved relative to TSP with time. Substitution values of RPR relative to TSP, denoted by S.V. (TSP/RPR) and defined as the ratio of P in TSP to P in RPR required to produce the same plant response during a specified period of time, were estimated by relating yields from RPR treatments to the yield response curve for different application rates of TSP. For the four trials combined, S.V. (TSP/SPR) increased from 0.32 in year 2 to 0.85 in year 6. S.V. values for NCPR were similar. The site which had the lowest S.V. values (average 0.20) for total production over six years was the site with highest pH (6.3) and lowest rainfall (712 mm). Corresponding S.V. values for the other sites were 0.50 to 0.78.Herbage P concentrations showed a similar pattern of RPR performance relative to TSP to that shown by DM production except at the highest application rate where TSP always supported much higher herbage P concentrations than RPR.The pattern of DM production from RPR relative to TSP was explained on the basis of a model involving soil P pools of undissolved fertilizer P and plant-available P respectively, with the hypothesis that P dissolved from RPR entered the plant-available P pool and was used with the same efficiency as P entering by dissolution of TSP. Model predictions of substitution values using directly measured RPR dissolution rates agreed well with observed substitution values.The advantage of RPRs in comparison to soluble P fertilizers for permanent pastures was considered to lie in their lower price and not in greater nutrient efficiency. Economic advantage was calculated in terms of the return on investment from establishing and maintaining a pool of RPR in the soil large enough to release the required annual amount of plant-available P compared with the cost of annual applications of soluble P fertilizer.

Patterns of, and a model for, dry matter yield response in grass/clover pastures to annual applications of triple superphosphate fertiliser

Abstract Pasture herbage dry matter (DM) responses to annual applications of triple superphosphate (TSP) at 0.5, 0.75, 1.0, and 2.0 times the calculated maintenance requirement were measured in 12 field trials for 6 years. The trials were on well‐established, permanent pastures which had typical fertiliser histories and represented major pastoral soils of North and South Islands of New Zealand. The DM response to TSP averaged over all rates and sites was 5% in Year 1, increasing to 23% in Year 4 with little change thereafter. At individual sites, average responses in total DM over 6 years ranged from 4 to 24%. Responses increased steadily with time at four sites, but at other sites changes with time were erratic. Mitscherlich curves were fitted to the 6‐year total DM production data for each site. However, the standard errors in the parameters of these fitted curves were very large, making it impossible to predict with any useful degree of accuracy the rate of TSP required for any specified yield. Respons...

Changes in soil Olsen P over six years with annual applications of triple superphosphate or reactive phosphate rock (Sechura)

Abstract The effects of annual applications at four different rates of either triple superphosphate (TSP) or Sechura phosphate rock (SPR) on the Olsen P test in soil under pasture were measured over 6 years at 13 sites in New Zealand. Averaged over the 13 sites, TSP applied at the calculated maintenance (M) rate maintained an almost constant Olsen P level throughout the 6 years. However, there were marked differences between sites, particularly at the 2.0 M application rate which at some sites raised Olsen P by over 30 μg P/ml soil but caused no increase at other sites. No obvious explanation such as soil group, phosphate retention, rainfall, or pH, or the TSP rates used could be found for these site differences. Olsen P tests were always markedly lower with SPR than with TSP; averaged over the 13 sites, even the 2.0 M SPR treatment barely maintained the initial Olsen P test values. This difference between TSP and SPR in their effect on Olsen P test contrasted with pasture production data (reported elsewh...

Effects and interactions of phosphorus and sulphur on a mown white clover/ryegrass sward 3. Indices of nutrient adequacy

Abstract Relationships were examined between relative yields of pasture with respect to phosphorus (P) and sulphur (S) applications, and clover herbage P and S concentrations and P/nitrogen (N) and S/N ratios in a P x S factorial fertiliser trial on a mown clover/ryegrass sward. The relationships between the yield of a treatment relative to the maximum yield obtainable with high rates of P alone (RY[P]) and both % P and P/N were affected by the application rate of S. S deficiency generally lowered the % P required for any given RY[P] but raised the P/N required. This latter effect was because S deficiency markedly lowered clover herbage N concentration. P deficiency had a similar but smaller effect on % S and S/N required for any RY[S]. S/N was much less sensitive to changes in clover S status than P/N was to changes in clover P status. This resulted from the strong stimulatory effect of S on clover % N. Relationships were also established between clover P and S adequacy indices and RY[PS] which is the he...

Effect of reactive phosphate rock on the pH of soil under pasture

Abstract Soil pH was measured over 6 years in 10 field trials in which superphosphates (SPs) and reactive phosphate rocks (RPRs) were applied annually to clover/grass pastures at rates equivalent to 0, 0.5, 0,75, 1.0, and 2.0 times the estimated amounts of phosphorus (P) required for maintaining near-maximum pasture production. Over the 6-year period, soil pH (0–75 mm soil depth) fell by an average of 0.16 units in control and SP treatments with no significant effect from rate of SP application. RPRs reduced the fall in pH and this effect increased with increasing RPR application rate, the fall in pH being virtually eliminated by the highest RPR application rate. The effect of RPRs on reducing the fall in soil pH could be largely accounted for by the difference in phosphate protonation compared with SPs, and the carbonate content of the RPRs.

Sechura phosphate rock supplies plant-available molybdenum for pastures

Abstract Sechura phosphate rock (SPR) applied annually as a slow-release phosphatic fertiliser in eight field trials (five in Northland and three in Southland) significantly raised molybdenum (Mo) concentrations in pasture herbage in each instance after 4—6 years. Mo concentrations in clover were raised from deficiency to sufficiency levels at two sites. Chemical analysis showed that SPR contained 43 ppm Mo; if applied annually for 4 years at rates equivalent to 30 kg P/ha per year this would provide twice the standard Mo recommended for a 4-year period. It is concluded that SPR can be an effective Mo fertiliser when applied annually at rates appropriate for maintaining phosphate status of pastures, but is unlikely to raise pasture Mo concentrations to levels harmful to grazing sheep.

Soil acidification through carbon cycling in legumes: A pot experiment examining the contributions from white clover, lotus, Caucasian clover, and lucerne

Abstract Soil acidification, induced by the growth of four legume species (white clover, lotus, lucerne, or Caucasian clover), was examined in a glasshouse pot trial at initial soil pH levels of either 4.5, 5.0, or 5.5. White clover and lotus out‐yielded lucerne and Caucasian clover at all soil pH levels, particularly at the lowest level of pH 4.5. The acidity produced as a result of this legume growth was shown to approximately correspond to the removal of excess cation over anion nutrients by the plants. Averaged over all three soil pH levels, plant excess cation concentrations decreased in the order: white clover > Caucasian clover > lucerne > lotus. With the exception of white clover, there was no evidence of any consistent change in excess cation concentrations as soil pH decreased, indicating that the rate of excess cation removal did not decrease as soil pH declined. Although white clover was shown to have a higher excess cation concentration than the other three legumes, plant yield was the major ...