Defining the relationships between pasture production and soil P and the development of a dynamic P model for New Zealand pastures: A review of recent developments

Abstract

A database was constructed comprising records from 2255 pasture phosphorus (P), potassium (K) and sulphur (S) field trials, of which 1799 included one or several rates of P. Subsets of this data were selected based on predetermined criteria to define the relationships between relative pasture production and available soil P (0-75 mm, Olsen P in µg P cm -3 soil)-the P production functions-for the major soil groups in New Zealand. These relationships, and their 95% confidence intervals, were defined using Bayesian statistics. For those soil groups for which there was sufficient data, the production functions were well defined and gave reasonably precise estimates of the relative pasture yield for a given Olsen P. the relative pasture production is most likely (P < 0.05) to be in the range 88-94% at Olsen P 25 and 98-100% at Olsen P 50. The shape of the production functions was similar for all soil groups-the relative pasture production increased with increasing Olsen P up to an asymptote-except the pumice soils and the podzols, which showed irregularities. The production function for the podzols was also flatter. There was good agreement between the empirically derived production functions and those generated from a dynamic P model. The Olsen P level required to achieve 97% maximum production was estimated for all soil groups. These ranged from 10 to 45 depending on soil group. The critical Olsen P levels were related to the soil anion storage capacity (ASC, a laboratory measure of P buffer capacity) and to soil volume weight (g cm -3 of sieved and dried soil), although not strongly. The field measured P buffer capacity (ΔP F )-the amount of soluble fertiliser P (kg P ha -1 ) required above maintenance to increase the Olsen P (0-75 mm) level by 1 unit-was estimated for selected trials. There was reasonable agreement between these estimates and those derived from the P model (ΔP M ), and these results indicated that ΔP decreases with increasing Olsen P. The results imply that factors other than those related to soil chemical properties affect the relationship between soil P and pasture production. The factors which determine the relationship between pasture production and soil P are defined and discussed. These were assigned to two categories: those factors which affect the ability of the soil to supply P for plant uptake and those that affect the ability of the plant to acquire soil P. It is concluded that further progress towards improving our ability to predict pasture responses to fertiliser P will depend on quantifying the latter effects. Based on these results and the development of a dynamic P model, an econometric P model was developed for New Zealand pastures which enables consultants to quantify the likely agronomic, financial and investment effects of any given fertiliser strategy on a given farm or block within a farm. This was not previously possible but is essential for the sustainable use of P fertilisers in pastoral farming.

Pasture production from a long-term fertiliser trial under irrigation

“…97%RY) was c. 19 mg/ml. This Olsen P concentration is just below the average of 20 mg/ml derived for sedimentary soils (Morton & Roberts 2010) and less than the critical level for sedimentary soils of 26Á32 mg/ml reported by Edmeades et al (2006), but falls within the scatter of field trial results presented in their paper. Furthermore, work by Sinclair et al (1997) and Gillingham et al (2007Gillingham et al ( , 2008 showed that there is little advantage in sheep pastures of Olsen P concentrations greater than 20 mg/ml.…”

“…5) Sinclair et al (1997) and Edmeades et al (2006). The relative yield of the RPR treated plots relative to the SSP asymptote are also presented in Fig.…”

“…Data from the trial for the early years (1952Á1989) was obtained from the database of P, S and K research trials set up in 1990 (Feyter 1993) and described by Edmeades et al (2006). The data was in the form of individual treatment means plus the standard error (SE) for the individual harvests as well as annual totals.…”

“…Much research has been carried out to ensure that current P fertiliser application rates are sufficient to maintain high pasture production via an optimal Olsen P concentration (Cornforth & Sinclair 1982, Sinclair et al 1997, Roberts & Morton 1999, Morton & Roberts 2010, Edmeades et al 2006. The majority of these field trials were conducted under mowing with stock excluded (e.g.…”

Effects of long-term exposure to enriched CO2 on the nutrient-supplying capacity of a grassland soil

“…However, it is somewhat surprising that an available P concentration of 18 μg ml −1 for the enriched CO 2 soil would limit plant growth. An analysis of a New Zealand P fertilizer trial database identified an Olsen P level of 12 μg ml −1 as a critical level for sandy soils, below which production would be reduced to less than 97% of relative yield (Edmeades et al 2006). Calibration of soil P levels and pasture production specific to the NZ FACE site would be needed to verify critical P levels for plant growth in this soil.…”