An overview of the New Zealand sulphur cycling model for recommending sulphur requirements in grazed pastures

Abstract

Estimation of sulphur (S) fertiliser requirements in grazed pastures is crucial in sustaining pasture and animal production and in ensuring efficient use of S fertiliser resources in New Zealand. Different S cycling models have been developed worldwide to estimate pasture S requirements in grazed pastures. This review examined the S model developed in New Zealand by the Ministry of Agriculture and Fisheries (MAF) with the major aim of identifying limitations associated with and future research requirements for improving the applicability of this model. The review showed that the MAF S model incorporates most of the processes which contribute to S gains and losses from the S cycle under steady state conditions. However, S transfer rates within and between soil, plant, and animal S pools are not considered. The contribution of soil organic S to pasture plants is incorporated into this model by using the empirical pasture development index (PDI), the MAF soil sulphate (S04 2 -) "Quick test" (i.e., O.OlM Ca(H 2 P0 4 hextractable soil S042~), and the soil organic S component in this test (i.e., 0.01 M Ca(H 2 P0 4 hextractable soil organic S). An empirical sulphate leaching index (SLI) is also integrated into the model for assessing SOi-leaching losses from the cycle.Although both PDI and SLI indices include some factors that could influence S04 2 -leaching and the A92062 Received 24 November 1992; accepted 1 September 1993 accumulation and mineralisation of soil organic S, there is a lack of quantitative data on these processes under field situations. The dynamic nature of soil organic S makes it difficult to generalise the extent of soil organic S mineralisation based on PDI, MAF soil S04 2 -Quick test, and MAF soil organic S test. The review reveals a need to calibrate this soil organic S test with field and laboratory data on soil S mineralisation, pasture production, and herbage S uptake. Research is also required to provide quantitative data on excretal S transfer losses and effects of grazing animals and soil microbial activity on the cycling of S fertilisers and excretal S in soilplant-animal systems.