Abstract

Summary. Attempts to measure the sulfur (S) requirements of pastures using existing soil test procedures have largely been unsuccessful due to the extractants either under- or over-estimating the availability of S from soil organic matter.

A range of extractants was used to assess the availability of S at 2 field sites on Kentucky yellow podzolic and Walcha krasnozem soils located on the Northern Tablelands of New South Wales. The experiments were conducted over a 2-year period on improved perennial pastures. The 7 extraction techniques used were: 0.1 mol Ca(H₂PO₄)₂/L (MCP_t); 0.1 mol Ca(H₂PO₄)₂/L, treated with activated charcoal (MCP_i); water (H₂O); 0.25 mol KCl/L, heated at 40˚C for 3 h (KCl-40); 0.25 mol KCl/L, heated at 100˚C for 4 h (KCl-100); 0.5 mol NaHCO₃/L, (NaHCO₃); and an acid digestion of the soil (total).

In both soils, the MCP_i technique extracted the least amount of S. The amount of S extracted by the MCP_t, H₂O and KCl-40 techniques was intermediate. The KCl-100 technique extracted greater amounts of S than these techniques. For the Kentucky yellow podzolic soil, the KCl-100 technique extracted the same amount as the NaHCO₃ technique but extracted less for the Walcha krasnozem soil.

The extraction techniques which varied least throughout the year were total and NaHCO₃ for both soils. The KCl-40 technique was as stable as these techniques for the Kentucky yellow podzolic soil but was less stable for the Walcha krasnozem soil. Nevertheless, the KCl-40 technique had a higher stability than the KCl-100, H₂O and MCP_t techniques, while the MCP_i technique was the least stable.

Pasture S content response to applied S fertilisers was related to dry matter production of each site, which was largely determined by the environmental factors (soil moisture, temperature and solar radiation). Seasonal changes in the level of soil S only modified this relationship for the Walcha krasnozem site when environmental conditions promoted mineralisation of soil organic S which was retained within the soil profile due to sulfate adsorption and resulted in an increase in inorganic soil sulfur. These findings suggest future research is required into field measurements of the ability of soil to supply S as determined by the rate of mineralisation and leaching in relation to the plant‘s demand for soil S.