SOIL-EXTRACTABLE SULFUR AND PASTURE RESPONSE TO APPLIED SULFUR - 2 - SEASONAL-VARIATION IN SOIL SULFUR TESTS AND SULFUR RESPONSE BY PASTURES UNDER FIELD CONDITIONS

Attempts to measure the sulfur (S) requirements of pastures using exis ting soil test procedures have largely been unsuccessful due to the ex tractants either under- or over-estimating the availability of S from soil organic matter. A range of extractants was used to assess the ava ilability 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 pere nnial pastures. The 7 extraction techniques used were: 0.1 mol Ca(H2PO 4)(2)/Ln (MCPt); 0.1 mol Ca(H2PO4)(2)/L, treated with activated charco al (MCPi); water (H2O); 0.25 mol KCl/L, heated at 40 degrees C for 3 h (KCl-40); 0.25 mol KCl/L, heated at 100 degrees C for 4 h (KCl-100); 0.5 mol NaHCO3/L, (NaHCO3); and an acid digestion of the soil (total). In both soils, the MCPi technique extracted the least amount of S. Th e amount of S extracted by the MCP,, H2O and KCl-40 techniques was int ermediate. The KCl-100 technique extracted greater amounts of S than t hese techniques. For the Kentucky yellow podzolic soil, the KCl-100 te chnique extracted the same amount as the NaHCO3 technique but extracte d 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 Kentuc ky yellow podzolic soil but was less stable for the Walcha krasnozem s oil. Nevertheless, the KCl-40 technique had a higher stability than th e KCl-100, H2O and MCP, techniques, while the MCPi technique was the l east stable. Pasture S content response to applied S fertilisers was r elated to dry matter production of each site, which was largely determ ined by the environmental factors (soil moisture, temperature and sola r radiation). Seasonal changes in the level of soil S only modified th is relationship for the Walcha krasnozem site when environmental condi tions promoted mineralisation of soil organic S which was retained wit hin the soil profile due to sulfate adsorption and resulted in an incr ease 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.