SULFATE AND MINERALIZABLE ORGANIC SULFUR IN PASTORAL SOILS OF NEW-ZEALAND .2. A SOIL TEST FOR MINERALIZABLE ORGANIC SULFUR

Annual pasture yield responses to sulfur, at sites where no sulfur had been added for at least 1 year, correlated well with both soil sulfat e and extractable organic-S. However, sulfate was far more subject to interference from external influences, particularly fertilisers, grazi ng animals, and rain. A soil test is advocated in which both soil sulf ate and organic-S are measured. Organic-S, extracted by phosphate (20 mM), is measured to indicate the long-term supplying power of the soil (also related to total organic-C and -S); and sulfate is measured to indicate any additions from external sources, which would have a short -term effect on pasture growth. The latter is estimated as the sulfate in excess of the quasi equilibrium value, based on the organic-S valu e. Potassium phosphate, in contrast to calcium phosphate, did not inhi bit the extraction of organic-S (and organic matter generally) at orga nic-C greater than or equal to 5%. Consequently, the organic-S extract ed by potassium phosphate correlated much better with pasture response to S, and is the recommended extractant. Both potassium and calcium p hosphates extracted the same amount of sulfate. The sulfate mineralise d during short-term incubation, and the organic-S extracted by 0.5 M s odium bicarbonate did not correlate with S response in pasture as well as organic-S extracted by potassium phosphate. The alkaline bicarbona te extracted about 5 times more organic-S than potassium phosphate, bu t extracted a similar amount of sulfate.