Sulfate and mineralisable organic sulfur in pastoral soils of New Zealand. II*. A soil test for mineralisable organic sulfur

Abstract

Annual pasture yield responses to sulfur, at sites where no sulfur had been added for at least 1 year, correlated well with both soil sulfate and extractable organic-S. However, sulfate was far more subject to interference from external influences, particularly fertilisers, grazing animals, and rain. A soil test is advocated in which both soil sulfate 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 value. Potassium phosphate, in contrast to calcium phosphate, did not inhibit the extraction of organic-S (and organic matter generally) at organic-C ≥ 5%. Consequently, the organic-S extracted by potassium phosphate correlated much better with pasture response to S, and is the recommended extractant. Both potassium and calcium phosphates extracted the same amount of sulfate. The sulfate mineralised during short-term incubation, and the organic-S extracted by 0.5 M sodium bicarbonate did not correlate with S response in pasture as well as organic-S extracted by potassium phosphate. The alkaline bicarbonate extracted about 5 times more organic-S than potassium phosphate, but extracted a similar amount of sulfate. * Part I. Aust. J. Soil Res. 1996, 34, 385–403.