A comparison of several procedures for estimating the sulphur status on soils

Abstract

The sulphur status of a number of soils was determined by several laboratory procedures. The chemical procedure which extracted amounts of sulphur showing the most satisfactory correlation with the sulphur-supplying ability of the soils (as measured by plant growth) was an extraction of the soil with a potassium dihydrogen phosphate solution containing 500 p.p.m. phosphorus. The superiority of this procedure over the other extraction procedures tested (cold-water extraction, hot-water extraction, acetate extraction, and Williams and Steinbergs's heat-soluble method) probably lies in its ability to displace adsorbed sulphate. The sulphate which was extractable by the phosphate solution was found to have decreased after cropping, by an amount which was closely related to, but not of the same order as, the sulphur taken up by the plants. The difference (uptake minus decrement), which represents the net mineralized sulphur, was small relative to the amounts of extractable sulphate initially present. Microbiological assay using Aspergillus niger was an equally reliable method for assessing sulphur status and had the advantage of simplicity. Except in two brigalow soils, the sulphur obtained by Aspergillus was very similar to that extracted by phosphate, which suggests that Aspergillus is able to obtain adsorbed sulphate.