Effect of extraction time and charcoal treatment on the adsorbed sulphate values of several New Zealand Topsoils

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Abstract
Results of experiments in which 12 New Zealand topsoils were extracted with 0.04 M Ca (H2PO4) for ½, 1 and 16 h, and extracts analysed for sulphate by the Johnson & Nishita reduction method with and without activated charcoal pretreatment, are reported and discussed. Non‐charcoal treated extracts gave higher sulphate levels than treated extracts. The difference in levels was ascribed to the presence of organic S, which was reduced during the Johnson & Nishita procedure and thereby incorporated into the sulphate determination in the non‐charcoal treated extracts. The amount of organic S extracted increased with extraction time. In general the amount of organic S involved was not large (2–3 ppm S), but was larger (up to 7.5 ppm S) in the higher S‐status topsoils, with high organic C content, formed from volcanic parent materials. Increases in extraction time also lead to varying increases in the amount of inorganic sulphate extracted (charcoal treated extracts). The implications of the results in relation to differences in experimental methods for determining the adsorbed sulphate status of soils are discussed and some suggestions made for improving the nomenclature for phosphate extractable sulphur in soils.