Elemental Sulfur Oxidation in Australian Cropping Soils

Sulfur fertilizers based on elemental sulfur (ES) have the advantage over sulfate-based fertilizers of releasing sulfate slowly over time, which effectively prevents leaching losses of sulfate from soils. The rate of S oxidation determines whether the applied fertilizer can meet the S demand of plants and this rate depends both on the soils' potential to oxidize ES and environmental conditions. In this study, we examined the capacity of 10 Australian cropping soils to oxidize ES under controlled environmental conditions (96-d incubation at 25°C and 70% field capacity). At the end of the incubation, 21 to 100% of applied ES was oxidized. The surface-based oxidation rate of ES was calculated from the decrease in ES concentration, and ranged from 5.1 to 51.7 μg cm-2 d-1. The oxidation rate was underestimated when estimated from the increase in concentrations of oxidation products (principally sulfate) extracted by CaCl2 due to incomplete extraction and immobilization of sulfate. The oxidation rate estimated from ES decrease was positively correlated with the initial soil pH (R2 = 0.54, P < 0.015). A regression equation including pH and organic C content as independent variables explained 79% of the variation in the oxidation rate. Oxidation rates of ES in these Australian soils were within the range of values reported in the literature for soils from other countries, measured using similar methods. Oxidation rates of ES measured by extraction of sulfate were often much lower than those derived from disappearance of ES, especially with short-term incubation methods. We hypothesize that discrepancies in oxidation rates between geographical areas observed in the literature, despite similar soil and environmental conditions, may be partly related to different methods used to determine the oxidation rate.