Dissolution of granulated low‐grade phosphate rocks, phosphate rocks/sulphur (Biosuper), and superphosphate in soil

Abstract The dissolution of two low‐grade phosphate rocks—Chatham Rise phosphorite nodules (CR) and Christmas Island ‘C’—after granulating with or without sulphur was studied by mixing these materials with soil and incubating at 25°c. A superphosphate treatment was included for comparison. Subsamples were taken at intervals over a period of 8 months, and were analysed for pH, sulphate, and phosphate in soil solution and Olsen's bicarbonate‐extractable phosphate. Greater acidity was produced when sulphur/phosphate rock mixtures were incorporated than with phosphate rock alone, owing to microbial oxidation of the elemental S to H 2 SO 4 . The oxidation of sulphur reached a maximum 3–4 weeks after incorporating CR/S and ‘C’/S. Addition of CR increased the bicarbonate‐soluble phosphate by a maximum of 24 μg/g, whereas CR/S increased it by 95 μg/g. Values similarly obtained for ‘C’ and ‘C’/S were 17 and 20 μg/g. Addition of superphosphate produced the highest level of bicarbonate‐soluble phosphate (maximum increase 177 μg/g) throughout the incubation period. No measurable difference in phosphate concentration of soil solution was found between rock phosphate and rock phosphate/S treatments. After 4 weeks of incubation CR increased the phosphate concentration in soil solution a little, whereas ‘C’ decreased it a little. Superphosphate maintained the highest concentration up to 15 weeks of incubation, after which the values were similar to CR.