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

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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 H2SO4. 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.