EFFECT OF CHANGES IN PH ON SULFATE AND PHOSPHATE RETENTION IN SOILS

Soils with differing capacities to sorb anions were incubated with lime, each to a range of pH values (4.5-6.8), and characterised for their ability to sorb phosphorus (P) and sulphur (S) by sorption isotherms, and "Quick Test" sulphate retention, and phosphate retention methods. Although both phosphate and sulphate sorption changed with pH, the effects of lime were much more significant for sulphate than for phosphate. Sulphate sorption was reduced in the soils studied as pH was increased, and this was also reflected in sulphate retention Quick Test results. In contrast, phosphate retention values showed little dependence on pH. These findings have implications for the mobility of sulphate in soils, the correlation between P retention and S retention values, and the methodology of sulphate retention tests.