ASSESSMENT OF METHODS FOR STUDYING THE DISSOLUTION OF PHOSPHATE FERTILIZERS OF DIFFERING SOLUBILITY IN SOIL .1. AN ISOTOPIC METHOD

An isotopic exchange method, based on tracer kinetic theory, was used to study the dissolution (F(in)) and retention (F(out)) of Phosphate (P) fertilizers in the soil. This method involves labelling of the soil with carrier-free P-32-phosphate ions, and monitoring changes with time of specific activity of isotopically exchangeable phosphate in the soil as extracted by plants, and of the amounts of readily exchangeable P. To assess this method, experiments were carried out to determine the rates of release and retention of phosphate from three fertilizers, monocalcium phosphate (MCP), ground North Carolina phosphate rock (NCPR < 150 mum), and 30% acidulated (with phosphoric acid) NCPR (NCPAPR), in two soils, Tekapo fine sandy loam and Craigieburn silt loam. MCP was applied at 75 and 150 mug g-1 soil; NCPR was applied at 150 and 750 mug g-1 soil; and NCPAPR was applied at 150 mug g-1 soil. After 1, 50 and 111 days of incubation, the treated soil samples were labelled with carrier-free P-32-phosphate ions, and perennial ryegrass (Lolium perenne) was grown to sample specific activity in these samples. Results showed that the changing patterns of exchangeable P in the soils as affected by fertilizer solubility and application rate could be clearly explained by the values of F(in) and F(out). With increasing period of soil-fertilizer contact, the P added in the form of MCP solution was rapidly transformed into non-exchangeable forms (high F(out) value). Exchangeable P in the NCPR and NCPAPR treated soils were maintained at steady concentrations for extended periods due to the continued release of P from the fertilizer material (steady F(in)) and lower rates of P retention by the soil (smaller F(out)). The dissolution rate of NCPR at the lower application rate was smaller in absolute terms, but greater in relative terms.