RESIDUAL PHOSPHATE FERTILIZER COMPOUNDS IN SOILS .2. THEIR INFLUENCE ON SOIL TESTS FOR AVAILABLE PHOSPHATE

Six phosphate (P) compounds, including soil minerals and fertilizer residues, were extracted with eight standard soil P-test reagents. In the absence of soil, the solubility of P differed greatly depending upon P compound type and extractant. For example an aluminium phosphate was almost completely soluble in Bray-1 reagent yet it was insoluble in Morgan reagent. P sorbed by calcium carbonate was insoluble in acid Bray-1 reagent but soluble in the alkaline Colwell soil test. In the presence of soil, the amounts of P extracted from the compounds were commonly reduced by sorption of dissolved P by the soil. For example, most dissolved P was sorbed by the soil from an ammonium sulphate solution whereas relatively little P was sorbed by soil from the Bray-1 reagent. The strong influence of the type of P compound on the solubility of P in soil test reagents might be partly responsible for the need for different calibration curves of P soil test versus plant yield for soils that have received different types of P fertilizer.