Residual phosphate fertilizer compounds in soils

Abstract

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.