Forms of phosphate in soils fertilized with rock phosphate and superphosphate as measured by chemical fractionation

Abstract

The forms of P in three Western Australian soils that had been fertilized up to 5 years previously with rock phosphates (RP) and superphosphate were determined by a P fractionation procedure using 1 M NH4Cl, 0.5 M NH4F, 0.1 M NaOH+1 M NaCl, citrate-dithionite-bicarbonate (CDB), 1 M NaOH, and 1 M HCl. P extracted by NH4Cl and NH4F from soils fertilized with superphosphate decreased with time since application. For soils treated with apatitic RP the amounts of fertilizer P extracted by 1 M HCl decreased with time since application. This decrease was probably due to loss of apatite grains from the top soil by eluviation, bioturbation and cultivation rather than continuing dissolution of RP. For these soils most of the fertilizer P (63-67%) remained in the soil as residual apatite 5 years after fertilizer application, and this P is soluble in 1 M HCl. Over the 5 years of the experiment there was no systematic transformation of one form of P to another in soils fertilized with apatitic RP. An average amount ranging over 20-37% of added P was lost from the 0-10 cm sampling depth for different fertilizers.