Investigating the nature of soil phosphate

Abstract AimsThe effectiveness of phosphate fertilisers decreases with increasing period of reaction. One explanation for this is that the phosphate (P) has changed its chemical form. Evidence for this is provided by fractionation schemes which include extraction with alkali, followed by extraction with acid. Our aim was to evaluate alkali and acid extraction procedures for their ability to reflect changes in phosphate form over time.MethodsWe measured the effect of period of reaction between P and an iron (hyroxy)oxide (goethite), an aluminium oxide, and a soil, using four methods. These are: extraction with acid, extraction with alkali, and extraction using two soil P fractionation schemes. ResultsPlots of P extracted by acid and by alkali through time were similar. This is not consistent with the idea that they extract different kinds of phosphate. With increasing period of reaction, more iron and more aluminium needed to be dissolved in order to dissolve the same amount of P. The rate of dissolution of iron and of aluminium decreased with increasing period of reaction. For fractionation methods, the amounts of P extracted from a soil by the first extraction steps decreased with time while the amounts extracted by the later steps increased. ConclusionsOur results are consistent with diffusion of P into the reacting materials rather than formation of different chemical forms. Phosphate diffused into defects and “repaired” them. Fractionation methods reflect increasing vigour of extraction rather than the presence of separate forms.