Effect of water: soil ratio on phosphate release: P, aluminium and fulvic acid associations in water extracts from Andisols and Andic soils

Summary The influence of water: soil ratio (w: s) on phosphorus fractions was studied in Andisols and Andic soils. Phosphorus was fractionated in 0.45‐μm filtered water extracts, and pH, monomeric Al and dissolved organic carbon (DOC) were determined. Speciation of dialysed extracts was also studied. Increasing w: s from 2.5 to 75 resulted in a release of dissolved reactive phosphorus (DRP), while DOC decreased significantly and pH increased moderately. For the 29 samples studied desorbed DRP =K (w: s)β, where β represents a P buffering term and ranged from 1 to 1.26 for DRP (mg kg−1). On dialysis, we observed a uniform partition between dialysable (DP) and non‐dialysable P (NDP) which was explained by the corresponding difference in DOC; these results were consistent with the existence of soluble P‐Al‐fulvic complexes. Study of solubility diagrams for determining solid‐phase controls on P activities in the extracts suggested that dissolution of Al‐phosphates could explain most of P release to water as free orthophosphate. Both this mechanism and organic P partition between solid and solution soil phases may be considered to reflect the main sources of P in Andisols: Al‐phosphate and organic‐Al‐P complexes.