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

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.