A literature review and evaluation of the. Hedley fractionation: Applications to the biogeochemical cycle of soil phosphorus in natural ecosystems

The Hedley fractionation recognizes plant-available forms (Resin Pi, Bicarb Pi, and Bicarb Po) and refractory forms (NaOH Pi, NaOH Po, sonic Pi, sonic Po, HCl Pi, Residual P) of soil phosphorus. This updated survey of the recent literature shows that the sequential fractionation proposed by Hedley et al. can also be used to separate forms of organically bound soil phosphorus from the geochemically bound fractions. We consider that biological P includes all the extracted organic fractions (Bicarb Po, NaOH o, sonic Po) and geochemical P includes the remaining fractions (Resin Pi, Bicarb Pi, NaOH Pi, sonic Pi, HCl Pi) and the Po and Pi in the Residual fraction. Data from the Hedley fractionation suggest that the contribution of geochemical versus biological processes to soil phosphorus availability varies with pedogenesis. The pool of primary phosphate declines and the NaOH and sonicated-NaOH phosphorus fractions increase as phosphorus becomes geochemically fixed to the iron and aluminum oxides in more highly weathered soils. The sum of organic-P fractions — biological P — is an increasing proportion of total available P as a function of soil development. Therefore, the Hedley fractionation provides a valuable index of the relative importance of biological processes to soil phosphorus content across a soil weathering gradient.