Hedley et al. [56] proposed a comprehensive scheme to fractionate soil inorganic and organic phosphorus based on chemical solubility. The scheme was based in part on the earlier work of Chang and Jackson [53] and involved sequential extraction of soil with anion-exchange resin, 0.5 M NaHCO3 at pH 8.5, 0.1 M NaOH, 0.1 M NaOH with ultrasonic dispersion, and 1.0 M HCl. A ‘residual’ fraction was determined by digestion of the residue in hydrogen peroxide and sulphuric acid. Plant availability was inferred on the basis of chemical stability. Thus, the weakly held organic phosphorus extracted in sodium bicarbonate was assumed to be more bioavailable than the strongly held organic phosphorus extracted by sodium hydroxide. The fraction extracted in sodium hydroxide after sonication was considered to be organic phosphorus held within soil aggregates, while the most stable forms of organic phosphorus were assumed to remain in the residual (unextracted) fraction. The original Hedley fractionation scheme has been extensively modified for use in different soils [10], but has been criticised for application to organic phosphorus (see below).