Phosphate sorption in allophanic soils and release of sulphate, silicate and hydroxyl

SUMMARY Phosphate (P) sorption and the concomitant release of sulphate (SO,), silicate (Si) and hydroxyl ion (OH) were determined on three allophanic soils from Spain, at different P concentrations. P effectively replaced SO 4 , Si and OH. However, at every stage of P sorption, the molar ratios of the total amounts of anions released (SO 4 + Si + OH) to that of the sorbed P were low. The amount of added P affected the relative proportions of SO 4 , Si and OH exchanged. At low concentrations of P, phosphate sorption was accompanied mostly by release of the adsorbed SO 4 with some Si. As more P was sorbed an increasing displacement of OH was also observed.