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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 SO4, Si and OH. However, at every stage of P sorption, the molar ratios of the total amounts of anions released (SO4+ Si + OH) to that of the sorbed P were low. The amount of added P affected the relative proportions of SO4, Si and OH exchanged. At low concentrations of P, phosphate sorption was accompanied mostly by release of the adsorbed SO4 with some Si. As more P was sorbed an increasing displacement of OH was also observed.

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The sorption of sulphate (SO:-) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO:were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO:was recovered than had been sorbed. This indicated a release of native SO:-. Sulphate replaced hydroxyl ions (OH-) and co-ordinated H,O molecules, as well as very small amounts of silicate (Si). No measurable amount of phosphate (P) was released. On average hydroxyl release accounted for 50% of SO:sorbed, the rest being accounted for by the increase in negative charge as measured by K+ adsorption. The results presented here are consistent with the sorption of SO:through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate.