EFFECTS OF SULFATE AND PH ON THE PLANT-AVAILABILITY OF PHOSPHATE ADSORBED ON GOETHITE

The adsorption of phosphate on metal (hydr)oxides may be influenced by the pH and by the adsorption of other ions. In this study, the influe nce of sulphate and pH on phosphate adsorption on goethite and the ava ilability to plants of adsorbed phosphate was examined. Maize plants w ere grown on suspensions of goethite with adsorbed phosphate, containi ng the same total amount of phosphate and either 0.11 mM or 2.01 mM su lphate at pH 3.7, 4.6 or 5.5. The uptake of phosphorus by the plants i ncreased with the larger sulphate concentration and decreasing pH. Mea n P uptake in the treatment with 2.01 mM sulphate and pH 3.7 was 55 mu mol plant(-1), whereas in the treatment with 0.11 mM sulphate and pH 5.5 it was 2 mu mol plant(-1). Batch adsorption experiments using P-32 and speciation modelling of ion adsorption showed that in the presenc e of sulphate, the phosphate concentration in solution strongly increa sed with decreasing pH, due to competitive adsorption between sulphate and phosphate on goethite. Modelled phosphate concentrations in solut ion in the uptake experiment were all below 0.6 mu M and correlated we ll with the observed P uptake. This correlation indicates that the str ong influence of the sulphate concentration and pH on the plant-availa bility of adsorbed phosphate results from the competition between sulp hate and phosphate for adsorption on goethite.