CALCIUM: ALUMINIUM EXCHANGE EQUILIBRIA IN CLAY MINERALS AND ACID SOILS 1

Summary Exchange reactions between 0.0 in AlCl 3 solutions of different pH and Ca‐saturated montmorillonite, vermiculite, illite, and soils from the Park Grass Experiment at Rothamsted and the Deerpark Experiment, Wexford, Ireland, showed that Al 3+ and Al(OH) 2 + were adsorbed from solutions of pH > 4.0 and Al 3+ and H + from solutions of pH < 3.0. When Al was adsorbed, the cation exchange capacity of Ca‐saturated soils and clays increased. Conventional Ca: Al exchange isotherms showed that Al 3+ was strongly preferred to Ca 2+ on all soils and clays. The equilibrium constant for Ca: Al exchange, K , was identical for soils before and after oxidizing their organic matter and did not vary, for any exchanger, with Al‐saturation or the initial pH of the AlCl 3 solution. This proved the validity of the procedure used for calculating exchangeable Al 3+ . K values for Ca:Al exchange favoured Al 3+ in the order: vermiculite > Park Grass soil > Deerpark soil > illite > montmorillonite. The influence of surface‐charge densities of the clay minerals on this order is discussed and a method proposed and tested for calculating the K value of a soil from its mineralogical composition.