THE EQUILIBRIA OF K:Al EXCHANGE IN CLAY MINERALS AND ACID SOILS ¹

Summary Conventional K: Al exchange isotherms for montmorillonite showed that Al 3+ was strongly preferred to K + in o‐oin solutions. The exchange coefficient, K' , calculated using the isotopically exchangeable K, was greater than unity and did not vary with the Al‐saturation or with the initial pH of the AlCl 3 solutions. Isotherms for vermiculite, illite, and soils in o·oin solutions also showed Al 3+ ‐preference but unlike those for montmorillonite were not asymptotic to q Al / q o = 1, q Al being the amount of adsorbed Al and q o the total adsorbed (Al + K), indicating that some of the isotopically exchangeable K could not easily be exchanged by Al 3+ ions; this difficultly exchangeable K (DEK) was estimated for each exchanger. K' values for vermiculite, illite, and soils were less than unity and did not vary with Al‐saturation or initial pH if the isotopically exchangeable K was corrected for DEK. This showed that K + was adsorbed more strongly than Al 3+ . Strengths of K + adsorption referred to Al 3+ as the counter‐cation were in order: soils > vermiculite, illite > montmorillonite.