Exchange isotherms determined for the K-A1 ion pair with montmorillonite, kaolinite and two soils were S-shaped. In dilute solutions (<0.05N), A1 was strongly preferred by exchange sites except at low levels of K-saturation. In more concentrated solutions (0.1 to 0.25N), K was preferred when its saturation was less than about 30%, while A1 was more strongly sorbed over the remainder of the isotherms. In 1N solution, K was bound more tightly than A1 over the entire range of ion saturation up to at least 90% K-10% A1.
In both dilute and concentrated solutions, A1 was preferred over Ca. In 1N solution, A1 was bound much more tightly than Na. Because of the strong sorption of K, competing with A1 in concentrated solution, KCl should be an effective displacing agent for exchangeable A1.
Exchange coefficients for the K-A1 reaction, calculated according to the “equivalent fraction” equation, varied greatly, both with electrolyte concentration and exchange site population. The isotherms, though not described by a simple equation, are regarded as valuable indices of the exchange behavior of acid clays and soils.