POTASSIUM-CALCIUM EXCHANGE EQUILIBRIA IN SOILS: THE LOCATION OF NON-SPECIFIC (GAPON) AND SPECIFIC EXCHANGE SITES

Several studies (e.g. Bolt et al., 1963; Beckett, 1964a) of the exchange equilibria between K and Ca on soil colloids or pure clay minerals have drawn attention to the presence of sites with different affinities for K. There is so far no evidence to indicate the location of sites with the highest affinity, though several workers (e.g. van Schouwenburg and Schuffelen, 1963) have assumed that they lie on the edge-faces of stacks of clay plates, or in the spaces between the expanded leaves of partially weathered stacks of clay plates. Earlier work has resolved the K: Ca exchange isotherms of soils and clays into a curved part at low values of aK/√aCa, attributed to exchange at sites with a high specific affinity for K, and an upper linear part commonly described by the Gapon equation and attributed to non-specific sites. The present work on soil clays and clay minerals shows that Na hexametaphosphate or changes in pH affect the curved but not the linear part; cetyl trimethyl ammonium bromide and changes in the amount or charge of exchangeable Al affect the linear but not the curved part of the isotherms. At low pH, the linear but not the curved part of the kaolinite isotherm obeys Schofield's Ratio Law. Grinding has more effect on the curved than on the linear part. So the specific sites are attributed to the edges or peripheral interstices of stacks of clay plates, and the non-specific sites to their planar surfaces. The specific sites take up K more slowly from solution than the non-specific sites. The isotherms of completely dispersed bentonites have no curved part. The specific sites are attributed to the wedge-shaped interstices opened between clay plates by weathering, from which exchange is diffusion-controlled. Added organic cations reduce the numbers of both kinds of site; peroxide treatment increases them. This paper confirms that the exchange sites with highest affinity for K are indeed associated with the edges, rather than the faces, of stacks of clay plates.