Cation exchange characteristics of the K:Ca saturated forms of five soils were measured at 25" C and 50' C. The rates of isotopic exchange of 4*K and 45Ca were too fast to be measured except that of '*K in the K:Ca Hanvell soil at 25' C. The slower isotopic exchange of K in this soil was attributed to the presence of a zeolite, clinoptilolite. The intra-particle diffusion coefficient, Di, of K in this soil increased with K-saturation to a maximum at about 40 per cent K, probably because of the 'blocking' action of the larger hydrated Ca ions at small K-saturations in clinoptilolite.The CEC, measured by isotopic exchange along the K:Ca adsorption isotherm, decreased with increasing temperature probably because some interlayer spaces collapsed.The standard free energy, enthalpy, and entropy changes were negative for the reaction Ca-soil+2K+ + zK-soil+Ca++. These results seem to show that K is more strongly bound than Ca by the soil and that the Ca-preference shown by the isotherm at small external electrolyte concentration is caused by entropy changes in solution.Calculated activity coefficients of the exchangeable ions changed with Ksaturation similarly at both temperatures but values at 50' C were smaller than at 25' C. Experimental Procedureindicated that the Ca ion was preferred alt k ough the standard free Some characteristics of the soils used, the methods by which soil samples were repared, the soil suspensions labelled and analysed were main expanding clay minerals in these soils (Brown, private correspondence) are vermiculite (Denchworth -zo%), chloritized vermiculite (Long Load -40%), montmorillonite (Newchurch -35%, Harwell -65%), and robably a 'mixed montmorillonite :vermiculite' (Sherborne -35 %). $he Harwell soil also contains considerable but unknown amounts of a zeolite, clinoptilolite, mainly in its coarse clay and silt fractions (Brown, 1965).

Isotopic exchange of '*K, ' "a, "Rb, and %a was used to measure the equilibria between some arable soils and mixed chloride solutions of the ion pairs, K-Ca, K-Rb, and K-Na; the results were interpreted by a thermodynamic treatment of the exchange isotherm (Gaines and Thomas, 1953). The cation exchange capacities of the soils were not constant for the three ion pairs and decreased appreciably with K-saturation in the K-Ca systems because Ca ions were trapped inside the interlayer spaces. That soils preferred K to Na in the K-Na systems and Rb to K in the K-Rb systems, was explained in terms of the standard free-energy changes of the exchange reactions.The activity coefficients of the adsorbed ions, f, changed with K-saturation differently for the three ion pairs: f~~ and faB decreased continuously with increasing K-saturation whereas jab remained almost constant. However, with increasing K-saturation, f~ increased in the K-Na systems, remained unchanged in the K-Rb systems, and first increased and then decreased in the K-Ca systems. These changes are interpreted in terms of the effect of the various ions on the interlayer space of z : I type clay minerals and the possible distribution of adsorbed ions between the Gouy and Stem layers. Introduction VARIOUS equations have been proposed to describe the equilibrium between an exchanger and its equilibrium solution. These include the essentially empirical Freundlich and Lan uir adsorption isotherms,

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