Potassium‐calcium exchange equilibria of the following representative Vertisols were determined: Houston Black, Montell, Beaumont, and Houston. Wyoming bentonite and the clay from a Miller soil were included for comparison. Exchange equilibria were expressed in terms of the Gapon equation. The Gapon coefficient K G of the soil clay fractions at high (>18%) K‐saturation were 4 to 5 times higher than for Wyoming bentonite and increased sharply with decreasing K‐saturation. On the basis of X‐ray diffraction data and total K analyses, it was concluded that the highly montmorillonitic clay fractions are interstratified with mica. Specific surface areas were determined relative to Wyoming bentonite. Charge densities (Γ) were expressed as the ratio's Γ soil clay/Γ Wyoming bentonite. Differences in the affinity for K at low K‐saturation were closely related to mica content. The shape of the K G vs K ‐saturation curves was explained by the hypothesis that three different sites are available for K adsorption: edge‐interlayer sites of mica, wedge‐shaped sites which might easily form between mica‐montmorillonite layers with graded K selectivities, and planar sites mainly on the montmorillonite. Total chemical analyses of the fine clay fraction of Houston Black and Montell indicated that the high charge density originates mainly in the octahedral layer.
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Knibbe et al. (1972) studied this question.