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Potassium-Calcium Exchange Coefficients in Clay Fractions of Some Vertisols¹

ABSTRACT

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 () were expressed as the ratio's soil clay/ 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.

¹ The results are from a Ph.D. dissertation by the first author, Texas A & M University, College Station, May 1968. A portion of this paper was presented in Div. S-2, ASA Annual Meeting at Washington, D.C., Nov. 1967.

² Formerly Graduate research Assistant and Professor, Texas A&M University, respectively. Presently Soil Scientist, USDA-ARS-SWC, Norfolk, Va. 23501, and Professor, University of Kentucky, respectively.

Received for publication September 24, 1971. Accepted for publication February 25, 1972.