THE THERMODYNAMICS OF CATION EXCHANGE INVOLVING SMECTITES AND SOIL CLAY FRACTIONS

Quantitative measurement of cation exchange of selected smectites was accomplished for the cation pairs: potassium-magnesium and calcium-magnesium. In both experimental situations the ionic strength was 5 x 10('-4) M. Two clays (SAZ (Cheto) and Chisholm (API 21)) were shown to have a high selectivity for potassium. These two clays were also shown to have a larger degree of tetrahedral isomorphous substitution. It is speculated that isomorphous substitution within the tetrahedral layer will cause potassium to be retained with a greater affinity than will isomorphous substitution within the octahedral layer. Soil clay fractions also demonstrated an enhanced potassium selectivity relative to the reference clays. This could not be attributed to isomorphous substitution within the tetrahedral layer since the soil clays exhibited mixed mineralogy and it was not possible to allocate the aluminum to a particular site.; Selectivity coefficients, which were calculated from an exchange isotherm, were shown to influence potassium uptake by corn. It was assumed that a smectite and the root system exert an equal chemical influence on potassium at the point where the plant could no longer utilize potassium. It was discovered that the selectivity coefficients may be used to predict the point where the plant can no longer exploit the soils potassium reservoir.