Effect of Hydrogen Clay on Calcium and Potassium Activities and Uptake by Excised Soybean Roots from Vermiculite and Kaolinite Suspensions

Libby vermiculite and Peerless kaolinite clay suspensions were H‐saturated by resin treatment, and then Ca‐ or K‐saturated by addition of the respective hydroxides. Reciprocal saturations of Ca‐H, K‐H, and Ca‐K with variable amounts of H‐clay added were prepared. Activities of Ca and K in the suspensions were measured by specific‐ion electrodes. Excised‐soybean root ( Glycine max L., var. ‘Corsoy’) uptake from the suspension phases of membrane‐equilibrated systems of both clays was measured using 45 Ca and 86 Rb as tracers for Ca and K, respectively. Cationic fractions active in Ca‐H and K‐H systems reached minimal values in kaolinite, but only when the H‐saturation reached almost twice the measured pH‐dependent CEC (30 vs 18%), while those in vermiculite did not reach minima when H‐saturation was nearly three times the measured pH‐dependent CEC (20 vs 7%). Additions of H‐clay to the Ca‐K systems progressively decreased cationic activities (or dialyzate concentrations) and uptake; the effect being more pronounced for Ca than for K. In general, the uptake of each cation was highly correlated with its effective concentration in solution. Failure of root uptake to reflect marked increases in Ca concentration of dialyzates when Ca saturation increased from 80 to 100% suggested that much of the Ca uptake at lower Ca saturations was the result of direct root‐clay contact in the mechanically‐shaken systems. Consequently, the uptake of Ca was more highly correlated with total concentration of Ca in the systems than with the dialyzate concentrations of Ca. Average Ca/K upake ratios were about 2.5 times higher from kaolinite than from vermiculite clays, and the ratios decreased significantly with increasing levels of H‐clay added in kaolinite but much less so in vermiculite.