Effect of Electrolyte Concentration on Donnan Systems and the Resulting Uptake of Cations by Plants

Donnan systems were prepared by dialyzing bentonite and illite clay suspensions against equal volumes of water. The electrolyte concentration was varied from 0 to 10 me. per liter. Ca 45 and Rb 86 were used as tracers for Ca and K in the systems. Greater amounts of Ca were absorbed by corn plants from the clay suspension phase than from the dialyzate phase provided that the electrolyte concentration was low enough. The differences in absorption from the two phases decreased as the electrolyte concentration was increased, and the uptake of Ca from the two phases was equal at about 0.1 symmetry concentration of electrolyte depending on the absorption time and saturation of the clay. A similar trend was observed for K absorption though less pronounced. Various measurements supported the theory that clays have a low degree of dissociation and thus give rise to relatively small Donnan effects. The apparent failure of Donnan theory to accurately describe clay‐water systems in studies by certain investigators was likely due to the assumption that the colloid is completely dissociated. In addition, differences or similarities observed in uptake of cations from suspensions and corresponding equilibrium dialyzates appear to be the result of variation in concentration of electrolyte present.