The contribution of sorbed sodium and calcium to the self‐diffusion of these cations in soils with different clay mineralogies

SUMMARY Self‐diffusion coefficients of sodium and calcium were measured on a range of moist subsoils. The ratio of concentrations of each isotope in the liquid phase and on the surface at the water content of the diffusion run was obtained from both the extrapolation of ratios measured at a range of soil:solution ratios, and, where possible, by measurement on expressed soil solution. The diffusion coefficients were considerably greater than would be predicted if only the liquid phase contributed to diffusion. Surface phase impedance factors, f s , were calculated for each soil. f s for sodium was between 1/5 and 1/2 of the liquid phase impedance factor, f L ; f s for calcium was between 1/9 and 1/20 of f L . Values of f s were not obviously related to the clay mineralogy, the proportion of internal to external cation adsorption, the volumetric water content, or the pH.