DISPERSION OF CHLORIDE, SODIUM, AND CALCIUM-IONS IN SOILS AS AFFECTED BY EXCHANGEABLE SODIUM

A sand-clay soil mixture was equilibrated in a column with various NaCl-CaCl2 solutions at constant normality (0.03N). After each equilibration, the percolating solution was labeled successively with 36Cl, 22Na, and 45Ca. The breakthrough curves of the radioactive species made it possible to calculate both effective pore volume and, at a given Darcy velocity, the Peclet number (P) for each ion. PCl was found to decrease markedly when the exchangeable sodium percentage (ESP) increased from 0 to about 20%. PNa and PCa were almost not affected by changing ESP. The following sequence was observed: PCa Na Cl. The discriminatory mixing power of soil towards the three ions likely results from changing size of montmorillonite tactoids and from demixing of adsorbed Na and Ca cations. © 1982 Soil Science Society of America.