The rate of cation diffusion in soil is influenced by the buffering effect of the adsorbed cation on the solution concentration, dC l /dC , i.e., the change in solution zinc concentration with respect to change in the total diffusible soil zinc concentration. This effect was studied with Zn using four silt loam soils having a wide range of initial exchangeable Zn contents and cation exchange capacities (CEC). Zinc adsorption isotherms for these soils had slopes that increased as CEC decreased. The effective zinc diffusion coefficient, D e , was measured at five Zn levels on each soil. Measured D e , correlated with dC l /dC (r 2 = 0.86). Values of D e calculated from known values for volumetric moisture content, tortuosity, and dC l /dC were larger than observed D e by factors of 5.5 to 18.2. Inclusion of CEC in a multiple regression of observed D e with calculated D e significantly increased r 2 from 0.87 to 0.90. An additional soil factor related to CEC needs to be included to accurately calculate D e values for Zn.
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Warncke et al. (1973) studied this question.