Prediction of Diffusion Coefficients from the Electrical Conductance of Soil

Electrical conductance may be used to predict salt movement through soil by diffusion. A two-component model was used which considers electrical conductance in soil to be a combination of a solution and a surface process. It has the form: ECₐ = ECwθT₈ + EC₈ where ECₐ is the specific conductance of the soil, ECw is the conductance of the solution, θ is the volumetric moisture content, ECᵣ is the surface conductance, and T₈ is the tortuosity. It was assumed that T₈ and EC₈ did not vary with changes in solution concentration, and these two parameters were calculated by measuring ECₐ and ECw on two soil samples which differed only in electrolyte concentrations. These values were used to calculate the equivalent conductance and the diffusion coefficients for Na⁺ and K⁺. The alternating current measurement of conductivity was frequency (f) dependent due to electrode-electrolyte interactions known as the Warburg impedance. The specific conductance was found by plotting the measured conductance against f⁻⁰.⁵ and extrapolating to zero. At low electrolyte concentrations, the conductivity of the soil varied as a linear function of the logarithm of frequency and this factor was evaluated before predicting diffusion coefficients from the conductance of the soil. The diffusion coefficient for K⁺ in four soils with 0.0133M KCl varied from 1.64 × 10⁻⁶ to 0.50 × 10⁻⁶ cm²/sec when computed from electrical conductivity but varied from 1.84 × 10⁻⁶ to 0.33 × 10⁻⁶ cm²/sec when computed from self-diffusion. The fraction of the electrical current carried by the 0.0133M KCl in the soil solution varied from 0.85 to 0.13 and was inversely related to the cation exchange capacity. A comparison of diffusion coefficients estimated from the self-diffusion of ⁴²K and ²⁴Na in concentrated and dilute systems indicates good agreement except for 0.0133M K⁺ in the Mexico Ap soil. The data suggest that diffusion measurements made by electrical conductance are more reproducible than those made by self-diffusion, but give estimates of the diffusion coefficient that are of similar magnitude.