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Soil Science Society of America Journal Abstract -

Estimation of Calcium Diffusion Coefficients from Electrical Conductance1


  1. B. L. Conkling and
  2. R. W. Blanchar2



Diffusion coefficients for Ca in various horizons of the Bardley, Lebanon, Marshall, and Putnam soils were calculated from electrical conductivity measurements. A diffusion model which separates ion movement in solution from that along soil surfaces provided estimates of Ca movement that agreed with estimates of self-diffusion measured using 45Ca. The equation has the form dq/dt = (ΛC/Λα) Dliq (dC/dx) + Dsur (dCe/dx), where dq/dt is the flux, ΛC/Λα the ratio of molar conductivities at the given concentration divided by that at infinite dilution, Dliq the diffusion coefficient in the liquid phase at infinite dilution, dC/dx the solution gradient, Dsur the diffusion coefficient along the soil surface, and dCe/dx the gradient of exchangeable concentration. Values of Dliq varied from 0.95 to 1.60 × 10−6 m2s−1 and Dsur from 0.61 to 1.18 × 10−6 m2s−1. Values of Dliq estimated at 0.01 mol/L CaCl2 concentration were 5 to 6% higher than those measured in 1 mol/L CaCl2 and values of Dsur are independent of salt concentration. The ratio of the quantity of Ca diffusing through solution to that along the particle surfaces was < 1 to 4 at 0.01 mol/L salt and approximately 4 to 1 at a 1 mol/L salt level. The estimated Ca diffusion coefficient along the particle surface was in all cases lower in the surface horizon samples than in those from a lower depth in the soil.

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