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Estimation of Calcium Diffusion Coefficients from Electrical Conductance¹

ABSTRACT

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 = (C/) D_liq (dC/dx) + D_sur (dC_e/dx), where dq/dt is the flux, C/ the ratio of molar conductivities at the given concentration divided by that at infinite dilution, D_liq the diffusion coefficient in the liquid phase at infinite dilution, dC/dx the solution gradient, D_sur the diffusion coefficient along the soil surface, and dC_e/dx the gradient of exchangeable concentration. Values of D_liq varied from 0.95 to 1.60 x 10^–6 m²s^–1 and D_sur from 0.61 to 1.18 x 10^–6 m²s^–1. Values of D_liq estimated at 0.01 mol/L CaCl₂ concentration were 5 to 6% higher than those measured in 1 mol/L CaCl₂ and values of D_sur 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.

¹ Contribution from the Missouri Ag. Exp. Stn., J. Ser. no. 9881 Approved by the Director.

² Research Assistant and Professor of Agronomy, respectively, Univ. of Missouri, Columbia, MO 65201.

Received for publication June 6, 1985.