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Calcium, Magnesium, and Potassium Diffusion Coefficients in Soil Estimated from Electrical Conductance

Dep. of Agronomy, 135 Mumford Hall, Univ. of Missouri, Columbia, MO 65211

*Corresponding author.

ABSTRACT

Diffusion coefficients for Ca, Mg, and K in various horizons of four soils were calculated from electrical conductivity measurements. The soil materials studied were surface and subsurface horizons of a limestone derived soil, Bardley (fine, mixed, mesic Typic Hapludalf); a fragipan soil, Lebanon (fine-silty, mixed, mesic Typic Fragiudalf); a loess derived soil, Marshall (fine-silty, mixed, mesic Typic Hapludoll); and a claypan soil, Putnam (fine, montmorillonitic, mesic Mollic Albaqualf). A diffusion model that separates ion movement in solution from that along soil surfaces was used to estimate Ca, Mg, and K movement in a mixed-ionic system. The equilibrium soil solution and exchangeable concentrations were estimated using the total salt concentrations, soil cation exchange capacity (CEC), and measured exchange constants. The molar conductivity of each ion in the soil solution and along surfaces was calculated using its published molar conductivity, estimated solution composition, exchangeable concentration, and the specific conductance of the soil measured at salt concentrations of 0.01 and 1 mol L^–1. Diffusion coefficients were computed from molar conductivity. For Ca soil solution, diffusion coefficients ranged from 0.78 to 1.64 x 10^–10, Mg from 0.69 to 1.46 x 10^–10 and K from 2.13 to 4.49 x 10^–10 m² s^–1. Surface diffusion coefficients for Ca varied from 0.20 to 0.44 x 10^–10, Mg from 0.18 to 0.39 x 10^–10 and K from 0.55 to 1.21 x 10^–10 m² s^–1. The quantities of Ca, Mg, and K that diffused across a boundary were calculated from these diffusion coefficients and were the same order of magnitude when compared with the quantities measured.

Contribution from the Mo. Agric. Exp. Stn. Journal Series no. 10721.

Received for publication January 23, 1989.