The effects of physical conditions on ion diffusion in soils measured by electrical conductivity
The four-electrode method modified for laboratory use was employed for electrical conductivity measurements of eight Scottish and two Greek Ca-saturated soils (< 2 mm fraction) over a range of soil solution conductivities (0.5 - 10 dS m-1 as CaCℓ2) and for a range of tensions (saturation to 8 bars). Soil bulk conductivity, κb(dS m-1), was linearly related with the soil solution conductivity κw (dS m-1) for the tensions used, the relationship being of the form, κb = 1/F_f κw + κs, where κs (dS m^-1) is the adsorbed Ca conductivity and 1/F_f accounts for tortuosity and available pore volume fraction for conductance. For each soil, 1/F_f was linearly related with the volumetric moisture content θ. It was found that for the type of the soils used, non-montmorillonitic soils and mean organic matter content 5% for the Scottish soils, the adsorbed Ca molar conductivity is only a small fraction (1 - 4%) of its infinite dilution value. Also κs was related to soil properties like clay content, total surface area, CEC and organic matter. Diffusion coefficients for Ca and Cℓ for the soil solution and for adsorbed Ca were calculated by the Nernst-Einstein relationship from the measured solution conductivities and from the κs values. For θ= 0.57 to 0.15 and soil solution concentration of 0.0025 - 0.0045M as CaCℓ2, Ca diffusion coefficients ranged from 2.67 to 0.062 x 10-10m2s-1 and Cℓ diffusion coefficients from 6.9 to 0.16 x 10^-10m^2s^-1. A concentration correction was applied. Adsorbed Ca diffusion coefficients ranged from 11.8 to 0.045 x 10^-12m^2s^-1 for θ= 0.52 tp 0.14. A bulk density effect was introduced by consolidating and compacting four of the soils with a static load of 0.45 kg cm-2. Soil bulk conductivities were measured at the same range of soil solution conductivities and for a tension range of 40 cm to 2 bars. Diffusion coefficients for Ca and Cℓ in the soil solution calculated using the Nernst-Einstein relationship were higher than the non-compressed soils on a tension basis, while for each soil on a common θ basis diffusion coefficients for Ca and Cℓ tended to increase slightly as bulk density was increased.