THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOIL

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Abstract
Summary: If an exchangeable ion in soil diffuses along a liquid and solid pathway, its diffusion coefficient may be expressed as image where D, v, f, C are diffusion coefficient, volume fraction, impedance factor, and concentration terms and the suffixes l,S refer to liquid and solid.The self‐diffusion coefficient of the ion is then image where D′, D′t, and D′s, are self‐diffusion coefficients. D and D′ will vary with concentration. In diffusion out of the soil to a zero sink, the appropriate average diffusion coefficient is, approximately, the self‐diffusion coefficient in the undisturbed soil. Diffusion of one ion species is influenced by other ions diffusing in the system through the diffusion potential set up. When ions are diffusing to plant roots, the diffusion potential is likely to be small.A more likely, though more complicated, expression for D than the first equation above is derived by assuming the ion to follow solid and liquid pathways in series as well as in parallel.