The solute diffusion coefficient in soil (Ds) and its dependency on the soil water content (θ), soil type, and compaction govern the transport and fate of dissolved chemicals in the soil vadose zone. Only a few studies have quantified solute diffusivity (Ds/D0, where Ds and D0 are the solute diffusion coefficients in soil and pure water, respectively) for variably compacted soils with different textures. We measured the Ds for KCl on five different soils from Japan: two volcanic ash soils (Andisols) at different bulk densities, two sandy soils, and a loamy soil. The Ds was measured across a wide range of θ using the half-cell method. The Ds/D0 values for Andisols with bimodal pore size distribution were comparatively lower than for the other soils. Opposite to the behavior for sandy soils, the Ds/D0 for Andisols at a given θ decreased markedly with increasing bulk density under wet conditions but increased with increasing bulk density under dry conditions. Data for all soil types including sandy soils with unimodal pore size distribution implied a two-region behavior when plotted as log(Ds/D0) vs. θ. We suggest that the similar behavior across soil types can be explained by regions of low and high water phase connectivity for relatively structureless soils and by high intraaggregate and low interaggregate water phase tortuosity for aggregated soils. Among a number of tested predictive models for Ds/D0, the Penman–Millington–Quirk model, which requires knowledge of only θ and total porosity, performed best across soil types.

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