Modeling diffusion and reaction in soils .4. New models for predicting ion diffusivity

Salt diffusivity as a function of soil-water content and soil-water characteristic curves was measured on three sieved soils of different texture, The data were used together with ion diffusivity data for sieved soils from literature to test different diffusivity models, No significant differences between salt-, counter-, and self-diffusion data were observed, suggesting an insignificant effect of diffusion type in most cases, Two new models for predicting ion diffusivity, based on the Campbell soil-water retention model parameter b, were proposed: (i) a model with a b-dependent threshold water content theta(th) (soil-water content where the ion diffusivity approaches zero) and (ii) a power function model where the power term eta, representing the liquid phase tortuosity, is a linear function of b, In both models, the diffusivity at the soil-water content equal to the total porosity was estimated from a simple impedance factor model, Both b-dependent models gave improved predictions of ion diffusivity compared with existing models,The b-dependent power function model with eta = 0.3b gave better predictions than both the theta(th)-dependent model and a recent model for gas diffusivity in undisturbed soil (Part III of this series, Soil Sci, 161:366-375) where the tortuosity term eta was best described as a function of b(-1), For use of the new b-dependent ion diffusivity models in the absence of soil-water characteristic data, it is shown that b can be fairly accurately estimated from soil texture.