MODELING DIFFUSION AND REACTION IN SOILS .7. PREDICTING GAS AND ION DIFFUSIVITY IN UNDISTURBED AND SIEVED SOILS

The classical Penman (1940) and Millington-Quirk (1960, 1961) diffusiv ity models were transformed into general form by introducing a tortuos ity parameter, m. Compared with measured diffusivities close to phase saturation (soil-water-and soil-air saturation for ion and gas diffusi vity, respectively), the Penman (1940) model was superior to the Milli ngton-Quirk models independent of diffusion type. The combined use of the Penman model to predict the diffusivity at phase saturation togeth er with a general Millington-Quirk model to predict relative decrease in diffusivity with decreasing phase content was labeled the Penman-Mi llington-Quirk (PMQ) model. The best fit of the new PMQ model to measu red data was obtained with m = 3 (high tortuosity) and m = 6 (medium t ortuosity) for gas diffusivity in undisturbed and sieved soils, respec tively, and m = 1 (high tortuosity) for ion diffusivity. Measurements did not suggest a significant difference between ion diffusivity in un disturbed, sieved, or aggregated soils. The differences in m-values be tween diffusion types are likely caused by different diffusion pathway s and geometries for ion and gas diffusivity as well as a large effect of soil heterogeneity and spatial variability on gas diffusivity. The PMQ model predicted gas diffusivity in sieved and undisturbed soil we ll, but a soil-type dependent model (Part IV of this series) was super ior for predicting ion diffusivity. The new models seem promising for more accurately predicting gas and ion diffusion and, therefore, for i mproving simulations of diffusion-constrained chemical and biological reactions in soils.