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GASEOUS DIFFUSION IN SHRINKING SOILS

To assess the effect of shrinkage of two zero-tilled clay soils on gaseous diffusion, we measured relative diffusivity on large samples (15.8 cm diameter, 10 cm high) that showed shrinkage cracks and on small samples (7.5 cm diameter, 5 cm high) that had no visible shrinkage cracks. The relationship between air-filled porosity and relative diffusivity was similar for both soils. Upon drying, both clay soils showed structural shrinkage followed by normal shrinkage. During structural shrinkage, relative diffusivity (Ds/D0) and air-filled pore space (α) were related by the expression Ds/D0 = α2]; with the onset of normal shrinkage and the appearance of shrinkage cracks, further increases in Ds/D0 were nearly equal to increases in α. At the same gravimetric water content, relative diffusivity decreased from the large to the small cores. This was more pronounced for the soil with the weak blocky structure than for the soil with the strongly developed block structure. Measurements of relative diffusivity in vertical and horizontal cores were similar when the cores were near saturation. As the air-filled pore space increased, however, vertical diffusivity of these zero-tilled soils was about twice as large as horizontal diffusivity. This anisotropic behavior is related to the larger vertical than horizontal shrinkage in the large cores. Both observations point to a difference in pore orientation or tortuosity in the vertical and horizontal direction. In plowed plots, horizontal relative diffusivity appeared larger than vertical diffusivity, but in the undisturbed subsoils the directional dependence of relative diffusivity was similar to that in the zero-tilled soils.