The Influence of Soil Crusts on Gaseous Diffusion

A method is described for forming crusts on soils in special containers in the laboratory using artificial rainfall, and for measuring gaseous diffusion through these soils without further disturbance of their structure. Unless the surface is completely impervious, the rate of diffusion through a soil does not depend solely on the properties of this layer. When various proportions of the soil surface were sealed with paraffin, the rate of diffusion was not reduced in proportion to the area covered. The effects of soil crusts upon diffusion depended to a very great extent on the soil moisture tension. There were no significant differences in the rates of diffusion through air‐dry silt loam having different degrees of crusting. Surface crusts on this soil restricted diffusion only at low moisture tensions; the wetter the soil, the greater the influence of the crust on diffusion. Except in the very wet range, even minor variations in soil moisture tension had more influence on diffusion than did the presence of a thin surface crust. A saturated soil crust could provide a very effective seal against diffusion at the time when soil aeration is most critical, i.e. during and for a short time after a rain or irrigation. Except during a period of complete sealing, however, crusted soils actually may permit more rapid diffusion and be better aerated than mulched or other uncrusted soils, because of differences in moisture content.