Scanning Electron Microscope Studies of Surface Crusts Formed by Simulated Rainfall

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Abstract
The micromorphology of the surface layer of three California soils, Columbia fine sandy loam, Wyo clay loam, and Yolo silt loam, undergoing crust formation by simulated rainfall was studied using the scanning electron microscope (SEM). Crusts formed at the time of ponding (early crust) and after 60 min rainfall were examined. At the early crust stage, crusts on all soils comprised a very thin compact surficial layer, about 0.05 mm thick, and a more open underlying layer of uniform pore and particle size distributions comparable to those of the undisturbed soil. At a later stage, the crusts on Yolo and Wyo soils consisted of an upper layer (about 0.1 mm thick) of high porosity, composed predominantly of individual sand grains and a lower layer of considerably reduced porosity, composed of a higher concentration of clay and silt sized particles. The lower layer, called the “washed‐in” zone, results from the accumulation of fine materials washed out in the upper layer. Columbia late crust also comprised an upper compact surface layer (about 0.05 mm thick) of well oriented platy particles, and a lower more open layer with particles uniformly reduced in size. Decreased apparent porosity at 0.3‐ to 0.8‐mm depth, measured directly on the SEM photomontages, indicated the presence of the “washed‐in” zone in the Yolo and Wyo late crusts and its absence in the Columbia. Mean pore size of the late crust Yolo sample was significantly reduced compared to the no‐crust treatment. Small changes in mean pore size were also observed for the other two soils.

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