CHARACTERIZATION OF SURFACE CRUSTS FORMED BY SIMULATED RAINFALL ON SIX CALIFORNIA SOILS (SCANNING ELECTRON MICROSCOPE)

Surface crusts formed on nine California soils by simulated rainfall were characterized, using the scanning electron microscope (SEM) technique and physical measurements. Crust properties measured were strength (modulus of rupture), bulk density, total porosity and pore size distribution, particle size distribution, degree of particle sorting and water infiltration rates.;The SEM photomontages revealed a detailed picture of the micromorphology of soil crusts which led to the reconstruction of the mechanism of crust formation in the soils examined. This mechanism consisted of the following processes outlined sequentially: (1) breakdown of soil aggregates by raindrop impact and slaking, (2) movement of fine particles into the soil profile thereby clogging pores and increasing bulk density of this layer, (3) deposition of suspended soil particles on the soil surface; particles may then assume parallel or random orientation depending upon their shapes and (4) with random orientation (especially of the irregularly shaped coarse particles) a very thin surficial layer with relatively higher porosity results. However, parallel orientation of particles on deposition results in compaction of the immediate surface layer.;The presence of a crust caused an increase in the strength, bulk density and particle sorting in the immediate surface of soils. It reduced the total porosity, sizes of coarse pores and the initial infiltration rates of soils. The extent of these changes was a function of the stage of crust development in each soil.;Particle size distribution was the single most important soil property influencing aggregate stability, crust strength and bulk density.;SEM observations were made on crusts at two stages of development (early and late stages) formed on three of the soils; Columbia, Wyo and Yolo. Apparent porosity measurements were made from the electron micrographs of the crusts. The rest of the measurements were made on the crusts themselves.;Percent change in soil porosity was found to be the best index for assessing the crusting propensity of soils. Using this index, Wyo soil was ranked highest and Columbia soil lowest in their crusting propensity. Yolo soil was ranked intermediate. Wyo and Yolo soils formed crusts of significantly higher strength (modulus of rupture) than Columbia soil. Crust initial infiltration rate of Columbia soil was higher than those of Yolo and Wyo soils.