PROCESSES OF STRUCTURAL CRUST FORMATION ON COARSE-TEXTURED SOILS

In order to understand better the mechanisms of structural crust formation on coarse-textured soils, mixtures of 92.5% sand and 7.5% Ca2+-kaolinite are exposed to simulated rainfall in experiments involving a range of sand particle sizes, drop diameters and drop kinetic energies. For each combination of these variables, clay illuviation is measured and the changes in surface structure are observed on thin sections. The intensity of clay illuviation is inversely proportional to sand particle size and is shown to be determined by both rainfall and drop kinetic energy. A washed-out layer forms at the surface of all samples exposed to drop impact. The lower boundary of this layer is generally marked by a relative concentration of clay micro-aggregates in the form of a band. These micro-aggregates are probably fragments derived from grain coatings initially present in the washed-out layer. They accumulate at a depth that increases, on average, with drop kinetic energy and with the particle size of the sand fraction. Based on similarities with natural crusts, it is proposed that clay-band formation in structural crusts formed on coarse-textured soils may be initiated by the displacement of micro-aggregates or other small particles from the washed-out layer, followed by their accumulation due to mechanical straining. The experiments also point to the potential importance of the particle size of the sand fraction on the morphology of structural crusts.