ILLUVIATION AND STRUCTURAL CRUST FORMATION ON LOAMY TEMPERATE SOILS

Dispersion and subsequent illuviation of fine particles into the pore (washing-in) have often been suggested as major processes causing th formation of soil crusts. Their actual role, however, is not clear with respect to structural crust formation on loamy temperate soils. Aggregates (2-5 mm) were sieved out of the Ap horizons of three cultivated Alfisols, ranging from 17.5 to 29.5% clay and 1.3 to 1.6 organic matter g-1. Crusts were formed by simulated rainfall with moderate kinetic energy (12.5 J m-2) and the induced structural crust were studied at all development stages using microscopic techniques In wet initial conditions (23.5-27.5 g g-1 water content), crusts developed under cumulative rainfalls ranging from 15 to 450 mm. Th stages were identified: bare silt grains first (i) formed meniscus-like bridges between aggregates at a depth of 5 mm below the surface, then (ii) deposited as infillings, which (iii) progressively clogged interaggregate packing voids up to the surface. Clay coatings were observed on the walls of interaggregate voids at depths of 10 to 20 mm. For initially dry samples, slaking-induced crusts developed after only 6 mm of rainfall without any separation of particles by size. Therefore, when the aggregates did not slake or coalesce, structural crusts were formed by silt illuviation. In this crusting process, the tops of surface aggregates were slowly eroded by raindrops and the resulting separated silt grains illuviated into the interaggregate packing voids. As a result, infiltration rate decreased.