A consistency index and rainwash mechanisms on Belgian loamy soils

Abstract

AbstractThe effects of soil crusting in Belgium are particularly conspicuous on the loamy plateaus and hill‐regions of southern Flanders, Brabant and Hesbaye. Field observations, made at the end of rainy winter seasons, permitted the making of a simple distinction between: (a) ‘stable’ soils showing crusting on less than 50 per cent of their total surface, without any evident slope wash, and (b) ‘unstable’ soils, crusting over more than 50 per cent of the total surface, with evident rainwash and basal colluviation.The Atterberg limits have been examined for both soil types. It was found that the upslope part of the liquid limit curves, in the range of low consistencies (high water contents), is steeper for stable than for unstable soils. So a distinctive consistency index C2–10 was deduced from the liquid limit curves, which may be used for prediction of the relative sensitivity of topsoils for crusting. Stable, well‐structured clods are more hydrophobic than unstable material. This was made clear by wettability tests under a rain simulator. Therefore more water must be added to remolded, stable material to lower its consistency, as is shown by the liquid limit curves.Crustability decreases and C5–10 values increase with increasing clay and organic matter contents, as one would expect. Stable soils are well aggregated. But rain simulator tests also showed that the detachment of this material by raindrop impact is high. Therefore, crumbs and clods of stable soils are relatively more sensitive to splash erosion. Their erosion results from the ejection of stable aggregates. Clods of unstable topsoils are compacted and flattened by raindrop impact, which causes internal liquefaction.Micromorphological analysis indicates that typical stable topsoils originate from well‐structured Ap‐horizons and from Bt‐horizons of grey‐brown podzolic soils (Alfisols). Laminated silt and mudcrust fragments characterize unstable topsoils which originate, at least partly, from slopewash deposits. The external stability of both stable and unstable clods is increased by compaction and by surficial concentration of gravel, coarse sands, root and other plant fragments. Thin sections of unstable material show the internal collapse of structures, due to liquefaction.A clear distinction must be made between the structural stability and the erodability of topsoils.