Soil crusting on sandy soils and its influence on wind erosion

Under rainfall, a crust forms at the surface of most soils. For soils rich enough in clay (clay content >5%), it is well known that a physical crust is responsible for a supply limitation of particles available for wind erosion. Sandy soils are very prone to crusting as well as to wind erosion. Indeed, structural ‘sieving’ crusts develop on sandy soils even after light rainfalls. This kind of crust shows a loose sand layer at the surface overlying a thin layer where fine particles are concentrated. The main objectives of this study were to determine whether such a crust formation limits the availability of (1) sand grains for entrainment into saltation and (2) fine particles for uptake into a vertical wind erosion flux (suspension in atmosphere of particles <20 μm diameter). For this, 1 ha (100×100 m) of sandy soil (sandy psammentic Haplustalf) was bared of all litter and vegetation cover to enhance wind erosion from February to June 1998 in Niger. Measurements of horizontal and vertical wind erosion fluxes were performed on this area before and after crusting that appeared immediately after the first rain event at the onset of the rainy season. Analysis of the experimental results show that in spite of an important reduction in the amount of loose coarse grains available for saltation, there is no supply limitation in such grains due to crusting. In the same manner, it is shown that although the fine-particle (<20 μm) content in the free sand layer drops to 1.5% after the first rain event, the vertical flux of fine particles is not supply limited after crust formation. Thus, it appears that unlike other types of crusts that form on soils with higher contents in fine particles, sieving crusts do not protect agricultural soils from wind erosion.