Clay particle differentiation in alluvial soils of southwestern Niger (West Africa)

In arid lands, textural and structural differentiation between the surface soil and deeper horizons is commonly attributed to mechanical transfer of finer material by wind and runoff, but other processes may be influential as well. This study was conducted to determine whether physico-chemical and mineralogical processes could participate in differentiation. The study area is in Sona (Niger Republ ic) on alluvial deposits of the Niger River. After a soil survey, soil samples ere collected from eight profiles. The particle-size distribution was determined by the sedimentation and centrifugation methods. The clay fractions were studied by x-ray diffractometry, chemical analyses, transmission electron microscopy, and energy dispersive x-ray analysis. The particle images were computed and displayed by an image analysis program. The surface horizons contained fewer particles 0.1 micrometer than the deeper horizons. However, they were relatively richer in 83-nm particles and these were corroded on the edges and (001) faces. From a mineralogical viewpoint, detrital kaolinite and smectite clays presented wider x-ray diffraction lines, indicating smaller, thinner particles with fewer silicate layers than in deeper horizons. Textural differentiation of the surface horizons was enhanced by weathering and dissolution of the smectite and kaolinite particles under acidolitic conditions caused by soil ultradesiccation