CLAY PARTICLE DIFFERENTIATION IN ALLUVIAL SOILS OF SOUTHWESTERN NIGER(WEST-AFRICA)

In arid lands, textural and structural differentiation between the sur face soil and deeper horizons is commonly attributed to mechanical tra nsfer of finer material by wind and runoff, but other processes may be influential as well. This study,vas conducted to determine whether ph ysico-chemical and mineralogical processes could participate in differ entiation, The study area is in Sona (Niger Republic) on alluvial depo sits of the Niger River, After a soil survey, soil samples were collec ted from eight profiles. The particle-size distribution was determined by the sedimentation and centrifugation methods. The clay fractions w ere studied by x-ray diffractometry, chemical analyses, transmission e lectron 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 mu m than the deeper horizons, However, they were relatively richer in < 83-nm particles an d these were corroded on the edges and (001) faces, From a mineralogic al viewpoint, detrital kaolinite and smectite clays presented wider x- ray diffraction lines, indicating smaller, thinner particles with fewe r silicate layers than in deeper horizons, Textural differentiation of the surface horizons was enhanced by weathering and dissolution of th e smectite and kaolinite particles under acidolitic conditions caused by soil ultradesiccation.