A hard sandy-loam soil from semi-arid Northern Cameroon: II. Geochemistry and mineralogy of the bonding agent

Summary The hardening of soils containing little clay content has often been related to the cementing of skeleton grains. In sandy-loam soils from the southern plain of the Chad basin the skeleton grains of the hard horizon are coated aiid bonded by wall-shaped bridges. We have studied the bonding agent by X-ray diffraction on extracted fractions (atomic ratio varies weakly with the morphology of the bridges. The bonding agent constituting the coatings and the wall-shaped bridges seems to be a mixture of predominantly Al-Fe beidellite with small contents of kaolinite, illite and quartz. A small content of fine clay minerals plays the major role of bonding agent and is responsible for hardness in sandy or sandy-loam soils.