Field study and simulation of geochemical mechanism of soil alkalinization in the sahelian zone of Niger

In some soils of the Niger valley, alkalinization and sodization are related to the concentration of alteration products of calco-alkaline gneiss to biotite, which releases significant amounts of cations and alkalinity. The results is an oversaturation of the soil solution with respect to calcite, which precipitates with a positive calcite residual alkalinity. Ca2+ molality decreases, while alkalinity and pH increases : Ca2+ desorption is accompanied by adsorption of Na+ on the exchange complex. K+, Mg2+, and Na+ concentrations in the soil solution are successively controlled by the formation of illites and smectite-type silicates, while kaolinite dissolves. These processes were successfully simulated according to the analytical data. They agreed with the significant increase of the alkaline reserve in the soils according to chemical, physical, and mineralogical alterations. These geochemical mechanisms were found to be involved in the formation of alkali soils at three sites of the region. (Résumé d'auteur