Soils of the terraces of the Niger River have locally undergone, prior to irrigation, a process of alkalinization. The use of the resulting nonsaline sodic soils [pHs 8.5-9.8 (s is "on saturated paste"), ECs = 2.2-3.2 dS m-1, SAR = 12-28 (mmol L-1)1/2, exchangeable sodium percentage (ESP) = 5-40] is greatly limited because of their alkalinity and sodicity. The mechanisms of degradation affecting the soil physicochemical properties, the water supply, and the mineral nutrition of crops were analyzed in the Lossa irrigation scheme in the Tillabery region in Niger. Increase in pH corresponds with an increase in the compactness and decrease in the permeability of the soils. In the subsoil a threshold effect is observed for an ESP ~10 or a pH ~8.5, and the hydraulic conductivity becomes very low, ~0.05 mm h-1. Reducing conditions inhibit the mineralization of organic matter, favor denitrification, and cause a deficiency in nitrogen. The simultaneous increase in pH diminishes nutrient availability and causes deficiencies in phosphorus, then in potassium and zinc, in the most alkaline soils. While maize yield is greatly affected by soil degradation, an aquatic forage grass (Echinochloa stagnina) seems to be more adapted to these nonsaline sodic soils.