Residual alkalinity as tracer to estimate the changes induced by forage cultivation in a non-saline irrigated sodic soil

Soil alkalinisation generally constitutes a major threat to irrigated agriculture in the semi-arid regions of west Africa. The improvement of sodic soils is generally difficult and expensive. However, a recent study in the Niger valley in Niger, reveals that a natural de-alkalinisation is possible under natural conditions in a semi-arid climate. Transformation of non-saline sodic soil into brown steppe soil type was recorded. On the same site, the cultivation of a Sahelian fodder grass, locally known as ‘Burgu’ was used on the sodic soil/brown steppe soil transition zone to accelerate this natural de-alkalinisation and characterise its mechanisms. The geochemical properties of both soil types were monitored before cultivation and 1 year after continuous crop cultivation. After cultivation and regular irrigation, the chemical properties of the former sodic soils were close to those of the surrounding brown steppe soils, which are better suited for agriculture. This modification of the sodic soil properties can be attributed to (i) the large amount of water supplied during cultivation that induced salt leaching. This is the main phenomenon responsible for the changes observed; (ii) the root activity that modified the acid–base equilibrium and consumes alkalinity.