The alkali soils of the middle Niger valley : origins, formation and present evolution

In Niger Republic, the regional development of irrigation along the river Niger is confronted with a large extent of alkali soils. For a sound management of the irrigated area, it appears necessary to understand the present evolution of these alkali soils. The study of one of these areas, located on the Liptako bedrock, reveals a concentric soil distribution : within a zone of brown soil with abrupt textural change and alkaline in depth (stagnic Solonetz), appears a ring of brown steppe soils (cambic Calcisol), around zones of alkali soils (Solonetz). These alkali soils are not saline but their soil solution is more concentrated than in brown steppe soils. The abrupt boundary between the brown steppe soils and the alkali soils is related with morphological and geochemical changes, resulting from calcite and fluorite precipitation in the soil solution. The detailed study of this contact demonstrates the current transformation of alkali soils into brown steppe soils. Therefore, it appears that soil alkalization is no longer in process on the terraces of the river Niger. The formation of brown steppe soils and alkali soils may be explained by the juxtaposition of two contrasted hydrological regimes in a former pond. This hypothesis is supported by aerial pictures and microscopic observations. It is also consistent with a recent study of the hydrology of Sahelian ponds and with considerations on the chemical quality of the runoff. (Résumé d'auteur