Physico-chimie de sols rizicultivés affectés par la salinité dans la basse vallée de la Rusizi au Burundi

Although salinization of Mugerero paddy soils is the main constraint to rice production, this process has not yet been investigated thoroughly. This work has been done to fill this gap. Physico-chemical analysis of soils of this area show that their exchange complex is mostly occupied by nearly equal percentages of Ca and Mg, while exchangeable sodium percentage vary up to 60% in some cases. Soil solution is characterized by large amount of sulfate or bicarbonate and sodium while chloride anion is poorly represented, and an alkaline pH. The Na - (Ca + Mg) exchange is well described either by a single Vanselow selectivity coefficient equal to 0.5 or by a single Gaines-Thomas selectivity coefficient equal to 0.3, while the Ca - Mg exchange is described by a single selectivity coefficient equal to 1.5. Moreover, analysis of correlations between yield components of rice and geochemical variables reveals positive effects of Ca and K ions on rice yield and negative effects of pH, salinity and / or soluble Na, SO4 or HCO3. The exploration of the cation contents of the soluble and exchangeable phases in five soil to water ratios shows that their variations as a function of water content are in conformity to the law of ion exchange and that the composition of solution and exchange phases for any moisture content can be estimated accurately from measurements carried out at 1/2 soil to water ratio using regression equations. Furthermore, incubation of the soil does not induce any important release of Fe and Mn in soil solution probably due to high values of pH. Moreover, potassium buffering capacity of these soils can be well predicted from clay content or sand content. Finally, soils of interest exhibit problems of salinity, sodicity and alkalinity that deserve to be keenly investigated for a best management in order to increase agricultural production.