Soil quality and rice productivity problems in Sahelian irrigation schemes

In irrigation schemes in theSahel, rice yields and cropping intensity are still far from their potential and some of the 10-20 year old irrigation schemes experience declining yields. The large investments in irrigation infrastructure, made to improve food security, to generate income, and to reduce rice imports, are currently at stake. Many farmers and researchers suggested that the rice productivity problems might (partly) be due to salt-related soil degradation. Sahelian irrigation waters contain little salt, but are relatively rich in carbonates (RA calcite > 0). In the hot and dry Sahelian climate, irrigation with such waters could lead to the formation of an alkaline (high pH) and sodic (high sodium content) soil, which has a low productivity. We investigated the causes of declining rice productivity in the irrigation schemes of Foum Gleita (Mauritania) and theSourouValley(Burkina Faso). We found that rice productivity problems in Foum Gleita were primarily caused by N and P deficiency, while Zn deficiency prevailed in theSourouValley. These nutrient deficiencies were caused by no or insufficient application of N, P and Zn fertilizers, in combination with low plant available N, P and Zn in the soil. The alkaline-calcareous nature of the studied soils likely contributed to the low availability of N through volatilization, and low availability of P and Zn through precipitation of or adsorption onto carbonate minerals. Application of ample N, P and Zn fertilizer increased yields at the study sites from 3-4 t ha -1 to 5-6.5 t ha -1 . However, the required chemical fertilizers are often expensive or not available on the localmarkets, which hampersadoption of improved fertilizer recommendations by the resource-poor Sahelian rice farmers. Application of organic amendments (a.o. rice straw) at a rate of 5 t ha -1 often improved yields at the study sites by 1-2 t ha -1 . In Foum Gleita, the yield increasing effect was attributed to improved N availability, while in theSourouValley, organic matter amendments improved Zn uptake.Although rice productivity problems correlated with the alkaline-calcareous nature of the soil, we found no evidence that irrigated cropping increased the alkalinity of these soils. On the contrary, this study and literature sources indicate that alkalinity problems were inherited from the parent material and/or from the prolonged cultivation of non-flooded crops. At the study sites inMauritaniaandBurkina Faso, concentration factors of irrigation waters are small (< 6). Moreover, detailed study in Foum Gleita showed that clayey rice soils seem to have a large buffer capacity against alkalinization. Adsorbed Ca 2+ at the exchange complex is desorbed with increasing salt concentrations in the soil solution, which leads to a decrease in soil solution alkalinity through precipitation of calcite. In addition, irrigated rice largely prevents the build-up of alkaline salts in the soil root zone through proton excretion. In a soil column trial, using soils from Foum Gleita and the Office du Niger (Mali), we found that irrigated rice cropping can be used to reclaim alkaline-sodic soils, provided some (natural) drainage is possible. These findings are in line with recent literature on changes in soils under flooded crops inMaliandNiger. Rice straw amendments accelerated the reclamation processes on soils that contained calcite.Anaerobic decomposition of straw increased pCO 2 , dissolution of calcite, and consequently Ca 2+ in the soil solution.The Ca 2+ exchanged with adsorbed Na + and the latter was leached together with (bi)-carbonates. Reduction of iron hydroxides could theoretically lead to ferrolysis and acidification of the upper horizons, but we found that the process was of little importance to the short term alkalinity changes of the studied soils.