Les sols de la moyenne vallée du fleuve Sénégal : caractéristiques et évolution sous irrigation

Les sols alluviaux de la moyenne vallée du fleuve Sénégal, développés en climat aride, sont l'objet d'aménagements hydro-agricoles visant à développer les productions irriguées. Pour l'instant, la riziculture est la production principale, complétée par du maraîchage. Ces dépôts fluviaux et fluvio-marins récents sont organisés en unités géomorphologiques. Les bourrelets de berge des multiples bras du fleuve Sénégal sont des formations de texture argilo-sablo-limoneuse. Les cuvettes de décantation des eaux de crue sont des formations argileuses, où les sols présentent des caractères vertiques prononcés. Leur minéraologie fait apparaître un mélange de smectites et de kaolinites. De relativement mauvaises relations entre CEC et teneurs en eau laissent quelques doutes sur les proportions de minéraux à charges variables. Ces sols sont naturellement salés selon un profil descendant, hérité du passé marin récent. Les sels sont très variables en composition et sans type dominant : chlorures et sulfates pour les anons, sodium, magnésium et calcium pour les cations. Estimer leur évolution sous irrigation pose une gamme de questions très large. La modification du régime hydrique est très et susceptible à elle seule de créer des conditions de pédogenèse différentes. Les sels neutres d'origine marine migrent dans les profils vers la surface. L'eau du fleuve, d'influence continentale, présente une alcalinité résiduelle calcite positive, qui fait craindre une alcalinisation. Les difficultés d'approche sont nombreuses, en particulier du fait de l'absence de données anciennes, mais aussi en raison de la relative jeunesse des périmètres irrigués, et de la nécéssité de diagnostiquer des évolutions complexes et encore peu marquées. Un ensemble de travaux a été conduit, pour préciser ces dynamiques et tenter de reconstituer l'évolution du sol sous culture depuis l'aménagement... (D'après résumé d'auteur

Les sols de la moyenne vallée du fleuve Sénégal : caractéristiques et évolution sous irrigation

Les sols alluviaux de la moyenne vallée du fleuve Sénégal, développés en climat aride, sont l'objet d'aménagements hydro-agricoles visant à développer les productions irriguées. Pour l'instant, la riziculture est la production principale, complétée par du maraîchage. Ces dépôts fluviaux et fluvio-marins récents sont organisés en unités géomorphologiques. Les bourrelets de berge des multiples bras du fleuve Sénégal sont des formations de texture argilo-sablo-limoneuse. Les cuvettes de décantation des eaux de crue sont des formations argileuses, où les sols présentent des caractères vertiques prononcés. Leur minéraologie fait apparaître un mélange de smectites et de kaolinites. De relativement mauvaises relations entre CEC et teneurs en eau laissent quelques doutes sur les proportions de minéraux à charges variables. Ces sols sont naturellement salés selon un profil descendant, hérité du passé marin récent. Les sels sont très variables en composition et sans type dominant : chlorures et sulfates pour les anons, sodium, magnésium et calcium pour les cations. Estimer leur évolution sous irrigation pose une gamme de questions très large. La modification du régime hydrique est très et susceptible à elle seule de créer des conditions de pédogenèse différentes. Les sels neutres d'origine marine migrent dans les profils vers la surface. L'eau du fleuve, d'influence continentale, présente une alcalinité résiduelle calcite positive, qui fait craindre une alcalinisation. Les difficultés d'approche sont nombreuses, en particulier du fait de l'absence de données anciennes, mais aussi en raison de la relative jeunesse des périmètres irrigués, et de la nécéssité de diagnostiquer des évolutions complexes et encore peu marquées. Un ensemble de travaux a été conduit, pour préciser ces dynamiques et tenter de reconstituer l'évolution du sol sous culture depuis l'aménagement... (D'après résumé d'auteur

salinity and organic amendment effects on methane production of a rain-fed saline paddy field.

International audienc

Yield of rice under water and soil salinity risks in farmers’ fields

Corresponding author. E-mail addresses: [email protected], [email protected] audienceThis study focused on the variability of rice yield under water and soil salinity risks in farmers’ fields in northeast Thailand.Arice plot was monitored in 24, 16 and 11 farmers’ fields during the rice seasons 2005, 2006 and 2007, respectively. The results emphasized that few plots were continuously submerged during the 2005 season, when rainfall was low. Drought significantly affected the rice yield, yield components and the internal efficiency (IE) of the absorbed nutrients, while slight soil salinity had the only significant effect of increasing the IE of potassium (IEK). In the very rainy 2006 and 2007 seasons, most fields were continuously submerged, and in contrast to 2005, the slight soil salinity that was recorded had significant effects not only on IEK, but also on rice yield, spikelet sterility and 1000-grain weight. The yield decrease due to drought was about 87% and that due to salinity was 20%. When neither salinity nor water were limiting, the soil nutrient supply was high enough to achieve about 80% of the maximum yield reported in the literature for the rice cultivar in this area. As both drought and salinity risks are hardly avoided by the current farmers’ management they should be taken into account in the way technical recommendations to farmers are formulate

Quantification and modelling of water flow in rain-fed paddy fields in NE Thailand: evidence of soil salinization under submerged conditions by artesian groundwater

Water flow and solute transport in soils forms an essential part in many groundwater hydrology studies. This is especially true for Northeast Thailand, where the agricultural land is affected by the soil salinity, which is a widespread and an increasing phenomenon affecting 25% of the agricultural land. Salinization appears as scattered discrete patches of 10-100 m2 in the lowlands, illustrated by white efflorescences during the dry season and bare soil during the cropping season. A field study was undertaken in farm plots to measure the water flow and solute transport within the soil surface and the vadose zone, both inside and outside a saline patch. The water flow was measured on the soil surface with lysimeters and infiltration rings, and was derived in the soil from the hydraulic gradients measured with tensiometers placed at different depths. The salt transport was evaluated with water traps also placed at different depths, where the soil water's electrical conductivity was measured throughout the rainy season. Field study results demonstrated that the accumulation of saline solutions in rain fed paddy fields, occurred mainly during the rainy season while the soil surface remained flooded. During this period the saline water table rose towards the soil surface independently of infiltration into the soil. It happened in specific places where the compacted soil layer, generally ubiquitous in the area at a depth of 40-50 cm, is interrupted. Therefore salinity appeareds in discret points as patches. Artesian upward flow already described in this area (Haworth et al., 1966; Williamson et al., 1989; Imaizumi et al., 2002) is most probably responsible for this water table rise, thereby affecting crop productivity. Numerical modelling of water flow using HYDRUS-3D further supported these results and showed that managing the depth of flooding within the plot can significantly reduce the outbreak of these saline plumes

Simulating the evolution of soil solutions in irrigated rice soils in the Sahel

In Sahelian countries, agriculture often relies on irrigation especially for rice cropping which has been introduced these last decades. As most of the water resources used for irrigation (mainly rivers and lakes) have an alkaline composition, rice cropping is often held as responsible for soil alkalinization. In order to evaluate the probability for this process to occur, we studied 3 different sites in West Africa where numerical simulation of the geochemical interactions between soil, irrigation water and groundwater were performed with PHREEQC. It was possible to simulate the evaporation of the ponding water, the mixing between groundwater and irrigation water, the lateral drainage (flushing) and the leaching, and finally the management of the crop residues. By performing prospective simulations over 60 cropping cycles it has been shown that soil alkalinization and sodication are not inevitable in these areas especially if the cropping is properly managed. Moreover, when straw is removed and the plot flushed before the new cropping season, alkaline soils can even be improved with rice cropping

Actual and potential salt-related soil degradation in an irrigated rice scheme in the Sahelian zone of Mauritania

Salt-related soil degradation due to irrigation activities is considered a major threat to the sustainability of rice cropping under semi-arid conditions in West Africa. Rice productivity problems related to soil salinity, alkalinity and topographic position were observed in an irrigated rice scheme in southern central Mauritania. Detailed study of soils in a toposequence revealed that highest topsoil salinity and alkalinity were found at the shallow soils (2.5 m) and salinity levels remained low due to leaching. Foum Gleita¿s irrigation water used is amongst the most alkaline in the Sahel. However, no clear indications of secondary salinization or alkalinization due to irrigation activities were observed. A comparison of historical data revealed no significant changes of topsoil salinity and pH over the last 30 years. The PHREEQC 2.0 model was used to study actual and potential development of soil salinity and alkalinity problems, by simulating excessive concentration of the irrigation water through evaporation. The evolution into a strongly sodic-alkaline solution due to precipitation of Mg-calcite and -silicate minerals did not fit with current composition of ground and surface water, which showed geochemical control of alkalinity at high concentrations. Incorporation of cation exchange processes, using a small (1.0 mmolc per 100 g dry soil) but calcium saturated CEC, resulted in a better fit with field data. Results indicate that the soil¿s buffer capacity to counteract alkalinization processes is large. However, the soil water and salt balance needs to be quantified in order to determine development rate and equilibrium levels of soil salinity and alkalinity for different soil type × water management combinations. This study does neither reject the hypothesis that salt-related soil degradation jeopardizes the sustainability of rice cropping in the Sahel, nor does it provide evidence for its verification. However, our results are in line with other studies in west Africa, in that current salt-related production problems are inherited, rather than being induced by irrigated rice cropping