Soil salinization under large-scale irrigation agriculture in Central Asia

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To analyze the potential risk of secondary soil salinization, an extensive soil survey was conducted with special reference to salt concentration in deeper layers of soils in different regions of Kazakhstan and Uzbekistan. A clear regional trend was found in terms of contents of soluble salts in the 100-200 cm layers of soils. 1) In the Hi watershed, the contents of soluble cations tend to be low. 2) In the Fergana regions, the contents of soluble cations and SAR are relatively low, but are increased in the lower watershed of Syr-Darya River. 3) In northern Kazakhstan, soluble cations are usually high in subsoils with a high proportion of Na. Thus the salt contents in deep soil layers are primarily determined by geochemical factors. There is a higher fundamental risk for secondary salinization in the lower watershed of Syr-Darya River and northern Kazakhstan than in the alluvial fans such as the Hi and Fergana basins.Then process and risk of soil salinization in rice-based system was analyzed in respective farm scale, with special emphasis on the effect of topography and groundwater level. This study examined the relationships between soil salinity, crop yield and affecting factors including local terrain for two farms. Both farms are characterized by a gentle slope, flat area and localized depressions. Most of the paddy fields showed wash-out of salts during the irrigation period except in waterlogged fields situated in depressions. Rice yield was adversely affected by the remaining soil salinity, deep ponding water and insufficient drainage capacity in the lower part of the irrigation system. Salt accumulation in the upland fields appeared to increase with cropping years, and was associated with groundwater depth that became shallower at the lower parts of the irrigation systems, adversely affecting the crop growth in most fields.