Potential risk of soil salinization in different regions of Central Asia with special reference to salt reserves in deep layers of soils

To analyze the potential risk of secondary soil salinization along with the development of irrigation agriculture, an extensive soil survey was conducted with special reference to the salt concentration in deeper layers of soils in different regions of Central Asia (Kazakhstan and Uzbekistan). In total, 41 soil profiles from different scales of distance, that is, from within respective farms to over different countries, were analyzed for soluble salt contents and particle size distribution. A regional trend was found in terms of the contents of soluble salts in the 100-200 cm layers of soils, which is considered to represent a potential risk of secondary soil salinization. The detailed findings were as follows. In the Ili watershed, the contents of soluble cations tended to be low. In the Fergana regions, the contents of soluble cations and sodium adsorption ratio (SAR) were relatively low, but increased in the lower watershed of the Syr-Darya River. It is considered, on a geochemical scale, that the most mobile species, such as Nasup(+) and Clsup(-), tend to be removed from the upper regions and concentrated in the lower watershed. In northern Kazakhstan, soluble cations were usually high in subsoils with a high proportion of Na. These salts were considered to be derived from fluvial deposits that extensively cover this region. Therefore, although rainfed agriculture is most common in this area, the risk of secondary salinization is high with the introduction of irrigation agriculture in the future. Thus, the salt contents in deep soil layers are primarily determined by geochemical factors, with the accessory factors of soil texture and topography (or water regimes) affecting the salinization risk in certain regions. In contrast to non-irrigated conditions, under irrigated conditions, the upper layers of soils usually accumulate higher concentrations of soluble cations compared to the 100-200 cm or even deeper soil layers, and as a result SAR increases, indicating a process of secondary soil salinization. There is a higher fundamental risk (i.e. by geochemical factors) for secondary salinization in the lower watershed of the Syr-Darya River and in northern Kazakhstan than in the alluvial fans, such as the Ili and Fergana basins. For sustainable maintenance of irrigation agriculture, further extension in the lower watershed of large rivers should be reconsidered.