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The development of large-scale irrigation agriculture in the arid region of Central Asia, which was initiated in the 1960s under the former Soviet Union, has resulted in soil salinization in irrigated farms and their vicinity. In order to establish an appropriate countermeasure against soil salinization, we investigated the distribution pattern of salt-affected soils and their forming processes in rice-based cropping fields and their surroundings in four former state farms in the R. Ili and R. Syr-Darya basins in southern Kazakhstan. In the farm along R. Iii, two types of distribution patterns of salts in the soil profiles were observed. The first process was observed in cultivated and uncultivated fields adjacent to the irrigation,canal, and characterized by an accumulation of gypsum and/or soluble salts at/near the soil surface, suggesting an upward movement of water with a high concentration of salts. The second process was observed in uncultivated fields far from canals. Salt accumulation was observed in deeper horizons in combination with a low-salt, alkalinized layer on top, indicating that the soils were formed by leaching with a positive reaction for residual sodium carbonate. In the surface soil layer under upland cropping, amounts of soluble cations was 16.6 +/- 3.4 (SE) cmol(c) kg(-1) in the farm located in the lower part of R. Syr-Darya, which was much higher than that in the farm along R. Iii (4.8 +/- 0.4 cmol(c) kg(-1)). In the latter, the annual rate of salt accumulation during the first year of upland cropping was roughly estimated to 45 kmol(c) ha(-1) year(-1), while in the second year the accumulation was about 15 kmol(c) ha(-1) year(-1). The contents of soluble salts decreased to 2.0 +/- 0.6 cmol(c) kg(-1) during the period of rice cultivation in the farm along R. Iii, showing that water-logging in this phase was effective to leach out the salts accumulated during the upland phase. The soils in the paddy field in the R. Syr-Darya basin showed, however, still a high salinity level, i.e. 5.6 +/- 1.2 cmol(c) kg(-1), presumably because of the incomplete drainage conditions. These results suggested that the rice-based irrigation agriculture studied is sustainable provided that an adequate supply of irrigation water and an appropriate drainage system are available. (C) 2000 Elsevier Science B.V. All rights reserved.