Abstract
To clarify the mechanism of secondary salinization, studies on water and salt behavior were conducted in an irrigation block where a rice-based cropping system has been practiced in the Lower Syr Darya river basin of Kazakhstan. Results are summarized as follows. 1) Due to excessive irrigation of rice fields with slightly saline river water, dissolved salts were deposited mainly in upland fields in the block and its periphery. Changes in salt accumulation rates were dependent upon the scale of annual changes in cropping patterns. 2) A remarkable finding was obtained on the salt behavior in saturated soil layers of rice fields. Salts from the deeper layer diffused upward in percolated water and thus increased soil water salinity in the shallow layer after the subsoil was saturated. This finding refutes the hypothesis that rice cultivation practiced in arid areas is effective for leaching salts already accumulated in the soil. 3) Because of the movement of percolated water from rice fields through underneath the field-drained bed, the rise of the groundwater table and the salt accumulation were accelerated in the adjacent upland fields. Thus, mixed cropping with rice and upland crops based on an eight-year crop rotation system in an irrigation block accelerates waterlogging and salt accumulation in upland fields.
