Abstract
More than 30% of the surface of the earth consists of arid and semi-arid soils, which are generally too dry to produce good yields. If enough fresh water is available and the soil condition is suitable, these soils can be irrigated and used for arable land. These regions are therefore thought to be new promising lands for food production in the future. For this reason in past dacades, much attention has been paid to arid and semi-arid soils and more agricultural development projects are being planned and carried out. However, many projects have failed in the past, and others fail even now, because some years after irrigation the salinity or alkali hazard increases. The soil profile investigations of salts accumulation related to the depth of ground water level, some field experiments using lysimeter and model experiments in the laboratory to clarify the mechanisms of salt accumulation are illustrated in this paper. From field investigations and lysimeter experiments of irrigated lands in Khuzistan State, Itan, the author presents the following cases to clarify the essential processes of soil salinization. Case 1 was caused by the formation of shallow groundwater due to seepage from earth canals and intensive irrigation such as basin irrigation. This case was the most dominant to soil salinization development process in arid irrigated lands. Case 2 was caused by the logging of irrigation water at the extremely hard pan formation under the plough layer caused by the compactness of tractor loading. This case was usually found in desalinized fields with clay-like texture. The hard pan is apt to become an impermeable layer logged with irrigation water,and provides a secondary site which functions to supply sodium salts to surface horizon. Case 3 was caused by the addition of salts to the soil irrigated by water with high electric conductivity. Chemical analyses of the irrigation water applied to arid land in Iran, for example, showed that about 1.8kg of the total dissolved salts were contained in 1 m5 of irrigation water. Case 4 was caused by cultivation under the low leaching fraction resulting from the extremely high evapotranspiration of the dry summer. The lysimeter experiment revealed that cultivation during periods of high temperature and low humidity reguires much irrigation water, as calculated by the leaching fraction, to keep soil salinity below a specified level. The mechanism of salt accumulation in arid lands was also examined through the experimental pedology view. Experiments were carried out by using soil columns saturated with salt solution which capillarily provides from the constant groundwater level.Most calcium and magnesium ions in the salt solution are adsorbed rapidly at the cation exchange sites of the soil when the solution contacts with the soil. Large amouuts of sodium ions, on the other hand, still remain in the from of free ions which moves easily with the soil solution. When the cation exchange sites adsorbing calcium and magnesium ions are dried out, there ions are detached from the sites and deposited on the surface of the soil column as salts. Furthermore sodium salt precipitation occurs directly from soil solution when the evaporation at the surface portion of the column proceeds.
