Salinization of Soils
Salinization of soils
Salinization of soil involves the processes of salt accumulation in the upper rooting zone so that many plants are inhibited or prohibited from normal growth. Salinization occurs primarily in the semi-arid and arid portions of the earth. Salinization is commonly thought to occur only in the hot climatic regions, but may be found in cooler to cold portions of the earth where precipitation is very limited. Where the annual precipitation exceeds about 20 inches (500 mm), there is usually adequate downward movement of salts through leaching to prohibit the development of saline soil . Occasionally salinization of soil will occur where there has been an inundation of land by sea water. Some areas of the world were once under sea water, but due to uplift the land is now many feet above sea level. These lands are common sources of "ancient salts" and may lead to the development of saline waters as these soils are slowly leached.
Natural sources of salts in soil are primarily from the decomposition of rocks and minerals through the processes of chemical weathering . With adequate amounts of moisture present, hydrolysis, hydration, solution, oxidation, and carbonation cause the minerals to decompose and release the ionic constituents to form various salts. The more common cations are sodium, calcium, and magnesium with lesser amounts of potassium and boron. The common anions are chloride and sulfate with lesser amounts of bicarbonate, carbonate, and occasionally nitrate.
Inherent factors of the landscape may also lead to salinization of soils. One of the more common factors is the restriction of water drainage within the soil, often referred to as soil permeability. Soil becomes less permeable because of genetic or inherited layers within the soil profile that have relatively high amounts of clay. Because of the extremely small sizes of the clay particles, the natural pore size is also very small, frequently being less than 0.0001 mm average diameter. The natural tortuosity and small size combine to severely reduce the rate of water movement downward which tends to increase salt buildup.
In addition, because of the soil pore size, saline waters may be transported upward through the processes of capillary action. While the rate of upward movement of water may be relatively slow, the process can deliver saline water from several inches below the surface of the land and create a zone of highly saline soil at the surface of the earth.
Salinization of soils has also occurred in areas where irrigation waters have been applied to lands that have not been subject to long-term natural leaching by rainfall. In several cases the application of relatively good quality water to arid soils with poor internal drainage has caused the soil to develop a higher water table which in turn has permitted the salts within the soil to be carried to the surface by high evaporation demand. In other cases water of relatively high salt content has been applied to soils without proper drainage, resulting in the development of saline soils. As the water is evaporated from the soil or the plant surfaces, the salts are left to accumulate in the soil.
Because of the increasing demand for food and fiber, many marginal lands are being brought into agronomic production. Proper irrigation management and drainage decisions must be considered on a worldwide and long-term basis to avoid a deleterious influence on the ability to produce on these soils and to determine the appropriate environmental considerations for disposal of salts from salinized soils.See also Arable land; Nitrates and nitrates; Runoff; Soil conservation; Soil eluviation; Soil illuviation
[Royce Lambert ]
Foth, H. D. Fundamentals of Soil Science. 7th ed. New York: Wiley, 1984.
Singer, M. J., and D. N. Munns. Soils: An Introduction. 5th ed. New York: Macmillan, 2001.
Richards, L. A., ed. Diagnosis and Improvement of Saline and Alkali Soils. USDA Handbook 60. Washington, DC: U.S. Government Printing Office, 1964.