An increase in salt content, usually of agricultural soils, irrigation water, or drinking water is called salinization. Salinization is a problem because most food crops, like the human body, require fresh (nonsaline) water to survive. Although a variety of natural processes and human activities serve to raise the salt contents of soil and water, irrigation is the most widespread cause of salinization. Almost any natural water source carries some salts; with repeated applications these salts accumulate in the soil of irrigated fields. In arid regions, streams, lakes, and even aquifers can have high salt concentrations. Farmers forced to use such saline water sources for irrigation further jeopardize the fertility of their fields. In coastal areas, field salinization also results when seawater floods or seeps into crop lands. This occurs when falling water tables allow sea water to seep inland under ground, or where aquifer subsidence causes land to settle. Salinization also affects water sources, especially in arid regions where evaporation results in concentrated salt levels in rivers and lakes. Remedies for salinization include the selection of salt-tolerant crops and flood irrigation, which washes accumulated salts away from fields but deposits them elsewhere.
Most of the world's people rely on irrigated agriculture for food supplies. Regular applications of water, either from rivers, lakes, or underground aquifers, allow grain crops, vegetables, and fruits to grow even in dry regions. California's rich Central Valley is an outstanding example of irrigation-dependent agriculture. One of North America's primary gardens, the valley is naturally dry and sun baked. Canals carrying water from distant mountains allow strawberries, tomatoes, and lettuce to grow almost year round. The cost of this miraculous productivity is a gradual accumulation of salts, which irrigation water carries onto the valley's fields from ancient sea bed sediments in the surrounding mountains. Without heavy flooding and washing, Central Valley soils would become salty and infertile within a few years. Some of California's soils have become salty and infertile despite flooding. Similar situations are extremely widespread and have been known since humankind's earliest efforts in agriculture. The collapse of early civilizations in Mesopotamia and the Indus Valley resulted in large part from salt accumulation, caused by irrigation, which made food supplies unreliable.
Salts are a group of mineral compounds, composed chiefly of sodium, calcium, magnesium, potassium, sulfur, chlorine , and a number of other elements that occur naturally in rocks, clays, and soil. The most familiar salts are sodium chloride (table salt) and calcium sulfate (gypsum). These and other salts dissolve easily in water, so they are highly mobile. When plenty of water is available to dilute salt concentrations in water or wash away salt from soil, these naturally occurring compounds have little impact. Where salt-laden water accumulates and evaporates in basins or on fields, it leaves behind increasing concentrations of salts.
Most crop plants exposed to highly saline environments have difficulty taking up water and nutrients. Healthy plants wilt, even when soil moisture is high. Leaves produced in saline conditions are small, which limits the photosynthetic process. Fruits, when fruiting is successful, are also small and few. Seed production is poor, and plants are weak. With increasing salinity , crop damage increases, until plants cannot grow at all. Water begins to have a negative effect on some crops when it contains 250–500 parts per million (ppm) salts; highly saline water, sometimes used for irrigation out of necessity, may contain 2,000–5,000 ppm or more. For comparison, sea water has salt concentrations upwards of 32,000 ppm. In soil, noticeable effects appear when salinity reaches 0.2%; soil with 0.7% salt is unsuitable for agriculture.
Another cause of soil salinization is subsidence. When water is pumped from underground aquifers, pore spaces within rocks and sediments collapse. The land then compacts, or subsides, often lowering several meters from its previous level. Sometimes this compaction brings the land surface close to the surface of remaining groundwater . Capillary action pulls this groundwater incrementally toward the surface, where it evaporates, leaving the salts it carried behind in the soil. Near coastlines such processes can be especially severe. Seawater often seeps below the land surface, especially when fresh-water aquifers have been depleted. When seawater, with especially high salt concentrations, rises to the surface it evaporates, leaving crystalline salt in the soil.
In such cases, the salinization of the aquifer itself is also a serious problem. Many near-shore aquifers are threatened today by seawater invasions. Usually seawater invasions occur when farms and cities have extracted a substantial amount of the aquifer's water volume. Water pressure falls in the fresh-water aquifer until it no longer equals pressure from adjacent sea water. Sea water then invades the porous aquifer formation, introducing salts to formerly fresh water supplies.
Rivers are also subject to salinization. Both cities and farms that use river water return their wastewater to the river after use. Urban storm sewers and sewage treatment plants often send poor quality water back to rivers; drainage canals carry intensely saline runoff from irrigated fields back to the rivers that provided the water in the first place. When dams block rivers, especially in dry regions, millions of cubic meters of water can evaporate from reservoirs, further intensifying in-stream salt concentrations.
The Colorado River is one familiar example out of many rivers suffering from artificial salinization. The Colorado, running from Colorado through Utah and Arizona, used to empty into the Sea of Cortez south of California's Imperial Valley before human activities began consuming the river's entire discharge . Farms and cities in adjacent states consume the river's water, adding salts in wastewater returned to the river. In addition, the Colorado's two huge reservoirs, Lake Powell and Lake Mead, lie in one of the continent's hottest and driest regions and lose about 10% of the river's annual flow through evaporation each year. By the time it reaches the Mexican border, the river contains 850 ppm salts, too much for most urban or agricultural uses. Following a suit from Mexico, the United States government has built a $350 million desalinization plant to restore the river's water quality before it leaves Arizona. The Colorado's story is, unfortunately a common one. Similar situations abound on major and minor rivers from the Nile to the Indus to the Danube.
Salinization occurs on every occupied continent. The world's most severely affected regions are those with arid climates and long histories of human occupation or recent introductions of intense agricultural activity. North America's Great Plains, the southwestern states, California, and much of Mexico are experiencing salinization. Pakistan and northwestern India have seen losses in agricultural productivity, as have western China and inland Asian states from Mongolia and Kazakhstan to Afghanistan. Iran and Iraq both suffer from salinization, and salinization has become widespread in Africa. Egypt's Nile valley, long northern Africa's most bountiful bread basket, also has rising salt levels because of irrigation and subsidence. One of the world's most notorious case histories of salinization occurs around the Aral Sea ,in southern Russia. This inland basin, historically saline because it lacks an outlet to the sea, is fed by two rivers running from northern Afghanistan. Since the 1950s, large portions of these rivers' annual discharge has been diverted for cotton production. Consequently, the Aral Sea is steadily drying and shrinking, leaving great wastes of salty, dried sea bottom. Dust storms crossing these new deserts carry salts to both cotton and food crops hundreds of miles away.
Avoiding salinization is difficult. Where farmers have a great deal of capital to invest, as in California's Central Valley and other major agricultural regions of the United States, irrigators install a network of perforated pipes, known as tiles, below their fields. They then flood the fields with copious amounts of water. Flooding washes excess salts through the soil and into the tiles, which carry the hypersa-line water away from the fields. This is an expensive method that wastes water and produces a toxic brine that must be disposed of elsewhere. Usually this brine enters natural rivers or lakes, which are then contaminated unless their volume is sufficient to once again dilute salts to harmless levels. However this method does protect fields. More efficient irrigation systems, with pipes that drip water just near plant roots themselves may be an effective alternative that contaminates minimal volumes of water.
Water can also be purified after agricultural or urban use. Purification, usually by reverse osmosis, is an expensive but effective means of removing salts from rivers. The best way to prevent water salinization is to avoid dumping urban or irrigation wastes into rivers and lakes. Equally important is avoiding evaporation by reconsidering large dam and reservoir developments. Unfortunately, most societies are reluctant to consider these options: reservoirs are widely viewed as essential to national development, and wastewater purification is an expensive process that usually benefits someone else downstream.
Perhaps the best way to deal with salinization is to find or develop crop plants that flourish under saline conditions. Governments, scientists, and farmers around the world are working hard to develop this alternative. Many wild plants, especially those native to deserts or sea coasts, are naturally adapted to grow in salty soil and water. Most food plants on which we now depend—wheat, rice, vegetables, fruits—originate in nondesert, nonsaline environments. When domestic food plants are crossed with salt-tolerant wild plants, however, salt-tolerant domestics can result. This process was used to breed tomatoes that can bear fruit when watered with 70% seawater. Other vegetables and grains, including rice, barley, millet, asparagus, melons, onions, and cabbage, have produced such useful crossbreeds.
Equally important are innovative uses of plants that are naturally salt tolerant. Some salt-adapted plants already occupy a place in our diet—beets, dates, quinoa (an Andean grain), and others. Furthermore, careful allocation of land could help preserve remaining salt-free acreage. Planting salt-tolerant fodder and fiber crops in soil that is already saline can preserve better land for more delicate food crops, thus reducing pressure on prime lands and extending soil viability.See also Salinization of soils
[Mary Ann Cunningham Ph.D. ]
Frenkel, H., and A. Meiri, eds. Soil Salinity: Two Decades of Research in Irrigated Agriculture. New York: Van Nostrand Reinhold, 1985.
National Research Council. Saline Agriculture. Washington, DC: National Research Council, 1990.
Scabocs, I. Salt-Affected Soils. Boca Raton, FL: CRC Press, 1989.
Shainberg, I., and J. Shalhevet. Soil Salinity Under Irrigation. Berlin: Springer-Verlag, 1984.