Salinization Research Paper

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Salinization, the process by which salts accumulate in soil, has long been (and continues to be) one of the world’s major challenges for sustaining agricultural production. In natural and managed ecosystems, salinization regulates plant and animal communities, and it determines the way in which water is circulated and distributed on and below the Earth’s surface and in the atmosphere.

Salinization is a natural and human-induced process by which soluble salts accumulate in soil. Salts enter soil systems mainly when they are dissolved in groundwater or irrigation water, when they are deposited from the atmosphere, and when minerals are weathered. Salts are further concentrated as water is removed from the soil via evaporation and transpiration (the uptake of water by plants). Slow-draining clayey soils are much more susceptible to salinization than are rapid-draining sands. Salt concentrations in soils tend to increase in the direction of surface or groundwater flow, that is, from uplands to low-lying depressions or bottoms. Salinization thus affects many of the Earth’s major biogeochemical cycles of chemical elements and compounds between the living and nonliving parts of the biosphere.

The predominant soil salts are chlorides and sulfates salts, which are generally balanced by sodium, calcium, magnesium, and potassium. Soils with elevated salts are known as “saline,” and those with sodium-dominating salt composition are known as “sodic” or “natric.” Sodium salts are particularly problematic in soils for physical, chemical, and biological reasons. Excess sodium can stress many plants, especially if calcium concentrations are low, is associated with very high pH (above 9) and causes conditions directly injurious to plants by worsening deficiencies in nutrients such as phosphorus, copper, iron, boron, manganese, and zinc.

High sodium also adversely affects soil’s physical and chemical properties, causing soil clays and organic matter to disperse into individual particles instead of remaining flocculated (i.e., attracted together in packages or aggregates of multiple particles of clay and soil organic matter). Sodium-enriched soils that are dispersed can be a near-complete barrier to water entry as dispersed clay particles block soil pores. Sodium-caused dispersion also reduces the rate of gas movement in soils. Soils that are sodium enriched readily become waterlogged and stress oxygen- demanding processes such as root and microbial activity. Only well adapted salt-tolerant plants and microbes are able to grow in salinized soils.

Plants range widely in their ability to tolerate salts in their root zone. Plants that readily accumulate or tolerate salts are called “halophytes” and include salttolerant grasses, herbs, and shrubs, species that are native to deserts, shorelines, and salt marshes. Most food crops are not halophytic and exhibit high sensitivity to salt, including many legumes, corn, rice, oats, and wheat. Crop plants that tolerate or benefit from halophytic properties include beets, date palms, spinach, jojoba, and barley.

Geographical Distribution of Salt-Affected Soil

Salinization occurs chiefly in arid, semiarid, and subhumid regions. Salt-affected soils are not a widespread problem in humid regions because precipitation is sufficient to dissolve and leach excess salts out of the soil into groundwater and eventually the ocean. Some saline soils occur along humid seacoasts where seawater inundates the soil.

The importance of salinization can be illustrated by the large areas soils that are enriched in salts throughout the world. Fully one-quarter to one-third of the world’s 1.5 billion hectares currently under cultivation are saline or sodic, a statistic that demonstrates the impact of salinization on world food production. Salinity is most problematic in cultivated soils that are irrigated. Irrigated lands with large areas of saline and sodic soils include Australia, India, Pakistan, Russia, China, the United States, the Middle East, and Europe. In some nations, salts seriously affect more than 50 percent of irrigated lands. Salinization will be a major problem for successfully doubling to tripling food production in the coming decades.

Reclamation of Saline and Sodic Soils

Salinization is an insidious problem, and early soil symptoms are often ignored. To control salinization and maintain irrigation-based agriculture over the long term requires soil analysis and careful monitoring of water and salt budgets (amounts) of local fields to river-basin scales. Such monitoring provides the technical basis for remedial action.

To diagnose soil salinity problems, soil samples are rinsed with water, and the water’s electrical conductivity is measured. The soil water’s conductance is linearly related to the concentration of soluble salts. The conductance at which the growth of many plant species is diminished by salts is well established. Sodium problems are also diagnosed from the ratio of the soil sodium to calcium.

Leaching with high-quality (dilute) water can potentially rid a soil of its salinity problems, but the reclamation of sodic soils is not straightforward. Because elevated sodium tends to disperse soil clays and greatly reduce the rate at which water moves through soil, the ratio of soil sodium to calcium needs to be decreased if sodium leaching is to be effective. Calcium, especially in the form of gypsum, tends to aggregate or flocculate clays, allowing sodium to be dissolved and to leach through the soil-rooting zone. Reclamation of sodic soils may, however, require large amounts of gypsum, sometimes several thousand kilograms per hectare. Gypsum additions can be costly, especially if the only sources of irrigation water have high sodium and salinity.

Reclamation of salt-affected soils requires a suitable system of disposal. In many cases the drainage is too brackish (salty) to be directly recycled. Moreover, in contemporary agricultural systems, drainage effluents also contain other constituents such as fertilizer nutrients, sediments, and pesticides, all of which may influence disposal. Nonetheless, irrigation effluents are often simply exported to streams or rivers, a practice that salinizes and pollutes the local water supplies. Receiving streams will become progressively saltier, and lower reaches may become unfit as a water supply for humans, animals, or crops. Adverse effects on aquifers and estuaries are direct and potentially serious. Improving the efficiency of water and salt budgets of irrigated lands is one of the highest priorities for contemporary agronomy (the branch of agriculture dealing with field-crop production and soil management).

Salinization Historically

Salinization affected the first great agricultural societies such as the Sumerian and Akkadian cultures, societies that developed on the alluvial soils of the Tigris and Euphrates Rivers. The region was initially developed into extensive agricultural fields with irrigated grain, forage, and palm production in the southern reaches of these rivers. Complex canal systems served both for transportation and irrigation, cities were built, and civilization flourished. Over time the irrigated lands shifted significantly northward along these rivers, a pattern frequently explained by a salinity-affected decline in agriculture in the south. Archeological records of the Sumerian civilization suggest that grain production also shifted over the centuries from the -preferred but salt-sensitive wheat to salt-tolerant barley.

The history of salinization in ancient Egypt contrasts greatly with that in Mesopotamia. Salt budgets and timing of annual floods along the Egyptian terraces of the Nile are much more conducive to longterm management than terrace soils of the Tigris and Euphrates. Alluvial soils along the Nile are relatively narrow and well drained, except in the Nile’s delta. The Nile’s river channel tends to be deeply incised along much of its course, ensuring that the rise and fall of the river affects relatively large fluctuations of the groundwater under river terraces. The Nile’s agricultural resources are generally considered to be nearly ideal, given the particular combination of high-quality water, relatively predictable flooding, soil fertility, nutrients, and organic matter. Salinization has not developed into an acute problem along the Nile, and irrigation-based cropping has continued for about five millennia.

Salinization has stressed other agricultural civilizations in the past. One of the largest of these developed in the Indus River valley of Asia, approximately coincident in time with ancient Sumeria. The area of Indus irrigation systems apparently exceeded those in ancient Egypt and Sumer. Relatively few records exist for these Indus civilizations, although several excavations partly tell their tale (at Harappa, for example). Although archeologists suggest that catastrophic floods, earthquakes, and soil erosion wore down these ancient civilizations, salinization was also probably a major problem. In the twentieth century, salinization greatly stresses irrigation-based agriculture which has spread across nearly 15 million hectares of the Indus River valley.

Historical patterns of salinization in Australia’s enormous Murray-Darling River basin are also instructive, given the ample documentation of the effects of land-use history on contemporary soils and agro-ecosystems. Although the basin covers about 15 percent of Australia’s total area, it provides far more than this percentage of the nation’s agricultural production, much of which is supported by irrigation. Since European settlement began in earnest by the mid-nineteenth century, extensive areas of deeply rooted Eucalyptus forests have been cut over and converted to relatively shallow-rooted annual crop systems, an ecosystem transformation that has reduced plant transpiration (the use of water by plants) and allowed a larger fraction of annual precipitation to percolate through soil and elevate local groundwater systems. Because the underlying groundwater is saline, substantial quantities of soluble salt have been mobilized into the upper soil and root zone. Evapotranspiration (loss of water from the soil both by evaporation and transpiration from plants) concentrates salts, and about 500,000 hectares of the Murray-Darling River basin are estimated to have saline groundwater within 2 meters of the soil surface. Australians refer to these circumstances as “secondary” salinization (i.e., human affected), reserving the term natural salinization to describe naturally occurring saline soils that occur in association with salt lakes in drier regions of the continent or adjacent to estuaries.

Salinization in the Future

Irrigated land today totals approximately 0.3 billion hectares of the 1.5 billion hectares of total cultivated land. About 35 percent of total crop output is currently derived from irrigated systems, a percentage that is expected to increase. Between the 1960s and the late 1990s irrigated land increased at the rate of about 3 percent per year. Five nations account for about two-thirds of the irrigated area: China, India, Pakistan, Russia, and the United States. Other nations dependent on irrigated systems include Egypt, Indonesia, Iraq, Jordan, and Israel. All indications are that in the future, irrigation management of water and salts will be increasingly important because global agricultural production is so dependent upon irrigation. The challenge, however, is that contemporary management of irrigated systems is far less than optimal and in fact presents serious concerns for the future. Several examples illustrate the challenges.

The extensive modern irrigation system along the Indus River, mainly in Pakistan, is one of the major agricultural regions of the modern world. Alluvium and associated native groundwater reservoirs are deep in this great basin, which drains the western Himalayas. Nearly 15 million hectares were under irrigation by the mid 1990s, with the length of irrigation watercourses, farm channels, and field ditches estimated to exceed 1.5 million kilometers. Early in the development of this massive system, saline groundwater tables were observed in the rooting zone of many fields, and by the 1960s a massive drainage project was launched that by the 1990s had benefited about one-third of the entire system. Currently, between 2 and 5 million hectares of irrigated fields are adversely affected by salts. Keeping this system operational will require an incredible continuity of effort, especially because the Indus drainage channels have such low elevation gradients (e.g., about a 0.02 percent gradient), which must transport water many hundreds of kilometers to the ocean outlet.

In modern Egypt the challenges of controlling salinization in irrigated agro-ecosystems have become similarly serious. From ancient times to the beginning of the nineteenth century, the human population of the Nile River valley totaled several million people, most of whom were supported by the river’s irrigation-based agriculture. Through most of this past, Egypt was able to export enormous amounts of food, a situation that is no longer the case. Modern Egypt has now about 60 million inhabitants, a total that is expected to grow to 90 to 100 million by 2030. Along with rising agricultural imports, Egypt is placing enormous demands on its irrigated soils for domestic food production. The Aswan High Dam, constructed in the 1960s, established management control over the Nile’s flood flows and thereby expanded opportunities for irrigation. On the other hand, with river flow controlled, the seasonality of river flow no longer regularly and flushes soils of salts. Particularly in the alluvial soils of the great Nile Delta, salts have grown to be an increasingly common problem. Future expansions of irrigation agriculture along with highly engineered drainage projects to control salts are widely contemplated.

Future problems with salinization are hardly confined to irrigated lands. Natural cycles of rainfall in arid and semiarid climates can be associated with expansion and contraction of saline and sodic soils. The saline ecosystems of Amboseli National Park in Kenya at the foot of Mt. Kilimanjaro provide a celebrated and much debated example. From the 1950s on, the vegetation of much of the Park has markedly changed from forested savannahs with Acacias to a landscape more dominated by salt-tolerant grasses and shrubs with only occasional trees. A long-standing hypothesis for the ecosystem transformation has been that when regional rainfall increased through the 1960s, highly saline groundwaters rose into the root zone of Acacia-dominated savannas, causing widespread tree decline. Whether or not the transformation of the Amboseli is explained by salinization, the Amboseli serves as an outstanding example of how future climate change in the decades and centuries ahead may affect salt balances and salinization across wide areas and thereby the structure and function of natural ecosystems.

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