Soil salination - AbsoluteAstronomy.com

Soil salinity is the salt content in the soil.

Causes of soil salinity

Salt-affected soils are caused by excess accumulation of salts, typically most pronounced at the soil surface. Salts can be transported to the soil surface by capillary

Capillary action

Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

transport from a salt laden water table and then accumulate due to evaporation

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

. They can also be concentrated in soils due to human activity, for example the use of potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

as fertilizer, which can form sylvite, a naturally occurring salt. As soil salinity

Salinity

Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

increases, salt effects can result in degradation of soils

Soils retrogression and degradation

Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a stable soil. Retrogression is primarily due to erosion and corresponds to a phenomenon where succession reverts back to pioneer conditions . Degradation is an evolution,...

and vegetation.

Salinization is a process that results from:

high levels of salt in the soils.
landscape features that allow salts to become mobile. (movement of water table)
climatic trends that favour accumulation.
human activities such as land clearing, aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...

activities and the salting of icy roads.

Natural occurrence

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

is a natural element of soils and water.
The ions responsible for salinization are: Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cl^-.

As the Na⁺ (sodium) predominates, soils can become sodic. Sodic soils present particular challenges because they tend to have very poor structure which limits or prevents water infiltration and drainage.

Over long periods of time, as soil minerals weather and release salts, these salts are flushed or leached out of the soil by drainage water in areas with sufficient precipitation. In addition to mineral weathering, salts are also deposited via dust and precipitation. In dry regions salts may accumulate, leading to naturally saline soils. This is the case, for example, in large parts of Australia. Human practices can increase the salinity of soils by the addition of salts in irrigation water. Proper irrigation management can prevent salt accumulation by providing adequate drainage water to leach added salts from the soil. Disrupting drainage patterns that provide leaching can also result in salt accumulations. An example of this occurred in Egypt in 1970 when the Aswan High Dam

Aswan Dam

The Aswan Dam is an embankment dam situated across the Nile River in Aswan, Egypt. Since the 1950s, the name commonly refers to the High Dam, which is larger and newer than the Aswan Low Dam, which was first completed in 1902...

was built. The change in the level of ground water before the construction had enabled soil erosion, which led to high concentration of salts in the water table. After the construction, the continuous high level of the water table led to the salination of the arable land

Arable land

In geography and agriculture, arable land is land that can be used for growing crops. It includes all land under temporary crops , temporary meadows for mowing or pasture, land under market and kitchen gardens and land temporarily fallow...

Dry land salinity

Salinity in drylands

Dryland salinity

Dryland salinity is salinity that occurs in a landscape that is not irrigated, as distinct from irrigation salinity and urban salinity.-Overview:...

can occur when the water table is between two to three metres from the surface of the soil. The salts from the groundwater are raised by capillary action to the surface of the soil. This occurs when groundwater is saline (which is true in many areas), and is favored by land use practices allowing more rainwater to enter the aquifer than it could accommodate. For example, the clearing of trees for agriculture is a major reason for dryland salinity in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops.

Salinity due to irrigation

Salinity from irrigation

Irrigation

Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and eventually begin to accumulate. Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must be leached out of the plant root zone by applying additional water. This water in excess of plant needs is called the leaching fraction. Salination from irrigation water is also greatly increased by poor drainage

Drainage

Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...

and use of saline water

Biosalinity

Biosalinity is the study and practice of using saline water for irrigating agricultural crops.Many arid and semi-arid areas actually do have sources of water, but the available water is usually brackish or saline . The water may be present in underground aquifers or as seawater along coastal...

for irrigating agricultural crops.

Salinity in urban areas often results from the combination of irrigation and groundwater processes. Irrigation is also now common in cities (gardens and recreation areas).

Consequences of salinity

The consequences of salinity are

detrimental effects on plant growth and yield
damage to infrastructure (roads, bricks, corrosion of pipes and cables)
reduction of water quality for users, sedimentation problems
soil erosion ultimately, when crops are too strongly affected by the amounts of salts.

Salinity is an important land degradation

Land degradation

Land degradation is a process in which the value of the biophysical environment is affected by one or more combination of human-induced processes acting upon the land....

problem. Soil salinity can be reduced by leaching

Leaching (agriculture)

In agriculture, leaching refers to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss.Leaching may also refer to ...

soluble salts out of soil with excess irrigation water. Soil salinity control involves watertable control

Watertable control

Watertable control is the practice of controlling the water table in agricultural land by subsurface drainage with proper criteria to improve the crop production.- Description and definitions :...

and flushing in combination with tile drainage

Tile drainage

Tile drainage is an agriculture practice that removes excess water from soil subsurface. Whereas irrigation is the practice of adding additional water when the soil is naturally too dry, drainage brings soil moisture levels down for optimal crop growth...

or another form of subsurface drainage

Drainage system (Agriculture)

An agricultural drainage system is a system by which the water level on or in the soil is controlled to enhance agricultural crop production.-Classification:Figure 1 classifies the various types of drainage systems...

. A comprehensive treatment of soil salinity is available from the FAO

Fão

Fão is a town in Esposende Municipality in Portugal....

.

High levels of soil salinity can be tolerated if salt-tolerant plants are grown. Sensitive crops lose their vigor already in slightly saline soils, most crops are negatively affected by (moderately) saline soils, and only salinity resistant crops thrive in severely saline soils. The University of Wyoming and the Government of Alberta report data on the salt tolerance of plants.

Regions affected

From the FAO/UNESCO Soil Map of the World the following salinised areas can be derived.