<P> Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization . Salts occur naturally within soils and water . Salination can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean . It can also come about through artificial processes such as irrigation . </P> <P> Salts are a natural component in soils and water . The ions responsible for salination are: Na, K, Ca, Mg 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 . </P> <P> 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 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 . </P> <P> Salinity in drylands can occur when the water table is between two and 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clearing of trees for agriculture is a major reason for dryland salinity in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops . </P>

Where does salt in the soil come from naturally
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