<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clearing of trees for agriculture is a major reason for dryland salinity in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops .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> <P> Salinity from irrigation 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 and use of saline water for irrigating agricultural crops . </P> <P> 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). </P>

When do soils become too saline for agricultural use