Soil salination
2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics
Soil salination is the accumulation of free salts to such an extent that it leads to degradation of soils and vegetation.
Salination is a natural 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
Salt is a natural element of soils and water. The ions responsible for salination are: Na+, K+, Ca2+, Mg2+ and Cl-
In some areas (for example in Australia), salinity is an inherent situation (enormous amounts of salts are stored in the soils). However, human practices have increased soil salinity by changing the natural balance of the water cycle, by allowing excess recharging of groundwater and accumulation through concentration.
One of the best examples of excess salination was observed 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.
Salinity from drylands 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the clearing of trees for agriculture is a major reason for drylands in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops.
Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (other than 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, in turn can lead to rising water tables, requiring drainage to keep the saline groundwater out of the root zone. If the water table rises too high, then natural soil evaporation will begin to draw the salts back upward into the soil profile. The problem is accelerated when too much water is added too quickly due to inefficient water use such as over-irrigation, applying more than is required for leaching, using bad estimates of evapotranspiration and poor system design, and is also greatly increased by poor drainage and use of saline water for irrigating agricultural crops. These practices result in the concentration of salts.
Salinity in urban areas often results from the combination of irrigation and groundwater processes. Cities are often located on drylands, leaving the rich soils for agriculture. Irrigation is also now common in cities (gardens and recreation areas).
The consequences of salinity are
- detrimental effects on plant growth and final 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 problem. One way to prevent excess salination would be the use of humic acids, especially in regions where too much irrigation was practiced. In soils with excess salts, humic acids can fix anions and cations and eliminate them from the root regions of the plants. A type of plant that is of particular interest for those in areas affected by soil salinity is the saltbush, which is able to tolerate saline conditions and draws salt up into its leaves.