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Soil salination

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Visible salt deposits on the former bed of the Aral Sea
   Enlarge
   Visible salt deposits on the former bed of the Aral Sea

   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^+, Ca^2+, Mg^2+ 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   since deep rooting of trees has been replaced by shallow rooting of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.

   Retrieved from " http://en.wikipedia.org/wiki/Soil_salination"
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   with only minor checks and changes (see www.wikipedia.org for details
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