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. Natural occurrence
Salt is a natural element of soils and water. The ions responsible for
salination are: Na+, K+, Ca2+, Mg2+ and Cl−.
As the Na+ 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 th——e 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. Dry land salinity
Salinity in 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, 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areas, since deep rooting of trees has been replaced by shallow rooting of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 can occur over time wherever irrigation occurs, since
almost all water 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.
Salinity in urban areas often results from the combination of irrigation and
groundwater processes. Irrigation is also now common in cities.
Consequences of salinity The consequences of salinity are
detrimental effects on plant growth and yield
damage to infrastructure 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. Soil salinity can
be reduced by leaching soluble salts out of soil with excess irrigation water.
Soil salinity control involves watertable control and flushing in
combination with tile drainage or another form of subsurface drainage. A
comprehensive treatment of soil salinity is available from the United Nations
Food and Agriculture Organization. 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 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. References
External links Article on water and salt balances in
the soil Download leaching model for saline soils
Salt of the Earth Documentary produced by Prairie Public Television