![Salinity](https://upload.wikimedia.org/wikipedia/commons/b/b0/Salinity.jpg)
Soil salinity is the
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
content in the
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
; 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
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...
or by the gradual withdrawal of an ocean. It can also come about through artificial processes such as
irrigation
Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...
and
road salt
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
.
Natural occurrence
Salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
s are a natural component in soils and water.
The
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s responsible for salination are:
Na+,
K+,
Ca2+,
Mg2+ and
Cl−.
Over long periods of time, as soil minerals
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
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. Salts may accumulate in dry regions, 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
Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Mediter ...
in 1970 when the
Aswan High Dam
The Aswan Dam, or more specifically since the 1960s, the Aswan High Dam, is one of the world's largest embankment dams, which was built across the Nile in Aswan, Egypt, between 1960 and 1970. Its significance largely eclipsed the previous Aswan Lo ...
was built. The change in the level of
ground water
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...
before the construction had enabled
soil erosion
Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, and ...
, 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
Arable land (from the la, arabilis, "able to be ploughed") is any land capable of being ploughed and used to grow crops.''Oxford English Dictionary'', "arable, ''adj''. and ''n.''" Oxford University Press (Oxford), 2013. Alternatively, for the ...
.
Sodic soils
When the Na
+ (sodium) predominates, soils can become sodic. The
pH of sodic soils may be
acidic
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
, neutral or
alkaline
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
.
Sodic soil
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 ...
s present particular challenges because they tend to have very poor structure which limits or prevents
water infiltration
Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meter ...
and drainage. They tend to accumulate certain elements like
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
and
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
in the
root zone at levels that maybe toxic for plants. The most common compound used for
reclamation
Reclaim, reclaimed, reclaimer, reclaiming or reclamation means "to get something back".
It may refer to:
* Land reclamation, creating new land from oceans, riverbeds, or lake beds
* Dedesertification, reversing of the land degradation in arid ...
of sodic soil is
gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...
, and some plants that are tolerant to salt and
ion toxicity
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
may present strategies for improvement.
The term "sodic soil" is sometimes used imprecisely in scholarship. It's been used interchangeably with the term
alkali soil
Alkali, or Alkaline, soils are clay soils with high pH (greater than 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico ...
, which is used in two meanings: 1) a soil with a pH greater than 8.2, 2) a soil with an exchangeable sodium content in excess of 15% of exchange capacity. The term "alkali soil" is often, but not always, used for soils that meet both of these characteristics.
Dry land salinity
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.
Salinity due to irrigation
Salinity from
irrigation
Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...
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. This water in excess of plant needs is called the
leaching fraction. Salination from irrigation water is also greatly increased by poor
drainage
Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic conditio ...
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 (gardens and recreation areas).
Consequences of soil salinity
The consequences of salinity are
* Detrimental effects on plant growth and yield
* Damage to infrastructure (roads, bricks, corrosion of pipes and cables)
* Reduction of water quality for users, sedimentation problems, increased leaching of metals, especially copper, cadmium, manganese and zinc.
*
soil erosion
Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, and ...
ultimately, when crops are too strongly affected by the amounts of salts.
* More energy required to desalinate
Salinity is an important
land degradation
Land degradation is a process in which the value of the biophysical environment is affected by a combination of human-induced processes acting upon the land.
It is viewed as any change or disturbance to the land perceived to be deleterious ...
problem. Soil salinity can be reduced by
leaching
Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to:
*Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amoun ...
soluble salts out of soil with excess irrigation water.
Soil salinity control
Soil salinity control relates to controlling the problem of soil salinity and reclaiming salinized agricultural land. The aim of soil salinity control is to prevent soil degradation by salination and reclamation of already salty (saline) soils. ...
involves
watertable control Watertable control is the practice of controlling the height of the water table by drainage. Its main applications are in agricultural land (to improve the crop yield using agricultural drainage systems) and in cities to manage the extensive underg ...
and
flushing
Flushing may refer to:
Places
* Flushing, Cornwall, a village in the United Kingdom
* Flushing, Queens, New York City
** Flushing Bay, a bay off the north shore of Queens
** Flushing Chinatown (法拉盛華埠), a community in Queens
** Flushing ...
in combination with
tile drainage Tile drainage is a form of agricultural drainage system that removes excess sub-surface water from fields to allow sufficient air space within the soil, proper cultivation, and access by heavy machinery to tend and harvest crops. While surface wate ...
or another form of
subsurface drainage
Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
.
A comprehensive treatment of soil salinity is available from the
United Nations
The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and international security, security, develop friendly relations among nations, achieve international cooperation, and be ...
Food and Agriculture Organization
The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
.
Salt tolerance of crops
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 (moderately) saline soils, and only salinity-resistant crops thrive in severely saline soils. The University of Wyoming
[Alan D. Blaylock, 1994, ''Soil Salinity and Salt tolerance of Horticultural and Landscape Plants.']
University of Wyoming
and the Government of Alberta
[Government of Alberta]
''Salt tolerance of Plants''
/ref> report data on the salt tolerance of plants.
Field data in irrigated lands, under farmers' conditions, are scarce, especially in developing countries. However, some on-farm surveys have been made in Egypt, India, and Pakistan. Some examples are shown in the following gallery, with crops arranged from sensitive to very tolerant.
File:berseem egypt.png, Fig. 1. Berseem (clover), cultivated in Egypt's Nile Delta, is a salt-sensitive crop and tolerates an ECe value up to 2.4 dS/m, whereafter yields start to decline.
File:wheat sampla.png, Fig. 2. Wheat grown in Sampla, Haryana, India, is slightly sensitive, tolerating an ECe value of 4.9 dS/m.
File: wheat gohana.png, Fig. 3. The field measurements in wheat fields in Gohana, Haryana, India, showed a higher tolerance level of ECe = 7.1 dS/m.
(The Egyptian wheat, not shown here, exhibited a tolerance point of 7.8 dS/m).
File:cotton egypt.png, Fig. 4. The cotton grown in the Nile Delta can be called salt-tolerant, with a critical ECe value of 8.0 dS/m. However, due to scarcity of data beyond 8 dS/m, the maximum tolerance level cannot be precisely determined and may actually be higher than that.
File: sorghum pakistan.png, Fig. 5. Sorghum from Khairpur, Pakistan, is quite tolerant; it grows well up to ECe = 10.5 dS/m.
File: cotton pakistan.png, Fig. 6. Cotton from Khairpur, Pakistan, is very tolerant; it grows well up to ECe = 15.5 dS/m.
Calcium has been found to have a positive effect in combating salinity in soils. It has been shown to ameliorate the negative effects that salinity has such as reduced water usage of plants.
Regions affected
From the FAO/UNESCO Soil Map of the World the following salinised areas can be derived.[R. Brinkman, 1980. Saline and sodic soils. In: Land reclamation and water management, pp. 62–68. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.]
See also
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References
External links
Article on water and salt balances in the soil
Salt of the Earth
Documentary produced by Prairie Public Television
Prairie Public Television is a state network of public television stations operated primarily by Prairie Public Broadcasting. It comprises all of the PBS member stations in the U.S. state of North Dakota.
The state network is available via flags ...
{{DEFAULTSORT:Soil Salinity
Soil science
Salts
Environmental soil science
Energy conversion
Water and the environment