Soil salinity

Visibly salt-affected soils on rangeland in Colorado. Salts dissolved from the soil accumulate at the soil surface and are deposited on the ground and at the base of the fence post.

File:Incrustação salina 1-PET Solos UESC.jpg

Saline incrustation in a PVC irrigation pipe from Brazil

Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization (also called salination in American English).<ref>Template:Cite encyclopedia</ref> Salts occur naturally within soils and water. Salinization 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 and road salt.

Natural occurrenceEdit

Salts are a natural component in soils and water. The ions responsible for salinization are: Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cl⁻.

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. 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 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 salinization of arable land.Template:Citation needed

Sodic soilsEdit

When the Na⁺ (sodium) predominates, soils can become sodic. The pH of sodic soils may be acidic, neutral or alkaline.

Sodic soils present particular challenges because they tend to have very poor structure which limits or prevents water infiltration and drainage. They tend to accumulate certain elements like boron and molybdenum in the root zone at levels that may be toxic for plants.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The most common compound used for reclamation of sodic soil is gypsum, and some plants that are tolerant to salt and ion toxicity may present strategies for improvement.<ref>Template:Cite book</ref>Template:Fv

The term "sodic soil" is sometimes used imprecisely in scholarship. It's been used interchangeably with the term alkali soil, which is used in two meanings: 1) a soil with a pH greater than 8.2, 2) soil with an exchangeable sodium content above 15% of exchange capacity. The term "alkali soil" is often, but not always, used for soils that meet both of these characteristics.<ref>Template:Cite book</ref>

Dry land salinityEdit

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 irrigationEdit

File:Salinity from irrigation.png

Rain or irrigation, in the absence of leaching, can bring salts to the surface by capillary action.

Template:Main article Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts.<ref>Template:Citation</ref> 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. Salinization 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 (gardens and recreation areas).

Consequences of soil salinityEdit

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,<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> especially copper, cadmium, manganese and zinc.

Soil erosion ultimately, when crops are too strongly affected by the amounts of salts.
More energy required to desalinate

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.<ref>Template:Citation</ref><ref name="Waterlog">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A comprehensive treatment of soil salinity is available from the United Nations Food and Agriculture Organization.<ref>Salt-Affected Soils and their Management, FAO Soils Bulletin 39 (http://www.fao.org/docrep/x5871e/x5871e00.htm)</ref>

Salt tolerance of cropsEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:See 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<ref name="Alan">Alan D. Blaylock, 1994, Soil Salinity and Salt tolerance of Horticultural and Landscape Plants. University of Wyoming Template:Webarchive</ref> and the Government of Alberta<ref name="Albert">Government of Alberta, Salt tolerance of Plants Template:Webarchive</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,<ref>H.J. Nijland and S. El Guindy, Crop yields, watertable depth and soil salinity in the Nile Delta, Egypt. In: Annual report 1983. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands.</ref> India,<ref>D. P. Sharma, K. N. Singh and K. V. G. K. Rao (1990), Crop Production and soil salinity: evaluation of field data from India. Paper published in Proceedings of the Symposium on Land Drainage for Salinity Control in Arid and Semi-Arid Regions, February, 25th to March 2nd, 1990, Cairo, Egypt, Vol. 3, Session V, p. 373–383. On line: [1]</ref> and Pakistan.<ref>R.J. Oosterbaan, Crop yields, soil salinity and water table depth in Pakistan. In: Annual Report 1981, pp. 50–54. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands, reprinted in Indus 24 (1983) 2, pp. 29–33. On line [2]</ref> Some examples are shown in the following gallery, with crops arranged from sensitive to very tolerant.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Crop Tolerance to Soil Salinity, Statistical Analysis of Data Measured in Farm Lands. In: International Journal of Agricultural Science, October 2018. On line: [3]</ref>

Graphs of crop yield and soil salinity in farmers' fields ordered by increasing salt tolerance.
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.
Wheat sampla.png

Fig. 2. Wheat grown in Sampla, Haryana, India, is slightly sensitive, tolerating an ECe value of 4.9 dS/m.
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).
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.
Sorghum pakistan.png

Fig. 5. Sorghum from Khairpur, Pakistan, is quite tolerant; it grows well up to ECe = 10.5 dS/m.
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.<ref>Template:Cite journal</ref>

Soil salinity activates genes associated with stress conditions for plants.<ref name="FH">Template:Cite journal</ref> These genes initiate the production of plant stress enzymes such as superoxide dismutase, L-ascorbate oxidase, and Delta 1 DNA polymerase. Limiting this process can be achieved by administering exogenous glutamine to plants. The decrease in the level of expression of genes responsible for the synthesis of superoxide dismutase increases with the increase in glutamine concentration.<ref name="FH"/>

Regions affectedEdit

From the FAO/UNESCO Soil Map of the World the following salinised areas can be derived.<ref>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.</ref>

Region	Area (10⁶Template:Nbspha)
Africa	69.5
Near and Middle East	53.1
Asia and Far East	19.5
Latin America	59.4
Australia	84.7
North America	16.0
Europe	20.7

ReferencesEdit

Template:Reflist

External linksEdit

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

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