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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). 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.
Over long periods of time, as soil minerals weathering 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 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.
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 Rhizosphere at levels that may be toxic for plants. The most common compound used for Land reclamation of sodic soil is gypsum, and some plants that are tolerant to salt and ion toxicity 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, 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.
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).
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 leaching model 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.Salt-Affected Soils and their Management, FAO Soils Bulletin 39 (http://www.fao.org/docrep/x5871e/x5871e00.htm)
Field data in irrigated lands, under farmers' conditions, are scarce, especially in developing countries. However, some on-farm surveys have been made in Egypt,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. India,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: [3] and Pakistan.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 [4] Some examples are shown in the following gallery, with crops arranged from sensitive to very tolerant. Crop Tolerance to Soil Salinity, Statistical Analysis of Data Measured in Farm Lands. In: International Journal of Agricultural Science, October 2018. On line: [5]
(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.
Soil salinity activates associated with stress conditions for plants. These genes initiate the production of plant stress 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.
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