Soil salinity#Sodic soils

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.{{citation needed|date=June 2020}}

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.{{Cite web|title=4. SODIC SOILS AND THEIR MANAGEMENT|url=https://www.fao.org/3/x5871e/x5871e05.htm|access-date=2023-02-08|website=www.fao.org}} 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.{{Cite book|last=Chesworth|first=Ward|url=https://books.google.com/books?id=EOYYM0-DAGQC&dq=boron+sodium+soil&pg=PA702|title=Encyclopedia of Soil Science|date=2007-11-22|publisher=Springer Science & Business Media|isbn=978-1-4020-3994-2|language=en}}{{fv|date=July 2023}}

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.{{Cite book|last1=Gupta|first1=S. K.|url=https://books.google.com/books?id=ANCBDwAAQBAJ&pg=PA2|title=Genesis and Management of Sodic (Alkali) Soils|last2=Gupta|first2=I. C.|date=2017-10-01|publisher=Scientific Publishers|isbn=978-93-87869-64-6|language=en}}

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.

Image:Salinity from irrigation.png

{{Main article|Environmental impacts of irrigation}}

Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts.{{Citation | title = Effectiveness and Social/Environmental Impacts of Irrigation Projects: a Review | series = Annual Report 1988 of the International Institute for Land Reclamation and Improvement | year = 1989 | pages = 18–34 | location = Wageningen, The Netherlands | url = http://www.waterlog.info/pdf/irreff.pdf |publisher=waterlog.info |url-status=live |archive-url=https://web.archive.org/web/20240216213433/https://www.waterlog.info/pdf/irreff.pdf |archive-date= Feb 16, 2024 }} 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).

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,{{Cite web |title=Saltier waterways are creating dangerous 'chemical cocktails' |url=https://phys.org/news/2018-12-saltier-waterways-dangerous-chemical-cocktails.html |date= December 3, 2018 |website=phys.org |language=en |url-status=live |archive-url=https://web.archive.org/web/20230709172409/https://phys.org/news/2018-12-saltier-waterways-dangerous-chemical-cocktails.html |archive-date= Jul 9, 2023 }} 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.{{Citation | title = Drainage Manual: A Guide to Integrating Plant, Soil, and Water Relationships for Drainage of Irrigated Lands | year = 1993 | publisher = Interior Dept., Bureau of Reclamation | isbn = 978-0-16-061623-5}}{{cite web |url=http://www.waterlog.info |title=Free articles and software on drainage of waterlogged land and soil salinity control |access-date=2010-07-28 }} 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)

{{main|Salt tolerance of crops}}

{{see|Maas–Hoffman model|Van Genuchten–Gupta model}}

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 WyomingAlan D. Blaylock, 1994, Soil Salinity and Salt tolerance of Horticultural and Landscape Plants. [http://ces.uwyo.edu/pubs/Wy988.pdf University of Wyoming] {{webarchive|url=https://web.archive.org/web/20100508143456/http://ces.uwyo.edu/PUBS/Wy988.pdf |date=2010-05-08 }} and the Government of AlbertaGovernment of Alberta, [http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex3303 Salt tolerance of Plants] {{Webarchive|url=https://web.archive.org/web/20100221211018/http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex3303 |date=2010-02-21 }} 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,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: [https://www.waterlog.info/pdf/segmregr.pdf] 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 [https://www.waterlog.info/pdf/pakistan.pdf] Some examples are shown in the following gallery, with crops arranged from sensitive to very tolerant.{{Cite web|title=Crop tolerance for soil salinity in farmers' fields|url=https://www.waterlog.info/croptol.htm|access-date=2023-02-08|website=www.waterlog.info}}Crop Tolerance to Soil Salinity, Statistical Analysis of Data Measured in Farm Lands. In: International Journal of Agricultural Science, October 2018. On line: [https://www.iaras.org/iaras/filedownloads/ijas/2018/014-0008(2018).pdf]

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.{{Cite journal|last1=Kaya|first1=C|last2=Kirnak|first2=H|last3=Higgs|first3=D|last4=Saltali|first4=K|date=2002-02-28|title=Supplementary calcium enhances plant growth and fruit yield in strawberry cultivars grown at high (NaCl) salinity|journal=Scientia Horticulturae|volume=93|issue=1|pages=65–74|doi=10.1016/S0304-4238(01)00313-2}}

Soil salinity activates genes associated with stress conditions for plants.{{cite journal |first1= Kamile |last1= Ulukapi | first2= Ayse Gul |last2= Nasircilar | title = The role of exogenous glutamine on germination, plant development and transcriptional expression of some stress-related genes in onion under salt stres | url = https://sciendo.com/article/10.2478/fhort-2024-0002 | journal = Folia Horticulturae | volume = 36 | issue = 1 | pages = 1–17 | date = February 2024 | pmid = | doi = 10.2478/fhort-2024-0002 | publisher = Polish Society of Horticultural Science | s2cid = 19887643 | doi-access = free }} 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.

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.

{{Portal|Water}}

• {{annotated link|Alkali soil}}
• {{annotated link|Arabidopsis thaliana responses to salinity|Arabidopsis thaliana responses to salinity}}
• {{annotated link|Biosaline agriculture}}
• {{annotated link|Biosalinity}}
• {{annotated link|Crop tolerance to seawater}}
• {{annotated link|Desalination}}
• {{annotated link|Environmental impacts of deicing salt}}
• {{annotated link|Halophyte}}
• {{annotated link|Halotolerance}}
• {{annotated link|Salinity in Australia}}
• {{annotated link|Salt tolerance of crops}}
• {{annotated link|Sodium in biology}}
• {{annotated link|Water softening}}
• {{annotated link|U.S. Salinity Laboratory}}

{{reflist}}

• [https://www.waterlog.info/pdf/balances.pdf Article on water and salt balances in the soil]
• [https://www.waterlog.info/leachmod.htm Download leaching model for saline soils]
• [https://www.youtube.com/watch?v=SYDme3KgO6g Salt of the Earth] Documentary produced by Prairie Public Television

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