Managing Saline Soil

Technical articles

Salinization Water management Saline soil Sodic soil Irrigation

Salinized plot

Soil salinization is defined by the FAO as the "phenomenon of increasing water-soluble salts in the soil." These can include sodium, potassium, magnesium, calcium, chlorine.

A soil is mainly considered salinized when the salt concentration is such that it has harmful effects on soil function and plants. This notably depends on the plant species present since some are more salt-tolerant than others.

In highly salinized soils, crop yields crops are reduced and sometimes agriculture becomes impossible.

Salinization is a very significant threat to agricultural soils worldwide, especially under arid and semi-arid climates. France is currently little affected, but the phenomenon is expected to spread as it is closely linked to the management of irrigation, drought, and sea level rise. This phenomenon is already widely observed in Spain.

Classification of salinized soils

Saline soil

• High concentration of soluble salts in the soil.

• Sometimes crystallize on the surface, forming a white crust on the soil.

• Persistent moisture several days after rain.

• Poor plant development : poor germination, burnt leaves,...

Sodic soil

• Sodium predominant among the salts.

• As a result, the soil is alkaline (pH between 8 and 10).

• Disintegration of clays and organic matter, sometimes creating a brown surface layer and making soils asphyxiating.

• Persistent moisture several days after rain.

• Sodium toxicity.

Saline-sodic soil

• High concentration of soluble salts where sodium is dominant.
• pH generally below 8.5.

These soils risk becoming sodic.

Impacts on soils

Normally, in healthy soil, clays are flocculated. That is, negatively charged clays cluster around positively charged ions like calcium. This allows aggregate formation and good soil structure.

Conversely, sodium promotes the dispersion of clay particles. This makes sodic soils unstable, prone to crusting, and compact. Such soils are difficult to work and unfavorable for seed germination and root growth.

Moreover, regardless of the dominant salts, the soil's osmotic balance is altered so water and nutrients are less available to plants. Additionally, some salts are toxic to plants, notably sodium.

Causes of soil salinization

Position of the salt wedge (boundary between fresh and salt water) in the case of low withdrawals.

Position of the salt wedge (boundary between fresh and salt water) in the case of high withdrawals.

Natural

• Marine spray and aerosols, even marine incursions onto land and into aquifers (e.g., in the Camargue delta).

• Dissolution of salty rocks and sediments by water or passage of brine through fresh water, then infiltration of salt-laden water into groundwater.

Anthropogenic

• Irrigation with poor quality water.

• Irrigation in insufficient volumes not allowing salt leaching.

• Disruption of aquifer balance and upconing of saline water into aquifers (poor groundwater management, drought, deforestation,...). This water can come from the ocean (see diagram) or deep groundwater.
• Industrial pollution ("wastewater, mining drainage, road drainage subjected to winter salting, potash mining, desalination" ).

Main keys to combat salinization

Prevention and remediation of soil salinization strongly depend on groundwater management (extraction volumes, water quality, etc.) and factory waste regulations among others. Thus, it does not rely solely on farmers' choices. However, they can implement certain best practices themselves.

Remediation methods are often costly, so it is advisable to prevent salinization if located in a sensitive area.

Note, in Overseas Islands and Corsica, salinization is mainly a marine phenomenon. But in mainland France, this is not the case: several inland groundwater tables are saline and thus can degrade soils.

Prevention includes :

• Monitoring irrigation water quality.
• Improving water use efficiency, especially if water quality is poor, to minimize salt input on plots.

• Providing over the long term more fresh water (irrigation and rain) than is evapotranspired and draining it, to leach the plots.

Warning : This classification is indicative and it is not recommended to use it to assess water suitability for irrigation. Suitability strongly depends on "usage conditions, notably crop, climate, soil, irrigation method, and management practices".

	Electrical conductivity dS/m	Salt concentration mg/L	Water type
Non-saline water	<0.7	<500	Drinking and irrigation water
Slightly saline water	0.7-2	500-1500	Irrigation water
Moderately saline water	2-10	1500-7000	First drainage water and groundwater
Highly saline water	10-25	7000-15,000	Second drainage water and groundwater
Very highly saline water	25-45	15,000-35,000	Very saline groundwater
Brine	>45	>45,000
Classification of saline waters. Source : FAO

If soils are already salinized, it is possible to grow salt-tolerant plant species. Some soil management methods also improve growing conditions.

• Leveling : uniform water application, thus better leaching. Uniform salinity control.
• Soil tillage : seedbed preparation, increased permeability.
• Deep plowing : in case of a deep impermeable layer, improving its physical state and infiltration capacity.
• Sowing on sloped beds or raised furrows : limits salt accumulation around seeds.

Field flooded with fresh water to rinse salt and make it cultivable.

It is also possible to remediate salinization :

• Leaching plots with fresh water and draining to remove salts from the root zone.

• Improving soil structure and water infiltration by adding organic matter.

• Restoring soil fertility with organic and chemical amendments.

• Reducing evaporation in favor of infiltration (mulching, drip irrigation).

• Deep drainage to maintain a superficial saline water table outside the root zone.

• Applying chemical amendments in sodic soils (gypsum CaSO42H2O) or sulfuric acid). These amendments contain calcium or release it from the soil, and calcium ions replace sodium ions bound to clays, allowing them to flocculate again and restore soil structure.

Note on drip irrigation: In the short term, drip irrigation is a good solution when irrigation water is salt-laden, as water input is reduced, thus salt input too. However, in the long term, this method seems to cause salt accumulation in the root zone because it does not allow soil leaching.

Annexes

This video made by the FAO (Food and Agriculture Organization of the United Nations) summarizes this article well :

References and sources

• FAO 2017. Voluntary Guidelines for Sustainable Soil Management Food and Agriculture Organization of the United Nations Rome, Italy Online: https://www.fao.org/3/bl813e/bl813e.pdf
• FAO, Salt-affected soils, Soil information portal, accessed online 24/08/2022 https://www.fao.org/soils-portal/soil-management/gestion-des-sols-a-problemes/gestion-des-sols-affectes-par-le-sel/more-information-on-salt-affected-soils/fr/
• AgroLeague, What are the benefits of gypsum and how to use it? May 9, 2022 https://www.agro-league.com/blog/quels-sont-les-interets-du-gypse-et-comment-les-utiliser
• Saadoune, Fatma Zohra (2016). Effects of gypsum and DS amendment on the physico-chemical and water characteristics of a saline soil of EL Hmadena (RELIZANE). http://e-biblio.univ-mosta.dz/bitstream/handle/123456789/7524/Document%20final.pdf?sequence=1&isAllowed=y
• Encyclopedia of the Environment - How do plants tolerate saline conditions? Soil salinization Potassium and Sodium: false twins! Biotechnology and salt tolerance of cultivated plants
• Wikipedia - Salinity management https://fr.wikipedia.org/wiki/Ma%C3%AEtrise_de_la_salinit%C3%A9