Salinity

Primary reference(s)

FAO, 1988. Saline soils and their management. In: Salt-affected Soils and their Management. Food and Agriculture Organization of the United Nations (FAO). Accessed 20 October 2020.

Richards, L.A. (ed.), 1954. Diagnosis and Improvements of Saline and Alkali soils. United States Department of Agriculture, Handbook 60. Accessed 20 October 2020.

University of California, 2020. Salinity measurement and unit conversion. Accessed 20 October 2020.

Additional scientific description

Salt-affected soils consist of saline and sodic soils, which occur in all continents and under almost all climatic conditions, but their distribution is relatively more extensive in the arid and semi-arid regions compared to the humid regions. Soil salinisation and sodification are major soil degradation processes threatening ecosystems and are recognised as being among the most important problems at a global level for agricultural production, food security and sustainability in arid and semi-arid regions. There are extensive areas of salt-affected soils on all the continents, but their extent and distribution have not been studied in detail (FAO, 2020a).

Salt-affected soils have serious impacts on soil functions leading to an array of consequences, including significant decreases in agricultural productivity, water quality, soil biodiversity, and soil erosion. Salt-affected soils have a decreased ability to act as a buffer and filter against pollutants. The degradation of soil structure and functions of global ecological systems such as hydrological, nutrient and biogeochemical cycles, impair the provision of ecosystem services, which are critical for supporting human life and biodiversity. Salt-affected soils reduce both the ability of crops to take up water and the availability of micronutrients. They also concentrate ions that are toxic to plants and may degrade the soil structure (FAO, 2020a).

Soluble salts most commonly present are the chlorides and sulphates of sodium, calcium and magnesium. Nitrates are present in appreciable quantities only rarely. Sodium and chloride are by far the most dominant ions, particularly in highly saline soils, although calcium and magnesium are usually present in sufficient quantities to meet the nutritional needs of crops. Many saline soils contain appreciable quantities of gypsum (CaSO4, 2H2O) in the profile (FAO, 1988).

Salt that accumulates in soil can come from a number of sources (NSW Department of Planning, Industry and Environment, 2019):

• Rainfall: airborne salts from ocean spray and pollution are dissolved in atmospheric moisture and deposited on the land in precipitation.
• Weathering: minerals that make up rocks break down and release ions that are able to form salts.
• Aeolian deposits: wind picks up and transports dust and salt from soil and lake surfaces and redistributes it across the landscape.
• Connate salt: during deposition, salt has been incorporated into marine sediments, or in areas of internal drainage, salt has accumulated over geological time due to transport and evaporative processes. These areas may later become sources of salt.

Salinity affects (NSW Department of Planning, Industry and Environment 2019):

• Farms: salinity can decrease plant growth and water quality resulting in lower crop yields and degraded stock water supplies. Excess salt affects overall soil health, reducing productivity. It kills plants, leaving bare soil that is prone to erosion.
• Wetlands: as salinity increases over time, wetlands become degraded, endangering wetland species and decreasing biodiversity. Where sulphate salts are present, there is an increased risk of acid sulphate soil formation.
• Rivers: increased volume (load) and/or concentration (electrical conductivity) of salinity in creeks and streams degrades urban water supplies, affects irrigated agriculture and horticulture, and adversely impacts on riverine ecosystems.
• Drinking water: when a source of drinking water becomes more saline, extensive and expensive treatment may be needed to keep salinity at levels suitable for human use.
• Buildings, roads and pipes: salinity damages infrastructure, shortening its life and increasing maintenance costs.
• Sports grounds: salty ground may lose all grass cover, making playing fields unusable.

Metrics and numeric limits

The salt concentration in water extracted from a saturated soil (called saturation extract) defines the salinity of this soil. If the salt content of this water is <3 g/l, the soil is said to be non-saline. If the salt concentration of the saturation extract is >12 g/l, the soil is said to be highly saline (FAO, 1985).

Soil salinity classes and crop growth (FAO, 1988):

Examples of drivers, outcomes and risk management

The amount of crop yield reduction depends on such factors as crop growth, the salt content of the soil, climatic conditions, etc. In extreme cases where the concentration of salts in the root zone is very high, crop growth may be entirely prevented. To improve crop growth in such soils the excess salts must be removed from the root zone. The term reclamation of saline soils refers to the methods used to remove soluble salts from the root zone. Methods commonly adopted or proposed to accomplish this include the following (FAO, 1988):

• Scraping: Removing the salts that have accumulated on the soil surface by mechanical means has had only a limited success.
• Flushing: Washing away the surface accumulated salts by flushing water over the surface is sometimes used to desalinise soils having surface salt crusts.
• Leaching: The most effective procedure for removing salts from the root zone of soils. Leaching is most often accomplished by ponding fresh water on the soil surface and allowing it to infiltrate.

The Food and Agriculture Organization of the United Nations (FAO) Global Soil Partnership and the Eurasian Soil Partnership (EASP), in collaboration with the Eurasian Center for Food Security (ECFS), announced a call in 2019 for short-term projects aimed at mitigation of or adaptation to soil salinity through the application of sustainable soil management practices. The objective of this initiative is to establish pilot projects in the field in Central Asian countries to test the viability and usefulness of different soil management practices for the adaptation or mitigation of soil salinity in a sustainable manner, with a duration not exceeding 12 months (FAO, 2019).

In order to achieve its mandate, the FAO Global Soil Partnership addressed five pillars of action to be implemented in collaboration with its regional soil partnerships (FAO, 2020b):

• Promote sustainable management of soil resources for soil protection, conservation and sustainable productivity.
• Encourage investment, technical cooperation, policy, education awareness around salinity in soil.
• Promote targeted soil research and development focusing on identified gaps and priorities and synergies with related productive, environmental and social development actions.
• Enhance the quantity and quality of soil data and information: data collection (generation), analysis, validation, reporting, monitoring and integration with other disciplines.
• Harmonise methods, measurements and indicators for the sustainable management and protection of soil resources.