We use the term 'salt-affected soil’ to refer to soils in which salts interfere with normal plant growth.
Salt-affected soils can be divided into saline, saline-sodic and sodic, depending in salt amounts, type of salts, amount of sodium present and soil alkalinity. Each type of salt-affected soil will have different characteristics, which will also determine the way they can be managed. [Technical definitions and classifications of salt-affected soils]
Extent of salt-affected soils
Several authors have attempted to estimate the extent of salt-affected soils in the last 30 years, for example:
Using the FAO/UNESCO soil map of the world (1970-1980), FAO estimated that globally the total area of saline soils was 397 million ha and that of sodic soils 434 million ha. Of the then 230 million ha of irrigated land, 45 million ha (19.5 percent) were salt-affected soils; and of the almost 1 500 million ha of dryland agriculture, 32 million (2.1 percent) were salt-affected soils as indicated in the below table. Figures included arable and non arable soils.
Asia and the pacific and australia
- Oldeman, Hakkeling and Sombroek estimated in 1991 that the total area affected by waterlogging was over 10 million ha and that affected by salinity was over 76 million ha. They did not distinguish between irrigated and rainfed areas.
- Dregne et al estimated in 1991 that about 43 million ha of irrigated land in drylands were affected by various processes of degradation, mainly waterlogging, salinization and sodication.
- Umali estimated in 1993 that by that time 1-1.5 million ha were lost to salinization every year.
- Ghassemi et al estimated in 1995 that salinization of irrigated lands
caused losses of annual income of about US$ 12 billion globally
- Nelson and Mareida estimated in 2001 that about 12 million ha of irrigated land may have gone out of production as a result of salinization.
- Data from FAO´s database Aquastat show that in some countries the area affected by salinity can be as high of 50% of the areas fully equipped for irrigation
Some of these estimations are the result of expert judgment or the aggregation of statistics which have been collected by different methods, therefore they are difficult to compare. There is still the need for data on the rate of change in areas afected by salinization, sodication and waterlogging at regional and global level.
Impacts of salinization, sodication and waterlogging
Salinization, sodication and waterlogging have negative impacts on agricultural production, the living conditions of farmers, the economy at different levels, the balance of ecosystems including the quality of natural resources. The impacts are often felt not only at the farm, but also at lanscape levels. In many countries they can pose serious problems to sustainable development. Examples of effects of salinization, sodication and waterlogging include:
Impacts on crop production
- Decline in soil productivity and crop yields
- Increased requirement and use of inputs including seeds, water and fertilizers
- Low crop yield per unit of input used
- Less choice in cropping systems, as farmers are forced to cultivate salt-tolerant crops which might not always be high income cash crops
- Reduced water use efficiency
- In cases of severe salinization and sodication land cannot be used anymore for production
Impacts on living conditions of farmers and the economy
- The reduction of yields results in less income and less food supply, especially in subsistence farming
- Working with salt-affected soils requires more labour to reclaim soils
- The use of more inputs and the reduction of yields result in less returns
- Lowered income and loss of land are often factors for the migration of farmers to cities
- Soil reclamation programmes are costly
- Rehabilitation programmes to improve the living conditions of those affected require high investments
Impacts on ecosystems
The impacts of salinization, sodication and waterlogging in ecosystems are still not fully understood but they seem to affect both terrestial aquatic ecosystems. From the available research and modelling it is believed that these processes may contribute to:
- Reducing the diversity of organisms
- Reducing the efficiency of nutrient cycling
- Reducing population sizes of previously dominant species
- Increasing the populations of salt tolerant organisms
- Changing disease patterns and prevalence in different species of plants, terrestrial and aquatic animals
- Increasing vector-borne diseases
- Salt-affected areas result in sparse vegetation that in turn leads to wind-blown dust storms. Health effects associated with dust may be a problem in salt-affected areas
Impacts on the quality of natural resources
- Salt-affected soils are fragil and more prone to other forms of degradation, e.g. wind and water erosion
- Wind-born salts can reach and damage vegetation, soils and water in nearby areas
- Water draining out of agricultural fields can increase the salt contents in groundwater and other surface water courses
- Wastewater from reclamation of salt-affected soils, if not disposed off safely, can contaminate other soils and water bodies
- In the case of sodic soils, the loss of organic matter weakens the strength of soil aggregates, increase the loss of nutrients in run-off, and increase carbon dioxide emitted to the atmosphere.
- Extreme conditions in sodic soils (pH and sodium salts)decrease water infiltration due to surface sealing and promote runoff during storm events
Management and rehabilitation of salt-affected soils
Salt-affected soils may form under a wide range of conditions, therefore, there is no single approach to reclaim or manage them to ensure agricultural production.
As a priority, it is fundamental to avoid the development of salt affected soils by using sound agricultural practices that consider the local conditions including soil type (and parent material from which it is formed), terrain elevation, type of crops, soil nutrient status, water quality, irrigation practices and drainage.
Once salt-affected soils have formed either through natural or human-driven processes, they need to be tested in order to determine the type of salt-affected soil and the degree to which it is affected. The earliest salt-affected soils are treated, the higher the probabilities to reclaim them and continue their production. Severely affected soils are often too costly to reclaim or very difficult to manage, even under biosaline agriculture.
Managing salt-affected soils for agricultural purposes requires a combination of approaches and technologies and the consideration of socioeconomic aspects under local conditions. Agricultural production in salt-affected soils is largely dependent on water availability, climatic conditions, crops and the availability of resources (capital, inputs and time).
In many cases salt-affected soils can be managed to restore to other uses different to crop production, which may also contribute to the economy of rural areas; e.g grassland restoration, recreation areas, cultivation of medicinal plants, restoration of biodiversity, etc.
Use the menu in the right to see general descriptions of the technologies used to manage/reclaim salt-affected soils. More information on management technologies can be found in the country members'pages.
Leaching. To prevent the excessive accumulation of salt in the root zone, irrigation water (or rainfall) must, over the long term, be applied in excess of that needed for ET and must pass through the root zone in a minimum net amount. This amount in fractional terms is referred to as the leaching requirement. Leaching requirements should be minimized as far as possible in order to prevent raising the groundwater and minimize the total load to the drainage system. Leaching requirements of 10 and 20 percent can be used depending on the degree of existing salinity.
Drainage. When underlying layers are permeable and relief is adequate, natural drainage may function well. Since such conditions are rare in areas where saline and sodic soils occur, a drainage system will usually be required.
Various types of drainage are used all over the world: surface drainage in which ditches are provided so that excess water will runoff before it enters the soil; subsurface drainage for the control of the ground water table at a specified safe depth, consisting of open ditches or tile drains or perforated plastic pipes; mole drainage where shallow channels left by a bullet shaped device pulled through the soil can act as a supplementary drainage system connected to the main drainage system (open or closed); and vertical drainage by pumping out excess water from tubewells when the deep horizons have an adequate hydraulic conductivity. The depth and spacings of the drainage system should be based on soil type (soil permeability, the existence of hardpans, or impermeable layer, etc.) and the local economic consideration.
Physical / mechanical management
Several mechanical methods have been used to improve infiltration and permeability in the surface and root zone and thus to control saline and sodic conditions, including land levelling, deep ploughing and tillage, subsoiling and planting procedures.
Land levelling to achieve a more uniform application of water for better leaching and salinity control.
Tillage for seedbed preparation and soil permeability improvement.
Deep ploughing is most beneficial on stratified soils having an impermeable layer. It loosens the soil aggregates, improves the physical condition of this layer and increases air space and hydraulic conductivity.
Planting procedures. Special planting procedures that minimize salt accumulation around the seed such as planting on sloping beds or raised furrows in single or double rows are helpful in getting better stands under saline conditions.
These include using chemical amendments and mineral fertilizer. Using chemical amendments neutralizes sodic soil conditions (exchangeable Na and any Na carbonate), followed by leaching for removal of salts derived from the reaction of the amendments with sodic soils. Gypsum, sulphur and sulphuric acid are commonly used. Since the benefits expected from reclamation of salt-affected soils would not be obtained unless adequate plant nutrients are supplied as fertilizer, the proper types and balanced amounts of mineral fertilizers should be used.
The biological practices include using organic matter, farm manure, growing legumes, mulching, crop residue and selection of salt-tolerant crops: Organic manure incorporated in the soil. This has two principal beneficial effects on saline and sodic soils: improvement of soil permeability and release of carbon dioxide and certain organic acids during composition. It also acts as a source of nutrients. Mulching to reduce evaporation losses and thus decrease or prevent soil salinization. Crop residue application. This is one of the easiest methods to improve water infiltration, especially for small farmers who do not have the resources to implement more costly corrective measures. Salt-tolerant crops. Judicious selection of crops that can produce satisfactorily under moderately saline or sodic conditions has merit in some cases. Barley, wheat, sugar beet, millet, rice, salt-tolerant forage and grasses for animal production are examples.
Institutional frameworks and policies
There is a wide range of other causes of salt-affected soils and constraints to the adoption of technologies to control salinity development and improve productivity. The most important of these is the insecurity of tenure. There is a need for confirmed land tenure to ensure that the land users have a continuing interest in the productivity of the land.
Traditional land rights have come under increasing strain as population density has grown and land has had to be increasingly fragmented as family numbers have grown and as members of each family require land to support their families. Problems of lack of secure tenure have been accentuated wherever land alienation has occurred. Land has often been cleared for development of "commercial" farming.
The displaced farmers may be restricted to areas of less inherent productivity than the lands they formerly occupied, and to which their traditional farming techniques are unsuited. Other constraints include lack of capital for required inputs, inadequate infrastructure for movement of people, marketing of produce and purchase of inputs, inadequate understanding of the sources of the problems and lack of awareness of the methods by which they may be resolved. Non-involvement of farmers in the development and evaluation of technologies for restoration of salt-affected soils is the obvious reason. Farmers themselves are the best extension agents but they first have to understand how they will benefit from any changes.
Assessment and monitoring tools
Determining the extent to which habitats are affected by salts and how they change over time is complicated because:
- the extent of salinity or sodicity can vary with distance and soil depth (e.g. due to change in soil type, terrain and hydrology)
- processes that leads to their formation can develop at different rates
- management practices can change them significantly (e.g. soil management, irrigation practices, water quality, drainage)
- equipment and laboratory services are in general needed
Data can be collected at different scales, from single fields to basin areas. This requires different approaches and tools. Assesing and monitoring water quality in salt-affected habitats is also fundamental and requires the availability of monitoring wells or regular sampling from water courses/irrigation schemes. In some areas the presence of wild vegetation that thrives on saline soils (halophytes) or salt-resistant species can also be used as indicators of salinization.
In summary the different tools for assessing and monitoring the extent of salt affected soils include:
- Vegetation: the presence of halophytes (literally "salt loving") plants is an indication of salinization. These plants include.
- Soil surface: white crusts indicate salinity; brownish-black crusts and loss often indicate sodic soils; seasonal or long period accumulation of water on soil surfaces can also be indicators of salinization and sodication
- Visual indicators of salinity and sodicity should be backed up by measurements, to establish the type of salt-affected soil, the degree and possible measures to reclaim it.
Measuring salinity and sodicity of soil and water samples in the laboratory:
- It is more accurate and results in a better picture of the status salt-affected soils, but it is only cost-effective when analysing small areas or few samples, as the procedures are time consuming and relatively expensive. This is done by measuring the electrical conductivity and determining the sodium adsorption ratio of soil and water samples.
- Assessing and Monitoring the Risk of Salinization in a Sicilian vineyard by the Geonics EM-38 (Universita di Palermo, Italy/Arizona State University, USA)
- Use of Aboveground Electromagnetic Induction Meter for Assessing Salinity Changes in Natural Landscapes and Agricultural Fields (Benemérita Universidad Autónoma de Puebla (Mexico)/Universidad de Valencia)
- Primary Soil Salinity, Sodicity and Alkalinity Status of Different Water Management Areas in South Africa (ARC- Institute for Soil, Climate and Water, South Africa)
- Advances in Assessment of Salt Affected Soils for Mapping and Monitoring in India and Management Strategies (Central Soil Salinity Research Institute, India)
- Salt-affected Soils in Thailand: Assessment and Monitoring of Salinization (Land Development Department, Thailand)
- An Overview of Salinity Problem in Iran: Assessment and Monitoring Technology (National Salinity Research Center, Iran)
- Advances in Assessment and Monitoring of Soil Salinization for Management of Salt-affected Habitats in Egypt (Ministry of Agriculture and Land Reclamation, Egypt)