Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. [1] Salts occur naturally within soils and water. Salination 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.
Salts are a natural component in soils and water. The ions responsible for salination 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 salination of arable land.[ citation needed ]
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. [2] 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. [3] [ failed verification ]
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. [4]
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.
Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts. [5] 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. Salination 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
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. [7] [8] A comprehensive treatment of soil salinity is available from the United Nations Food and Agriculture Organization. [9]
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 Wyoming [10] and the Government of Alberta [11] 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, [12] India, [13] and Pakistan. [14] Some examples are shown in the following gallery, with crops arranged from sensitive to very tolerant. [15] [16]
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. [17]
Soil salinity activates genes associated with stress conditions for plants. [18] 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. [18]
From the FAO/UNESCO Soil Map of the World the following salinised areas can be derived. [19]
| Region | Area (106 ha) |
|---|---|
| Africa | 69.5 |
| Near and Middle East | 53.1 |
| Asia and Far East | 19.5 |
| Latin America | 59.4 |
| Australia | 84.7 |
| North America | 16.0 |
| Europe | 20.7 |
Biosalinity is the study and practice of using saline (salty) water for irrigating agricultural crops.
A halophyte is a salt-tolerant plant that grows in soil or waters of high salinity, coming into contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs, and seashores. The word derives from Ancient Greek ἅλας (halas) 'salt' and φυτόν (phyton) 'plant'. Halophytes have different anatomy, physiology and biochemistry than glycophytes. An example of a halophyte is the salt marsh grass Spartina alterniflora. Relatively few plant species are halophytes—perhaps only 2% of all plant species. Information about many of the earth's halophytes can be found in the halophyte database.
Irrigation is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been developed by many cultures around the world. Irrigation helps to grow crops, maintain landscapes, and revegetate disturbed soils in dry areas and during times of below-average rainfall. In addition to these uses, irrigation is also employed to protect crops from frost, suppress weed growth in grain fields, and prevent soil consolidation. It is also used to cool livestock, reduce dust, dispose of sewage, and support mining operations. Drainage, which involves the removal of surface and sub-surface water from a given location, is often studied in conjunction with irrigation.
In geotechnical engineering, watertable control is the practice of controlling the height of the water table by drainage. Its main applications are in agricultural land and in cities to manage the extensive underground infrastructure that includes the foundations of large buildings, underground transit systems, and extensive utilities.
Soil salinity control refers to controlling the process and progress of soil salinity to prevent soil degradation by salination and reclamation of already salty (saline) soils. Soil reclamation is also known as soil improvement, rehabilitation, remediation, recuperation, or amelioration.
Well drainage means drainage of agricultural lands by wells. Agricultural land is drained by pumped wells to improve the soils by controlling water table levels and soil salinity.
Alkali, or Alkaline, soils are clay soils with high pH, a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and which can induce basicity. Sometimes these soils are also referred to as alkaline sodic soils. Alkaline soils are basic, but not all basic soils are alkaline.
Drainage research is the study of agricultural drainage systems and their effects to arrive at optimal system design.
SaltMod is computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge (hydrology) in irrigated agricultural lands, using different (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules. The water management options include irrigation, drainage, and the use of subsurface drainage water from pipe drains, ditches or wells for irrigation.
In agriculture, leaching is the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planting, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss. Leaching may also refer to the practice of applying a small amount of excess irrigation where the water has a high salt content to avoid salts from building up in the soil. Where this is practiced, drainage must also usually be employed, to carry away the excess water.
Surface irrigation is where water is applied and distributed over the soil surface by gravity. It is by far the most common form of irrigation throughout the world and has been practiced in many areas virtually unchanged for thousands of years.
An agricultural drainage system is a system by which water is drained on or in the soil to enhance agricultural production of crops. It may involve any combination of stormwater control, erosion control, and watertable control.
Agricultural hydrology is the study of water balance components intervening in agricultural water management, especially in irrigation and drainage.
The environmental impact of irrigation relates to the changes in quantity and quality of soil and water as a result of irrigation and the subsequent effects on natural and social conditions in river basins and downstream of an irrigation scheme. The effects stem from the altered hydrological conditions caused by the installation and operation of the irrigation scheme.
A leaching model is a hydrological model by which the leaching with irrigation water of dissolved substances, notably salt, in the soil is described depending on the hydrological regime and the soil's properties.
This page shows statistical data on irrigation of agricultural lands worldwide.
Irrigation is the artificial abstraction of water from a source followed by the distribution of it at scheme level aiming at application at field level to enhance crop production when rainfall is scarce.
The Central Soil Salinity Research Institute (CSSRI) is an autonomous institute of higher learning, established under the umbrella of Indian Council of Agricultural Research (ICAR) by the Ministry of Agriculture, Government of India for advanced research in the field of soil sciences. The institute is located on Kachawa Road in Karnal, in the state of Haryana, 125 km (78 mi) from the Indian capital of New Delhi.
Salt tolerance of crops is the maximum salt level a crop tolerates without losing its productivity while it is affected negatively at higher levels. The salt level is often taken as the soil salinity or the salinity of the irrigation water.
Crop tolerance to seawater is the ability of an agricultural crop to withstand the high salinity induced by irrigation with seawater, or a mixture of fresh water and seawater. There are crops that can grow on seawater and demonstration farms have shown the feasibility. The government of the Netherlands reports a breakthrough in food security as specific varieties of potatoes, carrots, red onions, white cabbage and broccoli appear to thrive if they are irrigated with salt water.
Water use in alluvial fans refers to irrigation systems using the water resources in alluvial fans, mainly river floods and groundwater recharged by infiltration of rain or river water, to enhance the production of agricultural crops.
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