Description

Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization.[1] Salt is a natural element of soils and water. Salination can be caused by natural processes such as mineral weathering or the gradual withdrawal of an ocean. It can also be caused by artificial processes such as irrigation.

Causes / history

The excess accumulation of salts, typically most pronounced at the soil surface, can result in salt-affected soils. Salts may rise to the soil surface by capillary transport from a salt-laden water table and then accumulate due to evaporation. They can also become concentrated in soils due to human activity, for example the use of potassium as fertilizer, which can form sylvite, a naturally occurring salt. As soil salinity increases, salt effects can result in degradation of soils and vegetation.

Salinization as a process can result from:-

high levels of salt in water.landscape features that allow salts to become mobile (movement of water table).climatic trends that favor accumulation.human activities such as land clearing.

Several factors affect the amount and composition of salts in soils:

Irrigation water quality - The total amount of dissolved salts in the irrigation water, and their composition, influence the soil salinity. Therefore, various parameters, such as source water EC and its minerals content should be tested.

Fertilizers applied - The type and amount of fertilizers applied to soil, affect its salinity. Some fertilizers contain high levels of potentially harmful salts, such as potassium chloride or ammonium sulphate. Overuse and misuse of fertilizers leads to salinity buildup, and should be avoided.

Irrigation regimen and type of irrigation system - The higher the water quantity applied, the closer soil salinity is to irrigation water salts concentration. When the soil dries, the concentration of salts in the soil solution is increased.

Since salts move with the wetting front, the salts accumulate in specific profiles according to the irrigation regimen and the type of irrigation used. For instance, when irrigating using sprinklers, water and salts move deeper, according to the soil's infiltration capacity and the water quantity, until they stop at a certain depth. When using drip irrigation - there is also a lateral movement of water and salts.

Field's characteristics and agricultural history - A poorly drained soil might reach salinity level that is harmful to the plants and to the whole crop. A soil that was not flushed after a previous growing cycle might contain high level of accumulated salts.

Impacts

Impacts of salinity

As a result of rising water tables in irrigated and non-irrigated areas or the use of saline water supplies—salinity can have significant impacts on the following aspects.

Agricultural production

Water moves into plant roots by a process known as osmosis, which is controlled by the level of salts in the soil water and in the water contained in the plant. If the level of salts in the soil water is too high, water may flow from the plant roots back into the soil. This results in dehydration of the plant, causing yield decline or even death of the plant. Crop yield losses may occur even though the effects of salinity may not be obvious. The salt tolerance of a specific crop depends on its ability to extract water from salinised soils. Salinity affects production in crops, pastures and trees by interfering with nitrogen uptake, reducing growth and stopping plant reproduction. Some ions (particularly chloride) are toxic to plants and as the concentration of these ions increases, the plant is poisoned and dies.

Water quality

The most significant off-site impact of dryland salinity is the salinisation of previously fresh rivers. This affects the quality of water for drinking and irrigation—with serious economic, social and environmental consequences for both rural and urban communities. High levels of salts may affect the taste of drinking water. Chloride in particular has a low taste threshold. Sodium and magnesium sulfate levels in drinking water may produce a laxative effect and reduce the suitability of a water supply for grazing animals.

Ecological health of streams

Salt interacts with in-stream biota (animals and plants), changing the ecological health of streams and estuaries. The greatest threat to biodiversity is from the loss of habitat—both on land and in water. Riparian zones are particularly at risk as they occupy the lowest parts of the landscape where much of the saline groundwater is released to the surface. Salts also help fine materials (such as suspended clay particles) to flocculate, allowing more sunlight to penetrate rivers. This may lead to more harmful algal blooms if there are suitable environmental conditions.

Terrestrial biodiversity

Much of the natural vegetation of salt-affected areas has been destroyed or damaged. This has caused major changes to the landscape and biodiversity including the destruction of remaining natural habitat in many agricultural areas and the fragmentation of many wildlife corridors.

Soil erosion

Dryland salinity is closely linked to other soil degradation issues, including soil erosion. Salinity is often associated with prolonged wetness and lack of surface cover and therefore increases the vulnerability of soils to erosion.

Flood risk

Shallow water tables can increase the risk of flooding. Soils in this situation have limited capacity to absorb rainfall, resulting in high rates of run-off. This can result in damage to roads, fences, dams, agricultural land and wetlands.

Infrastructure and fixtures

Impacts include large decreases in the lifespan of road pavements when groundwater levels rise to within 2 metres of the pavement surface. As in other situations, capillary action will assist to draw the salt-laden water to the surface. Salt also corrodes and destroys the properties of bitumen, concrete and brick structures. Damage to infrastructure including houses, roads and playing fields, has been particularly high in a number of cities and towns. Salinity damage has also occurred to country roads and farm tracks and buildings.

Irrigation

All irrigation water contains some salts, which may remain on the soil surface or on leaves of plants after evaporation. Therefore, any irrigation system has the potential to deliver an increased amount of salt to the soil.

The risks

The problems from this are greatest in drier environments, where rates of evaporation are usually very high. Salts are less likely to be leached from the soil in low rainfall areas and therefore poor quality irrigation water with high levels of salts will have a greater impact on the soil. Excessive amounts of water applied by irrigation may move past the root zone and contribute to rising water tables. Leakage from irrigation channels may also contribute to rising water tables. Sodic irrigation water contains a high level of sodium salts compared to calcium and magnesium salts. It may result in soil dispersion, with consequent soil surface sealing, crusting, erosion, poor water entry and poor seedbeds.

How to avoid

The use of efficient water application and monitoring systems can minimise many of the problems associated with irrigation. Additional costs may include surface levelling, lining drainage channels, subsoil drainage schemes, pumping to lower water tables and mixing saline water with water of better quality.

Social

Salinity can also affect people directly in a number of ways including:

cost to rural communities of declining populationloss of business (both existing and potential)cost of rural restructure when farms become unprofitableincreased health problems due to stress on families affected by change.

Salinity may also indirectly affect people by reducing the quality of the natural environment (for example, where the numbers and variety of wildlife decrease in salinised natural wetlands).