Salinity, or salinization, is the build up of salt in soil and water. It reduces crop yields in about 50% of the world’s irrigated land areas.
Salinity is caused by several factors – mostly to do with human activities such as agriculture and cattle and sheep farming. Irrigation is the main offender, the clearing of land is another.
Salinity through irrigation:
Irrigation is the practice of pumping large amounts of water, usually either from ground water supplies or rivers, onto agricultural land. This water is almost always slightly saline. The water is absorbed into the soil, or evaporates, and either way it deposits the salt it carries in the soil. There are methods by which the salt deposits can be lessened, but they are often fairly ineffectual.
Salinity from rising water table:
When land is cleared in order to grow crops or graze domestic animals, the removal of the trees allows the water table to rise. The water table can rise to near, or even above the soil surface. The ground water is usually saline, having absorbed soluble mineral salts from the rocks it moves through. As it rises, it becomes more saline, and brings that salt to the surface. This salt can remain in the area, or move to the base of a catchment area. The latter results in extreme salinity in a few areas.
The effects of salinity:
Plants and animals have varying levels of tolerance for salinity. Even a low level of salinization can reduce the agricultural yield in an area, or render it unsuitable for some crops or animals. Higher levels of salinization can render a region barren, killing all the vegetation, and turn it into a slick mud wasteland, or a desert covered in salt crystals. Salinity can spread – the salt will move with runoff or through the water table and affect other areas. Once an area has reached a critical level of salinization, it is extremely susceptible to erosion, and very difficult to reclaim.
Fixing the problem:
Once an area is severely salinated, there are several steps that can be taken in order to reclaim the land. Unfortunately, these steps are often not cost effective. Domestic grazing animals must be removed from the area, to reduce the impact on the affected soil. Often ceasing any irrigation can be helpful. The area should then be planted with salt tolerant grass species – if the middle of the salt affected area cannot support these species, planting the edges is recommended. The grasses slightly lower the area’s salinity, meaning that larger plant species can be established. It is possible to use this method of succession to re-establish tree species, which will then act to lower the water table and reduce the amount of salt being added to the area. There are also various drainage and leaching methods.
Since salinity in soils and water is very difficult to fix, preventative strategies need to be implemented. Agriculture the world over is dependent on irrigation – which does indeed allow crops to be grown in areas where naturally they could not exist. However, the huge amount of irrigation needed to support these crops causes severe salinity. In order to reduce this, agriculture needs to return to more natural methods – growing crops in areas that are at least slightly suited to them. For example – the farming of rice in Victoria, Australia, requires huge amounts of irrigation. It seems counter-intuitive to grow a water-based plant in a very dry environment. There are also different methods of irrigation – for example, ensuring that irrigation is only carried out at night, not in the heat of the day when the water evaporates quickly. This seems logical, so it is surprising how many places do not follow this plan.
Salinization of land reduces crop yield, and causes economic loss due to rendering land unusable. It is difficult and expensive to reverse, and occurs mainly due to unsustainable agricultural practices. Agricultural methods need to be reviewed in order to slow the gradual salinization of much of the Earth’s agricultural land.
Acknowledgements: notes from university lectures, and