Soil salinity (the content of salts in soil) and salinization (an increase of salinity due to various reasons) is one of the most common forms of soil degradation and is considered to be a major cause of desertification. This problem has been intensified by climate change with rising temperatures and decrease of precipitation.
Soil salinity occurs when water-soluble salts accumulate in or near the upper horizons of the soil. These salts contain K+, Mg+, Ca+, Cl–, SO42-, CO32-, HCO3– and Na+ ions (mostly these salts are sodium chlorides and sodium sulphates). When soils accumulate the exchangeable sodium, this process is called sodification. If Mg+ and Ca2+ ions are accumulated in the soil, an alkalization process takes place and the soil becomes alkaline (with a pH above 8.5).
As a result of salinization, soil structure degrades and is more exposed to water and wind erosion, soil compaction, soil crust formation, soil desertification, soil fertility decreases, agronomic productivity decrease, the soil buffering capacity against pollution decreases, plant nutrient uptake is impaired, soil biota biodiversity is reduced and also groundwater quality is affected.
Where does soil salinization take place?
Soil salinization is especially particularly common in arid areas. In the arid and semi-arid climate, soil salinity is observed in 50% of the irrigated fields. In the Aral Sea Basin, the Euphrates Valley and the Nile Valley the soil salinity problems are particularly acute as 70% of all agricultural land of the Nile Valley, what corresponds to 1.2 million ha, is saline. It is estimated that the total amount of soil affected by salinization in the world is around one billion ha. In Europe, saline soils are found in the Caspian region, Ukraine, Romania, Hungary, the Mediterranean region, Spain. In the European Union, soil salinization affects around 1 million hectares of land.
Soil salinization has also been a historical problem, it is known that it was also a problem in Babylon. Salinity is most common in areas with low rainfall and high evaporation, in soils where salt leaching is prevented, in areas where inappropriate irrigation measures are performed and excessive use of water resources are carried out.
In Latvia, soil salinity occurs when salts are used in winter to spread roadways for snow and ice defrosting.
What affects salinization?
Salinization can be divided into primary and secondary salination.
Primary salinization is associated with the accumulation of salts caused by natural processes. Factors that affect it are climate and weather conditions, soil bedrock, ground cover, vegetation type, relief, geological processes, hydrological conditions – surface and groundwater and their flow, wind. For example, as the groundwater level rises in soils with saline rocks, minerals and bedrock, the dissolved salts move together with the groundwater – Na+, Cl–, Mg2+, Ca2+ ions rise through the soil capillaries – and accumulate in the soil and its upper horizons as water evaporate. In this process also a solid salt crust on the top of the soil can be formed. Salts can be transported from the sea coast by the wind to places and soil with less salt concentration. Also, seawater level changes can lead to salinization (in Latvia this occurs in the vicinity of Kuiviži). As a result of climate change with rising temperatures and decrease of precipitation, salts may accumulate in the soil.
Secondary salinization is associated with anthropogenic factors, such as inappropriate irrigation measures – using water with a high salt content or not providing appropriate drainage; discharge of highly saline wastewater into the soil; use of mineral fertilizers; excessive use of water resources, leading to depressive funnels into which seawater can flow in.
How to solve it?
As a simple solution, salted soils are desalinated by rinsing, which causes leaching, but in heavy soils, it is difficult to reduce saline pollution. In regions with high rainfall, salinity may decrease, for example, soils that have become saline as a result of a tsunami may recover fairly quickly when there is intensive rainfall. It is important to find solutions not only for the successful management of saline soils but also to prevent soil salinization.