The soil environments in the arid and semi-arid regions of the world are quite different to those in the temperate and colder regions. In dry regions of the world, the problem is not one of leaching out of nutrients but of accumulation of too many soluble salts and too much sodium in relation to other elements. Two main chemical processes contribute to salinisation. One is salinisation itself which is the accumulation of large amounts of soluble salts in the soil, usually chlorides, carbonates and sulphates of sodium, magnesium and calcium. The other is sodicity, a term used for the accumulation of large amounts (more than 10 per cent) of sodium in the soil. Although seen by some as separate processes these two processes are considered here together as salinisation, a major degrading soil process in arid and semi-arid climates.
Salinisation is one of the most widespread soil degradation processes on the earth and has been a feature of soils for thousands of years. It is largely, though not solely, connected with areas in which the ratio of precipitation to evapo-transpiration is less than 0.75. This climatic regime with a very low rainfall and high evaporation means that there is little or no washing out of salts and they can accumulate in the upper parts of the soil coinciding with the root zone. In evaporative conditions the salts can also be deposited on the soil surface. Human induced salinisation has been affecting soil for centuries. It is considered by some to have contributed to the fall of some of the great Empires. In the last few decades there has been a marked increase in the areal extent of salinisation and many countries now suffer from the problem. It is now thought to affect more than one billion hectares of land. It occurs mainly in the arid and semi-arid regions of Asia, Australia, South America and the United States of America. In Europe it affects mainly the Iberian Peninsula (Spain and Portugal), the Ukraine and the Carpathian and Caspian regions
There are three main sources of salinisation. Some is natural and has developed over millions of years, and is associated with a combination of the climate and parent rocks from which and under which the saline soils have formed. Hydrological conditions and landscape position are also important and many of these soils have developed in inland evaporative basins. Low lying basins into which feed waters from the surrounding higher areas are particularly vulnerable to salinisation. Salinised soils also occur in coastal zones where they have formed under marine incursions. Events such as tsunamis transfer large amounts of saline water inland where in addition to the other catastrophes it causes for people, it will also damage the coastal soils almost irreparably. Human-induced salt accumulation is, however, the main reason for the recent increase in salinisation. Irrigation has become a widely used practice for providing water to crops in arid areas and many of these waters are laden with salts which thus enter the soil.