The Reference Soil Group of the Solonchaks includes soils that have a high concentration of `soluble salts' at some time in the year. Solonchaks are largely confined to the arid and semi-arid climatic zones and to coastal regions in all climates. Common international names are `saline soils' and `salt-affected soils'.

The total extent of Solonchaks in the world is estimated to be between 260 million (Dudal, 1990) and 340 million hectares (Szabolcs, 1989), depending on the level of salinity that is taken as diagnostic. Solonchaks are most extensive in the Northern Hemisphere, notably in arid and semi-arid parts of northern Africa, the Middle East, the former USSR and central Asia; they are also widespread in Australia and the Americas.

The most extensive occurrences of Solonchaks are in inland areas where evapotranspiration is considerably greater than precipitation, at least during a greater part of the year. Salts dissolved in the soil moisture remain behind after evaporation/transpiration of the water and accumulate at the surface of the soil (`external Solonchaks') or at some depth (`internal Solonchaks'). The Reference Soil Group of the Solonchaks is heterogeneous by nature. Solonchaks may differ in

the content and depth of salts in the soil;

the composition of accumulated salts or

the mineralogy of salt efflorescences.

The horizon differentiation of Solonchaks is normally determined by other factors than their high salt content. Many saline soils in waterlogged backswamps are Gleyic Solonchaks; without their salic horizon they would have been Gleysols. Likewise, Mollic Solonchaks may have the appearance of a Chernozem, Kastanozem or Phaeozem, and Calcic and Gypsic Solonchaks are basically strongly saline Calcisols and Gypsisols. Saline Histosols, Vertisols and Fluvisols occur as well; they are not classified as Solonchaks because Histosols, Vertisols and Fluvisols key out before Solonchaks.