Bui, E. N., L. Krogh. R. S. Lavado, F.O. Nachtergaele, T. Tóth and R. W. 
Fitzpatrick. 1998. Distribution of sodic soils: the world scene. pp19-33. in 
Sumner M.E. and R. Naidu (eds.) Sodic soils. Oxford University Press. New York.

The preceding discussion suggests that sodic soils can occur over a wide range 
of climates, but generally under seasonally constrasted conditions. Among the 
environmental conditions that promote the genesis of sodic soils are the 
presence of shallow saline groundwater, textural discontinuities during the 
deposition of eolian, glacial, alluvial, or colluvial sediments, the occurrence 
of perched water tables within 1 m of the surface, low slope gradients, and 
endorheic or impeded drainage. Salts dissolved in groundwater as well as the 
weathering of  Na-containing minerals, such as albite, in granitic or 
sedimentary deposits are the sources of Na. The classical theory of Solonetz 
formation is clearly limited in its applicability on a worldwide basis. More 
generally, it appears that restricted water movement and soil moisture dynamics 
that fluctuate between aquic and ustic or aridic, within 1 m of the surface, in 
a landscape where a source of  Na exists, promote the formation of natric 
horizons or sodic properties. Soil can become sodic as a result of anthropogenic 
processes that change their water balance, such as irrigation and drainage of 
saline soils, and land clearing. Many soils that would not fit classical sodic 
soil specifications exhibit sodic behavior and cover a large but undetermined 
area of the world. Because of their agricultural importance, more attention 
needs to be devoted to these soils in the future.