ABSTRACT
Salinity and sodicity are considered separately from other chemical properties because of their common occurrence in arid regions and the special problems they cause in soil and water management. Soils with high sodium levels are here referred to as 'sodic' soils, rather than the older term of 'alkali' soils, to avoid ambiguity; sodic soils may or may not be strongly alkaline (ie have a high pH). Saline soils occur where the supply of salts, for example from rock weathering, capillary rise, rainfall or flooding, exceeds their removal, for example by leaching or flooding. Thus they tend to coincide with areas where evapotranspiration exceeds precipitation and where there is no lengthy rainy season. Irrigation is required for crop growth in these areas although it may itself induce salinisation unless salts are leached regularly and water-tables are kept low by adequate drainage. Failure to do this has contributed to the downfall of many irrigation schemes.
