World’s Soil Resources

availability and inhibit plant growth. Sodic soils generally have a low salt content but a high ESP, which causes
dispersion of clay particles and results in deterioration of the soil structure. These soils generally have low air
and water permeability and a pH above 8.2.
Salinity problems are encountered in all climates and are a consequence of both natural (primary) and
human-induced (secondary) processes. Soil salinity and sodicity problems are more common where rainfall
is insufficient to leach salts and excess sodium ions out of the rhizosphere. Salt-affected soils often occur
on irrigated lands, especially in arid and semiarid regions, where annual rainfall is insufficient to meet the
evapotranspiration needs of plants and to provide for leaching of salt. In humid areas, soluble salts are carried
down through the soil profile by percolating rainwater and ultimately are transported to sea.
Although salt-affected soils are widespread and an increasingly severe problem, no accurate recent
statistics are available on their global extent. The best available estimates suggest that about 412 million ha are
affected by salinity and 618 million ha by sodicity (UNEP, 1992), but this figure does not distinguish areas where
salinity and sodicity occur together. The Soil Map of the World (FAO/UNESCO, 1980) depicted a similar extent
Continent
Saline soils
(million ha)
Sodic soils
(million ha)
Total
(million ha)
Africa 122.9 86.7 209.6
South Asia 82.3 1.8 84.1
North and Central Asia 91.5 120.2 211.7
Southeast Asia 20.0 - 20.0
South America 69.5 59.8 129.3
North America 6.2 9.6 15.8
Mexico/Central America 2.0 - 2.0
Australasia 17.6 340.0 357.6
World total 412.0 618.0 1030
Table 6.6 Distribution of salt-affected soils in drylands different continents of the world. Source: UNEP, 1992.
of 953 Mha affected by salinity (352 million ha) and sodicity (580 million ha). Table 6.6 shows the distribution
of dryland salinity in different continents.
Human-induced salinity, mainly caused by irrigation without adequate drainage, affects a much smaller
area than natural salinity. According to GLASOD, the extent of human-induced salinity is about 76 million ha
(Oldeman, Hakkeling and Sombroek, 1991) of which 52.7 million ha occurs in Asia. In Europe, significant parts
of Spain and areas in Italy, Hungary, Greece, Portugal, France and Slovakia are also affected by human-induced
salinization.
In 2006 the global area equipped for irrigation stood at 301 million ha. At present in developing countries,
irrigated agriculture covers about one fifth of all arable land, but accounts for nearly half of all crop production
and 60 percent of cereal production. About 70 percent of the world area equipped for irrigation is in Asia where
it accounts for 39 percent of the cultivated area. India and China each have 62 million ha equipped for irrigation
(FAO, 2011). An estimated 60 million ha (or 20 percent of the total irrigated area) are affected by soil salinity,
of which 35 million ha are located in four countries e.g. Pakistan (3.2 million ha), India (20 million ha), China (7
million ha) and the United States (5.2 million ha). Other countries with large amounts of salt-affected lands
in irrigation districts include Afghanistan, Egypt, Iraq, Kazakhstan, Turkmenistan, Mexico, Syria and Turkey
(Squires and Glenn, 2011).
Australia has the largest extent of naturally sodic soils of any continent (Table 6.5).
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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