Page 153 - Soil Degradation, Conservation and Remediation

142 5 Chemical Soil Degradation

when they are acidified, they are ultimately abandoned. The global distribution of acid
sulfate soils as obtained from data of the FAO-UNESCO Soil Map is shown below.

Region Area (M ha)
Africa 3.70
Asia and Far East 6.70
Latin America 2.10
North America 0.10
World total 12.60

5.4 Soil Salinization

A soil is said to be saline if the electrical conductivity of its saturation extract (EC e )
(saturating the soil with water and taking the extract by suction) is greater than
4 dS m (decisiemens per meter). Saline soils contain excess soluble salts, generally
−1
chlorides and sulfates, with some carbonates and bicarbonates, of sodium, potassium,
calcium, and magnesium. Soil salinity is harmful for plants barring the halophytes;
it causes water stress through osmotic disturbances in plant tissue and by toxicity
of some salt constituents. Seed germination, plant growth, and yield of crops are
considerably reduced by soil salinity. Sometimes it causes crop failure.
Some soils are naturally saline. They are formed by processes called primary
salinization or natural salinization. Some soils are made saline by mismanagement
of soil and crop, particularly improper irrigation and drainage, that is, changing the
hydrologic balance. This is known as secondary salinization or human-induced salini-
zation. Salinization, both natural and human induced, may occur in two climatic
settings – arid and semiarid and humid regions. In arid and semiarid regions, scarcity
of water due to low rainfall and high evaporation does not allow necessary leaching
of salts. Moreover, there is a net capillary rise of water which brings salts to the surface
soil. In humid areas, on the other hand, excess irrigation or poor drainage cause the
groundwater table to rise to the root zone of plants and make the soil saline.
Oldeman et al. (1991) estimated that worldwide 76.6 M ha lands are affected by
human-induced salinization, but they did not differentiate salinity in the irrigated
and non-irrigated rainfed areas. A survey made by Ghassemi et al. (1995) revealed that
vast areas of India, China, Pakistan, and Central Asian countries are affected by
salinity due to over irrigation and lack of drainage facilities. Some soils have become
saline due to the use of saline water for irrigation. Table 5.7 gives areas affected by
different degree of salinization in different continents.
Two important approaches to soil salinity management are (1) selection of salt-tolerant
crops and (2) removal of excess salts from the root zone. Several salt- tolerant crops
can be grown. Removal of salts by irrigation and drainage and drawing the
salty water safely may be financially impracticable in some situations. Whether
the reclamation will be cost-effective or not depends on salinity level, cost of water
and labor, crop return, etc. Reclamation is relatively easy if the soil is saline alone,
but difficult if it is saline – sodic or sodic.