Page 154 - Soil Degradation, Conservation and Remediation

5.4 Soil Salinization 143

Table 5.7 Global extent of human-induced salinization
Light Moderate Strong Extreme
6
6
6
6
Continent (10 ha) (10 ha) (10 ha) (10 ha) Total (10 ha)
6
Africa 4.7 7.7` 2.4 – 14.8
Asia 26.8 8.5 17.0 0.4 52.7
South America 1.8 0.3 – – 2.1
North and Central America 0.3 1.5 0.5 – 2.3
Europe 1.0 2.3 0.5 – 3.8
Australia – 0.5 – 0.4 0.9
Total 34.6 20.8 20.4 0.8 76.6
Oldeman et al. (1991), with permission
5.4.1 Management of Saline Soils

Saline soil management involves growing salt-tolerant crops and leaching of salts
below the root zone. If there is a salt crust on the surface of the soil, salt farming may
be more profitable than cropping. For cropping, decrusting may be done mechanically
and with soil flushing. However, decrusting followed by flushing has not been very
successful for increasing crop yields. But some soils may need decrusting before
leaching. Effective leaching may be achieved by flooding and draining soils. Some
soils are only slightly saline. For shallow-rooted crops, the salts may be driven
below the root zone by temporary leaching. This technique will need less water
than normal leaching. Furrow irrigation with suitable seedling placement may also
be satisfactory. Drip irrigation at the root area also dilutes salts and keeps the salts
apart. If the soil is considerably saline, it needs removal of excess soluble salts by
thorough leaching. However, a reliable estimate of the quantity of water required
to accomplish salt leaching is required. The salt content of the soil, salinity level to
depth to which reclamation is desired, and soil characteristics are important factors
determining the amount of water needed for reclamation. A useful rule of thumb is
that a unit depth of water will remove nearly 80 % of salts from a unit soil depth.
Thus, 30-cm water passing through the soil will remove approximately 80 % of the
salts present in the upper 30 cm of soil. To leach soluble salts in irrigated soils, more
water than required to meet the evapotranspiration needs of the crops must pass
through the root zone to leach excessive soluble salts. This additional irrigation
water has typically been expressed as the leaching requirement (LR). Leaching
requirement was originally defined as the fraction of infiltrated water that must pass
through the root zone to keep soil salinity from exceeding a level that would significantly
reduce crop yield under steady-state conditions with associated good management
and uniformity of leaching (Rhoades 1974):

EC
LR = iw ,
EC dw
where LR is leaching requirement, EC iw is electrical conductivity of irrigation water,
and EC dw is the electrical conductivity of drainage water. Several leaching