Page 21 - Curriculum Visions Dynamic Book

irrigation: the application of water to fields to help plants grow during times when natural rainfall is sparse.
salinisation: the concentration of salts, especially sodium chloride, in the upper layers of a soil due to poor methods of irrigation.
Irrigation water
 Under irrigation, it is vital that the salt is flushed out of the soil. This is because water used for irrigation will often have come from groundwater supplies and in its long passage through rocks it will have picked up a higher salt content than rainwater.
Farmers are often tempted to put on just enough water to feed the roots with the water they need, but this is a very short-sighted policy because unless the salt is flushed away it will pollute the soil.
A good irrigation system will have deep drains to make sure the water is carried from the soil as well as sprinklers to apply the water.
 In some places irrigation waters are so salty that significant amounts of salt are left in the bottom of a beaker after the water has been evaporated.
Soils most at risk
It is likely that many areas of desert were once fertile before
salts built up in them because of poor irrigation. Modern areas badly affected include the Murray–Darling basin of Australia, the Indus Valley of Pakistan and the Colorado Basin of North America.
 Soils that have suffered salt damage may not be suitable for use for over 20 years even if irrigated properly thereafter.
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