Irrigated Land Damaged by Salt

May 6, 2002

Irrigation produces much of the world's food, but about a tenth of the world's irrigated land has been damaged by salt, according to the United Nation's Food and Agriculture Organization, and this has become a profound threat to food security. Although only 17% of all cropland is irrigated, it provides 40% of the world's food.

FAO says some regions have the wherewithal for much more irrigation, especially small-scale schemes. But much existing irrigated land is threatened by salinization -- a build-up of salts in the soil. This lowers yields and can damage the land beyond economic repair. Salinization is reducing the world's irrigated area by 1-2% every year, hitting hardest in the arid and semi-arid regions. "No one is really certain of the figures, but it seems that at least 8% of the world's irrigated land is affected," says FAO water expert Julián Martínez Beltrán. "In the arid and semi-arid regions, it's somewhere around 25%."

As rocks and soils are worn away by water, small quantities of the mineral salts they contain are carried into rivers and aquifers and then into irrigation water. If too little water is used on a field, the salts are not washed away, remaining in the soil.

But the worst danger to the soil is from too much water. This causes water logging, which raises the water table. The soil then acts like a sponge, drawing water up into the root zone by capillary action. This effect can draw water upward by around 1.5 meters, depending on the soil. The water then evaporates, leaving salt around the roots, which interferes with the ability of the roots to absorb water. This process takes place especially quickly in arid regions.

Martínez Beltrán points out that farmers should use the amount of water the plants really need, plus a little extra to ensure that salts are leached out -- but no more. Farmers should also find out whether or not they have a salinity problem. "In some cases the only sign is falling yields, which may not be obvious for some time," he says. "But there can be visible symptoms, such as a light crust on the soil and a soft feeling to it."

Using just a bit more water than the plants need -- but not too much -- reduces salinity by leaching salts past the root zone and into aquifers, which carries them away, provided there is sufficient natural drainage. Ditches or underground pipes can take saline water away. Up to a third of water logged and saline land could be reclaimed with better drainage management, using a variety of strategies to address the local situation.

Badly salinized land that can no longer sustain agriculture can sometimes be rehabilitated by flooding and drainage. Although often expensive, this approach may be economic, depending on the value of the land and crops, according to FAO.