Although only 17 percent of all cropland is currently irrigated, it provides 40 percent of the world's food. Some regions have scope 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 percent 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 percent 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 percent."

What causes salinity?

As rocks and soils are worn away by water, small quantities of the mineral salts they contain are carried into rivers and aquifers, and thus 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 waterlogging, 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 metres, 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.

What can be done?

Mr 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."

There are strategies for preventing or correcting salinization:

Leaching: 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.

Drainage: Ditches or underground pipes can take saline water away. Up to a third of waterlogged and saline land could be reclaimed with better drainage management, using a variety of strategies to address the local situation. For example, over the last 30 years, Egypt's national drainage programme has confronted waterlogging and salinity through the use of different types of drains as well as pumping stations. In addition to aiding drainage, this strategy allows reuse of drainage water.

Flooding: 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. (There has been speculation that this type of rehabilitation may qualify for support under the Kyoto Protocol's Clean Development Mechanisms because farmland returned to production removes carbon from the atmosphere). In cases where the land can still grow something, farmers can plant a rehabilitation crop that tolerates some salinity and uses a lot of irrigation water, such as rice.

More efficient use of irrigation water: Irrigating with sprinklers can use water more efficiently than surface irrigation, but it can also deliver salts right onto the plant itself if the irrigation water is saline. Drip irrigation, which involves delivering a metered amount to the area around the plant itself, is better yet.

Some crops are more salt-tolerant than others. But sometimes farmers -- and even whole regions -- need to rethink their crops. This may even turn out to be more profitable. In Cape Verde, for example, farmers have been switching from thirsty sugar cane to high-value horticultural crops, such as tomatoes, watered by drip irrigation. National horticulture production tripled to 17 000 tonnes between 1991 and 1999.

"We know that salinization is serious, and farmers and technicians need better information to develop appropriate strategies," says Mr Martínez Beltrán. FAO Irrigation and Drainage Papers contain guidelines on assessing soil salinity at field level and on drainage design to help prevent salinization. FAO has also produced training manuals suitable for extension agents and advanced farmers. Studies are under way to determine the feasibility of using remote sensing (satellite images) to map salt-affected areas.

Almost 800 million people lack adequate nutrition in the developing world -- and salinization could be threatening up to 10 percent of the global grain harvest. Sustainable irrigation and drainage management is an issue that must be addressed at once.