Problem: Loss of moisture from the soil is the primary challenge. It arises from the disappearance of vegetation, whether through drought, deforestation, overcultivation or overgrazing.

Dani-Jeske/Still Pictures

Solutions: The ancient practice of terracing - planting in steps cut into slopes - allows water to soak into fields as it flows downhill, and prevents erosion. Variations on this theme helpfarmers to retain moisture in degraded lands. Contour bunding - embankments built into the earth along the contours of a slope - holds onto rainwater in Niger. In Burkino Faso, the embankments - known as diguettes - are reinforced with linesof stones. And in the Philippines and Thailand, farmers strengthen bunds by planting them with deep-rooted vetiver grass.

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Rangeland can be set aside to let it recover. Shepherds in Morocco, for example, were encouraged to form cooperatives and then compensated for allowing parcels of land to recuperate. The vegetation recovered very quickly, and is now grazed in controlled rotation.

Coping with salinization

Problem: Overirrigation causes salts to build up in the soil, reducing its fertility.

Carole Hodgson

Solutions: Drip irrigation - where water is fed through perforated tubing drop by drop, directly to the roots of the plants - is highly effective. Less evaporation greatly reduces the build-up of salt and the waste of water. It can wash salt away from plant roots and can even be done with saline water, as the method prevents salt from touching the leaves of plants. The technique is used in the United States, Mexico and Australia, and is being encouraged in Egypt, Eritrea, Jordan and elsewhere.

Dan Porges/Still Pictures

In Pakistan, farmers are adapting to saline land by growing salt-resistant plants such as pistachio trees and barley. Researchers in the region are also investigating salt-resistant plants which can bind the soil and provide cattle forage, and are also a source of cooking oil.

Israel is taking a high-tech approach, developing genetically modified melons, grapes and tomatoes that tolerate saline conditions.

Problem: Topsoil and sand loosened by the loss of vegetation blow into arable areas and encroach on cities. This is particularly problematic when degradation on the fringes of deserts destroys the green belts that had held down the soil and sand and provided a buffer zone.

Solutions: One approach is to stabilize the sand - such as by using straw grids to hold it down, planting drought-tolerant shrubs in dunes, or even spraying with petroleum. Another is to slow the wind: trees make excellent windbreaks, which shield young plants, anchor soil and help to retain moisture.

Mark Lynas/Still Pictures

China calls its blowing sands - the world's worst - 'yellow dragon', and is fighting them with the Green Great Wall, planting more than 3.5 million hectares of forest in a 4,500-kilometre network of belts stretching from Beijing to Inner Mongolia. The project is now in its fifth year but is expected to take decades to complete.

Problem: Land impoverished by overcultivation, erosion and drought needs to have its nutrients restored.

Solutions: The Zai technique of pit planting - which originated in Mali and has been adopted by farmers in Burkina Faso, captures rain and run-off, keeps seeds and mulch from being washed away, concentrates nutrients and helps improve depleted soil. Pits 20 to 40 centimetres wide and 10 to 20 centimetres deep are dug at 1-metre intervals during the dry season, and organic matter is gradually built up inside. After the first rain's fall, they are covered with a layer of soil, into which seeds are sown.

J.P. Delobelle/Still Pictures

Legumes like pigeonpea - grown in India and Africa - add valuable nitrogen to soil. Agroforestry - planting trees among crops and herds - can have many benefits. Trees drop nitrogen-rich leaves and so boost soil fertility, prevent erosion and provide shade, fuelwood, fruit, fodder and timber.