BBC News Online: Sci/Tech

Soil loss threatens food prospects

Scientists say impoverishment of the soil is a major threat to the Earth's ability to feed itself.

They have found that nearly 40% of the world's agricultural land is seriously degraded.

The damage has already had "a significant impact" on the productivity of about 16% of the planet's farmland.

And its economic and social effects have been much more significant in developing countries than in rich ones.

The scientists, from the International Food Policy Research Institute (IFPRI), analysed the world's agro-ecosystem using data from satellites, maps and tabular data sets.

Multiple causes

Working with colleagues from the World Resources Institute (WRI), they formed part of a larger international initiative, the Millennium Ecosystem Assessment.

They say the seriously degraded areas include 75% of central America's cropland, 20% of Africa's, and 11% of Asia's. The researchers identify several causes:

• erosion, two-thirds of it caused by water and the rest by wind. One estimate says that topsoil is being lost at least 16 times faster than it can be replaced, with the formation of 2.5 cm needing from 200 to 1,000 years
• damage from mechanical tilling can cause soil compaction
• repeated cropping can deplete soil nutrients, which need replacing, something poor farmers can seldom afford
• poor water management on irrigated cropland is a big problem: inadequate drainage can lead to waterlogging or salinization, where salt levels build up to poison the soil. Between 10% and 15% of all irrigated land suffers from these two problems.

The director-general of IFPRI, Dr Per Pinstrup-Andersen, said: "Small farmers ought to have access to drought-tolerant seed, insect-resistant seed, pest-resistant seed, the kinds of things small farmers need in order to increase production."

The scientists say food production is also threatened by competition for water.

Remedies possible

Dr Pinstrup-Andersen said: "We have to learn to utilise natural resources much more sustainably than we have done in the past.

"That means, for example, allocating water where water is most needed, without lowering the water table, without misusing the available water."

But the scientists say there are ways of halting and even reversing the cycle of soil degradation and loss.

Methods of farming that minimise tilling and its potential for erosion are coming into wider use in countries like Morocco, the Philippines and Thailand, as well as parts of sub-Saharan Africa and south America.

The methods include contour farming, terracing, and using plants as natural barriers.

There is also a growing realisation that erosion and other soil problems are threats to entire watersheds, and need to be tackled on that basis.

Geographic information systems are enabling scientists to prepare maps which show farmers where they can get the highest yields with the least environmental damage.