BOSTON — THIS year has brought some good news in the fight to save the earth. Measuring their successes by the acre, soil experts with the United Nations, the World Bank, and the US Soil Conservation Service say they are learning how to hold back sand dunes and keep steep, erodible hillsides intact. And, unlike the huge engineering projects of the past, the leading edge of today's soil conservation work is as simple as planting a tree.
But the scale of the problem remains huge.
In the last 50 years, the Earth has lost 11 percent of its vegetated land to human-induced soil degradation, an area about the size of China and India combined, according to a 1991 UN Environment Programme (UNEP) study. Much of the lost soil is irretrievable. In the same period, the number of mouths to feed in the world has more than doubled.
The UNEP report, produced in conjunction with the International Soil Reference and Information Center in Wageningen, Netherlands, helped alert people to the severity of the problem, says Richard Grimshaw, a soil conservation expert at the World Bank in Washington. But, he adds, ``it all comes back to the farmers, what they can do to improve the soil.''
According to a study released last year by the Food and Agriculture Organization (FAO) in Rome, soil degradation occurs most often when people clear-cut forested hillsides for timber or fuelwood, when they allow their cattle to compact soil or chew vegetation down to the roots, or when farmers grow only one type of crop (monoculture) on a piece of land, without allowing the soil to regain nutrients. As more land is cleared for development, wind and water erosion sweep away tons of topsoil; degradation also occurs when pollutants are dumped or when poor irrigation allows salt buildup.
But Wim Sombroek, a soil expert at FAO, says: ``It's time to show that not every influence of man is negative. Traditional farming methods have actually helped the soil by terracing hillsides and building up soil matter through the addition of composted vegetable matter and manure.''
The problem with some of these traditional methods, Mr. Sombroek adds, is that they are labor-intensive, requiring long hours for distant benefits. Some farmers, for instance, balk at the costs of planting rows of antierosion vetiver grass on sloping fields, a method that begins to pay for itself in higher yields only after three years.
In contrast to the grand engineering projects of the past, many of the new projects are simple, relying on native vegetation:
* In Senegal, UNEP's Sahel Office encouraged villagers from Thiambene Till to plant native trees as windbreaks and enclosures for their domestic animals in 1986. Seven years later, the hedges have allowed natural vegetation to grow back and erosion has been reduced.
* In Sri Lanka, the Tea Research Institute of St. Coombs planted vetiver grass hedges and leguminous shrubs between tea rows on the contour of hillsides, a method that has helped add soil nutrients and deter erosion.
For its part, the US government, chastened in the Dust Bowl of the 1930s for ignoring the warnings of its own soil experts, has made remarkable strides in cutting erosion. This June, in its National Resources Inventory, the US Department of Agriculture said that better farming techniques and conservation policies had saved the US 1 billion tons of soil per year between 1982 and '92. (Plowless farm, below.)
But even such good news is tempered, Sombroek says: ``The US has hidden degradation - the loss of organic matter [such as manure or compost] in the soil, and the overuse of fertilizers.''