2. Past: What not to do with dirt

3. Present: On thin ground?

4. Future: Dig in

Cover crops, like this one at the USDA's 17-acre certified organic research plot in Salinas, Calif., are a popular way to fight erosion. Photo by Scott Bauer, USDA

Ray Weil, a soil scientist from the University of Maryland, took clumps of soil from this experimental farm in Ethiopia: some from the forest and some from the cultivated field. In the lab, a simple experiment -- dropping the clods in beakers of water -- shows the effects of erosion on soil. The sample on the left in each picture is from the field, and the sample on the right from the forest. The sponge-like qualities of healthy soils are absent in the farmed soil, which means less water for the plants and more to wash nutrients away as runoff. Photos: NASA.

Four elements -- carbon (C), oxygen (O), hydrogen (H), and nitrogen (N) -- make up 96 percent of living matter. For ecosystems to work, these nutrients must be continuously recycled. Since intensive agriculture leaves soil short on nitrogen, farmers typically replenish their fields with fertilizer. Between 1945 and 1985, the use of nitrogen fertilizer increased twenty-fold. Is there a way to take better advantage of nature's natural nitrogen cycle? Diagram: EPA

Soil erosion is a major contributor of phosphorus to streams. This photo shows bank erosion during a flash flood on the Rio Chama near Chamita, New Mexico. Photo by Lisa Carter, USGS.

World hunger rates fell in that period, but hundreds of millions remain underfed. The global population is now 6.2 billion and rising. And, scientists say, Green Revolution methods -- hot issues themselves -- aren't likely to increase yields further. In other words, more of the same won't do. "The global agricultural enterprise...has gone from being a minor source of off-site environmental degradation 35 years ago to becoming the major source of nitrogen and phosphorous loading to terrestrial, freshwater, and marine ecosystems," wrote ecologist David Tilman in 1999 (see "Global environmental impacts" in the bibliography).

That leaves two options: cultivate more land or find better ways to use existing cropland. More than a third of the world's land is already occupied by agriculture, so most scientists favor the latter choice. The biggest obstacle within it, says David Pimentel, is still erosion.

The end of soil
"When soil is severely eroded," Pimentel explains, "there's very little organic matter. There are few earthworms and insects, and few holes that allow water to percolate through. The sponge is gone. More water runs off the land." When that water runs off soils teeming with fertilizers and pesticides, pollution happens. Since two-thirds of erosion is caused by water (the rest by wind), runoff pollution is a serious concern.

In the United States, great pains have been taken to reduce erosion since the 1930s. Some policies remove lands from production permanently through conservation easements. Others, like the Conservation Reserve Program, offer incentives for farmers to give fields a breather. In addition to land set aside, an additional 1.3 million hectares of land was treated with erosion control measures in 2001, according to the U.S. Department of Agriculture. But how successful these efforts have been is under question. In 1995, David Pimentel and his colleagues published a paper in Science estimating erosion rates worldwide. The numbers were staggering.

Each year, water and wind clear 75 billion tons of soil from the land, most of which comes from agricultural land.

That loss degrades farmland, eventually leaving it useless.

Thirty percent of the world's farmland has become unusable in the last 40 years due to erosion.

Each year, about 10 million hectares of eroded cropland are abandoned worldwide.

Erosion rates are highest in Asia, Africa and South America, averaging 30 to 40 tons per year, per hectare, of land.

"Since publishing our 1995 paper, the USDA has reduced the erosion rate on cropland to 10 tons per hectare, per year. This is still 10 times the sustainability level," Pimentel notes. What's more, he says, the soil erosion rate on pasture land has held steady at six times the rate at which soils naturally replenish.

Erosion is still "neglected from the point of view of farmers and scientists and, especially, decision makers" he says.

Feeding the fertile Earth
"Agriculture does not improve soils," says Ward Chesworth of the University of Guelph in Ontario. "For more than 10,000 years we have conducted a long agricultural experiment with the biosphere by using modified annual grasses to mine the soil for plant nutrients.

"Like all mining operations, this one will eventually fail unless we can find a way of conserving the fertility of soil that does not depend on the diminishing natural resources that we currently use for that purpose."

By necessity, crops sop up nutrients from the soil. Historically, these nutrients have been returned by plowing harvested crop residue back into the ground and with fertilizers. But intensive plowing causes erosion, and some nutrients -- nitrogen, most notably -- are difficult to replenish.

Erosion and losses in soil fertility are inevitable products of conventional agriculture, together constituting the "problem of agriculture," says Wes Jackson, director of The Land Institute in Salinia, Kan.

Annual grasses like wheat, rice, corn, rye, and barley account for 70 percent of the human diet, Jackson says. Historically, these annuals have been planted in monocultures -- one crop per field. It's a bad model, Jackson says, if preserving the land is an ultimate goal. Monocultures leach more nutrients, are invaded by more weeds, and attract more disease and pests more than fields planted with more than one crop.

At the Land Institute, Jackson and his colleagues plant many crops on a field to maximize nutrient cycling without relying on fertilizers.

"There are 16 or 18 elements necessary for life," he explains. "And the most important thing that manages these elements are the roots of plants. In nature, many different kinds of root architectures exist in one place so that specialization is possible within the soil ecosystem. Some long tap roots may be good at cycling one element, and some fibrous roots may be good at something else. It's a division of labor at the ecosystem level."

Jackson hopes mimicking nature's methods will help farmers worldwide. Soil fertility is a particular problem in Africa, where small-scale farmers have removed large quantities of nutrients from soils without using enough manure or fertilizer to replenish them. On average during last 30 years, Africa has lost 22 kilograms of nitrogen, two kilograms of phosphorous, and 15 kilograms of potassium annually, per hectare of cultivated land (See "Soil Fertility and Hunger in Africa" in the bibliography).

What about "sustainable agriculture?"
Jackson is part of a growing group of scientists and farmers who are shunning the old agriculture model. Consider, for instance, the growing popularity of "sustainable agriculture," a catchall phrase including any technique that maintains yields and natural resources, like soil, over time. It's a tricky science, but some of the methods used -- old and new -- appear to work:

Windbreaks: Planting trees along field borders of their fields can reduce wind erosion, especially in flat regions.

Cover crops: Planting winter cover crops like fall rye or winter wheat act as a ground cover and protect vulnerable soil from eroding.

Grassed waterways: In some fields the land forms natural depressions that collects excess water. To keep the soil in these depressions from being washed away, farmers plant strips of grass that absorb the water.

Contour cultivation: Planting crops to follow the contours of a field provides furrows at an angle to the slope of the field. The irregular surface interrupts the flow of water, reducing erosion.

Strip cropping: Alternating a field with strips of different crops can do the same thing, as well as reducing the pests that monocultures invite.

Conservation tillage: Leaving stalks and leaves of harvested crops on fields protects the underlying soil from wind and rain until a new crop is planted in the spring. It include no-till farming (leaving all crop residue on the soil) and ridge-tillage (molding the soil into ridges, planting seeds between them).

Where do we go from here?

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