Assault of the earth.

• Author: Wilken, Elena
• Position: Soil supply

THE APPETITES OF GROWING POPULATIONS ARE SQUEEZING THE LIFE OUT OF THE EARTH'S FINITE SUPPLY OF GOOD SOIL. NOW THE QUESTION IS WHETHER WE CAN REBUILD IT FASTER THAN IT DISAPPEARS.

To be human is to be a creature of the soil: that message is contained in creation stories from all over the world. In Genesis, for instance, man is formed "from the dust of the ground." The soil's mysterious vitality is a subject for science too; in the lightless world beneath our feet, death becomes life, and the renewal of the soil itself - a process so slow it's usually indiscernible in a human lifetime - proceeds in rhythms still largely unknown. But in the developed world, at least, those creation stories seem to be lost on us. By and large, we no longer honor our relationship to the soil. Soil has become simply one more resource - a substance necessary for crop production and for holding up buildings. We take it for granted, and fail to notice that it's disappearing.

For most of human history, we could afford to remain ignorant of how the soil worked. When farmers exhausted the productivity of a field, they could usually bring another one into production. But now that human populations are pushing into every nook and cranny of the globe, we no longer have the option of moving on. Virtually all of the world's most productive cropland is already in cultivation. It's true that in many areas, some of the best land is producing cash crops instead of food for local consumption. But even so, the basic trends are clear: if the amount of land in production remains constant over the next 40 years, farmers will nearly have to double their yields to feed the growing population. And as we try to grow more and more food - by cultivating marginal cropland, by intensifying production, by using more powerful technologies - our soils deteriorate. And our ignorance of how to heal them becomes increasingly dangerous.

In general terms, soil seems a pretty simple affair: it consists of a large mineral component and a small measure of organic material, plus water and air. The mineral content comes from bedrock, which is broken into particles by chemical reactions and the movement of water. Decaying material on the surface and dead roots underneath make up the organic matter, which is broken down by microbes and invertebrates into a web of debris, or humus. Worms, roots, rodents, and the soil's other denizens tunnel through it, mixing the minerals from below with the organic matter from above. This process gives the soil its denSity and texture, or "structure." Good structure requires a high proportion of organic material and a loose, porous consistency; it allows for drainage, aeration, and a high nutrient load.

The surface of the earth looks inert and solid, but soils are in constant flux. Just as soil formation is a continuous process, so is degradation. Soils erode; their nutrients leach out; their complement of life waxes and wanes. In undisturbed systems the rate of formation is slightly higher than the rate of erosion, allowing for soil build-up. But human activity, particularly agriculture, increases degradation without enhancing formation.

Degradation has been a part of agriculture from its beginnings. Archaeologists have, for instance, been able to trace the development of farming in the Mesoamerican civilization around Mexico's Lake Plazcuaro by studying the lake's sediments, which eroded off the surrounding farmland. By the 13th century, the sedimentation rate was six times its pre-agricultural level.

Today, farmers face the challenge of slowing the processes of degradation and incorporating their corollary - soil creation - into agriculture. But despite the consensus on the need for conservation, planning is hampered by a high degree of uncertainty. It's very difficult to assess the scope of the problem, or to predict the effect that various solutions might have on agriculture and the environment. Degradation is measured in terms of compaction, erosion, nutrient loss, loss of organic matter, and decreased microbial activity. These points of reference are the same for all soils, but the cumulative effect may vary widely over even a small patch of ground. A single hectare can hold three or four different soil types, each with its own set of vulnerabilities. One area might be especially susceptible to water erosion and compaction, for instance, while another might suffer from a drop in microbe populations. A global measurement of degradation would have to account for the role of each variable within each agricultural system.

The problem is further complicated by our ignorance of natural changes: not every form of degradation is caused by human activity, and large natural cycles are probably futile to combat. In 1984, for example, the U.N. Environment Programme claimed that the sands of the Sahara were being pulled southward by excessive livestocking, deforestation, and over-grazing around watering holes. But satellite photographs eventually showed that this instance of desertification was largely the result of a sustained drought. In marginal climates, a few centimeters of rain can make the difference between relatively lush vegetation and a barren landscape. Certainly much of sub-Saharan Africa is under increasing pressure from grazing and agriculture, and some areas are severely degraded. But the simple appearance of degradation is not a sufficient basis for...