SOME AGRONOMISTS, trying to make a good thing out of global warming, suggest that with rising temperatures we will soon be able to farm northward into Canada and Siberia, and higher on the slopes of mountains. The changes in climate, they predict, will stimulate immediate fresh soil building.
It isn’t so. The virgin, never-cultivated soils of the north and the heights are stiff and stony. Even though reaction rates double with each ten-degree rise in temperature, a fertile soil does not appear overnight. The geologist William Fyfe has studied this question, observing volcanoes at different latitudes to see how fast their fresh lava turns to soil. Even in Hawaii, he finds, it takes at least one thousand years for the first centimeter of fresh soil to form. So if the Northwest Territories turned into Mauna Kea, we would still have to wait perhaps ten times ten centuries to plant the wheat.
Virgins are particular. They call forth the husbands appropriate to them. In the tundra of the Northwest Territory, it takes 120 square miles to feed a single human being. Not grain or corn, but caribou, is the transformer that converts the energy of this thin soil into food for a man.
A tundra soil does not look productive. Rainforest soils, on the other hand, dress richly, but only to conceal their underlying poverty. In fact, the better virgin soils there—for example, in parts of the Amazonian rainforest—are in disguise, cloaked not with behemoths, but with slender acai, becacu, and mora trees. The great 150-foot trunks of the acapu, the caju-acu, and the jurana trees stand on far poorer soils. The native peoples know this, choosing sites for their shifting slash-and-burn fields by looking for the slender trees. The colonos, recent immigrants brought from the coast under government programs, burn off the large trees, assuming falsely that they must have grown on the better soils. To make matters worse, while the natives grow locally adapted cassava in their plots, the colonos import nutrient-hungry and ill-adapted rice, corn, and beans. Of the thousands of plots laid out to either side of the Trans-Amazon Highway to a distance of sixty miles, only a small fraction have been claimed.
Even the virgin soils of the temperate forest are delicate creatures. A temperate forest lives largely by recycling its own masses. If you remove the litter, you break the chain, and the nutrient-poor soil beneath cannot supply the deficit.
Consider the growth of a redwood tree on the Northern California coast. On the day Christ was born, the seed falls in sand deposited at the bend of a river. The diving roots send out sugars that attract microbes that feed there, living, dying, and decaying. The young tree begins to drop its foliage, bark, and cones, along with the bodies of insects and other animals that live in it. Organic acids in the soil water leach the iron and aluminum from the sand and begin to carry it deeper into the ground, creating distinguishable soil horizons. Two thousand years later, the soil is thirty feet deep, it erodes less than one millimeter per century, and the redwood rises 350 feet into the air. We picnic beneath it, leaving an apple core and some cheese rinds as offerings.
Yet the matter that has accomplished this miracle has been largely recycled. If all the litter that had fallen on the forest floor in that time had remained there, the tree would be standing sixty feet deep in the stuff. Instead, the mix of acid humus and still-integral fallen needles, cones, and twigs is a mere four inches thick, and has never been thicker. Its steady decay has fed the tree.
To clear-cut the forest is to break the millennial rhythm of events. Soil destruction is rapid and can be catastrophic. The alkaline ash of burned-over trunks masks these effects for a few years, since it amounts to an all-at-once shot of plant nutrients, but soon it produces accelerating deterioration. The alkali makes silt and clays disperse, shattering the soil’s structure and forming a tight crust. Falling water runs off the surface, carrying away nutrients with it. Furthermore, the soil, exposed to the rays of the sun, quickly becomes hotter and digests the leftover organic matter, whose growth-promoting nitrogen soon bleeds through the soil and disappears.
But the most desirable virgin soils are not from the forest. In a forest, at least two thirds of the organic matter resides above the level of the soil. In a prairie, the proportion is reversed. The great prairie soils of the American Midwest and of the Russian steppes are rich with the remains of millennia of dense, sinuous roots that have lived and died in a soft mineral accretion formed from windblown silts high in calcium and other bases. Not only has the root mass given its organic residue for fertilizer; its sugars have also attracted a vast microflora, and its polysaccharides have glued smaller grains of soil into larger aggregates that permit easy passage of food-bearing water and air.
In the 1930s, Hans Jenny studied virgin prairie soils side by side with cultivated soils in Missouri. He found that after only sixty years of cultivation, with zero erosion, the farm soils had lost one third of their organic matter. As a result, there was no longer the same level of glue to bind the soil into aggregates, and instead of rising like yeasted dough, it was collapsing into heavy slabs.
Only by replacing what you take can you keep a soil fertile. As geneticist Wes Jackson suggests, nature must be the standard. To be responsible to the soil is to respond to its gifts with our own.