The bowels of earth

Natural History, June, 1998 by Evan Eisenberg

In The Ecology of Eden, philosopher, classicist, and biologist Evan Eisenberg examines the strategies living things have used in their struggle for nutrients, living space, and offspring. Drawing from a broad range of cultural and scientific topics, he examines the place the myth of Eden holds in the Western psyche.

From The Ecology of Eden, by Evan Eisenberg. Copyright [C] 1998 by Evan Eisenberg. Reprinted by permission from Alfred A. Knopf, Inc.

Excerpt

Soil is an extraordinary thing, unlike anything else on or in the earth. But it is such an occult thing--hidden from us even when we dig into it, slipping through our fingers in more ways than one--that we feel the need to compare it to something else. If I had to choose two aboveground things that soil is most like, I would choose a great city and a mess of entrails.

The city part is easy enough. Soil is, to put it mildly, densely settled. One teaspoon of good grassland soil may contain 5 billion bacteria, 20 million fungi, and 1 million protists. Expand the census to a square meter and you will find, besides unthinkable numbers of the creatures already mentioned, perhaps 1,000 each of ants, spiders, wood lice, beetles and their larvae, and fly larvae; 2,000 each of earthworms and large myriapods (millipedes and centipedes); 8,000 slugs and snails; 20,000 pot worms, 40,000 springtails, 120,000 mites, and 12 million nematodes.

These citizens are often in motion, hurrying along the vast expressways made by moles, the boulevards of earthworms, the alleys between particles of sand or clay, and the dank canals that these alleys often become. Certain districts and certain intersections--mainly close to the roots of plants--get especially busy. The citizens move in the dark, sniffing at chemical trails. They are constantly doing business with one another. They traffic in molecules: minerals, organic compounds, packets of energy. Their interactions are sometimes friendly, sometimes competitive, often predatory.

Aristotle called earthworms the intestines of the earth, and he was not wrong. The soil itself, though, is a tangled slaw of intestines, some as thick as your arm, some a few molecules wide. The business of the soil is digestion and assimilation, and the creatures of the soil are always enlarging the surface area where these processes can take place.

Fungi, ranging in size from the great mycelial clot beneath a toadstool to tiny, threadlike microbes, send out long fingers called hyphae that penetrate dead tissue. Besides physically roughing up the tissue, they exude enzymes that digest it chemically, so that the simplified nutrients can be absorbed into the fungi. If you could somehow unravel the fungal hyphae in a single ounce of rich forest soil, they might easily stretch two miles.

A like figure might be racked up by filamentous bacteria called actinomycetes, which, under the electron microscope, look like something midway between antlers and bean sprouts. Among the most plentiful of all soil organisms, they give damp soil and damp basements their heavy, heady smell. They also secrete enzymes that break down dead tissue into nutrients, which are then absorbed along the surface of the branched, springy filaments--although, like all bacteria, they lack the fungi's ability to break down tissue by brute force.

Roots, which do not digest but merely absorb, are not quite so rich in surface area, but they come close. The root system of a single, four-month-old rye plant was found to have a surface area of 639 square meters--130 times the surface area of the aboveground plant--all of it packed into about six liters of soil. Much of this expanse is accounted for by root hairs, which sprout like a scraggly beard near the growing tip of the root. If a mere stripling of an annual plant can so pack the soil with roots, rootlets, and root hairs, one can imagine what lies beneath the sod of a mature perennial grassland or the litter of a forest.

Most plants, though, are unsatisfied with the mileage they get out of rootlets and root hairs. They enlist the help of fungi that invade or cling to the roots and branch out into the soil. These unions of roots and fungi, known as mycorrhizae, not only increase the surface area of the root network but also give the plant access to certain nutrients, such as phosphate, that it would be hard put to gather on its own. In return, the fungi get a trickle of sugar.

Fungi maximize surface area--or, to be precise, the ratio of surface area to volume--by stretching out. Bacteria, fungi's only peers in decomposition, achieve the same end by staying very small. As any object gets bigger, its volume increases faster than its surface area. (Volume increases with the cube of the radius; surface area with the square.) To put it even more crudely, the bigger something gets, the farther its insides get from the outside world. If you want to exchange energy and matter with the outside world as efficiently as possible, you had better stay small.

It is no accident that, apart from the soil, the other great habitat for bacteria on land is the guts of humans and other animals. Not only bacteria's small size but also their chemical virtuosity--the range of enzymes they can produce, the unlikely stuff from which they can pluck energy--make them invaluable in your bowels, as well as in the bowels of the earth.