Soil

Soils teem with life. In fact, more creatures live below the surface of the earth than live above. Among these soil dwellers are bacteria, fungi, and algae, which feed on plant and animal remains breaking them down into humus, the organic component of soil, in the process. The numbers of these microscopic soil organisms is vast—a gram of soil, which would fit into a peanut shell, can contain from several hundred million to a few billion microorganisms. The importance of their actions to the health of the soil is equally large.

Bacteria are the most abundant life form in most soils and are responsible for the decay of the residue from crops. Certain bacteria convert ammonia in soils into nitrogen, a fundamental plant nutrient. Some algae perform the same function (assuring a nitrogen supply) in rice paddy soils. Algae are numerous on the surfaces of moist soil. Fungi teem in soils, and range from several celled fungi to the large wild mushrooms that grow on moist soil. Fungi are capable of decomposing a greater variety of organic compounds than bacteria.

Nematodes are also abundant in most soils, and these eel-shaped, colorless worms are slightly larger than bacteria, algae, and fungi. An acre of soil may hold as many as 1 million nematodes. Most nematodes feed on dead plants, but some are parasites, and eat the roots of crops such as citrus, cotton, alfalfa, and corn.

Ants abound in soils. They create mazes of tunnels and construct mounds, mixing soils and bringing up subsurface soils in the process. They also gather vegetation into their mounds, which, as a result, become rich in organic matter. By burrowing and recolonizing, ants can eventually rework and fertilize the soil covering an entire prairie.

Earthworms burrow through soils, mixing organics with minerals as they go, and aerating the soil. Some earthworms pull leaves from the forest floor into their burrows, called middens, enriching the soil. The burrowing of the 4,000 or so worms that can inhabit an acre of soil turns and aerates soil, bringing 7-18 tons of soil to the surface annually.

Larger animals inhabit soils, including moles, which tunnel just below the surface eating earthworms, grubs, and plant roots, loosening the soil and making it more porous. Mice also burrow, as do ground squirrels, marmots, and prairie dogs; all bringing tons of subsoil material to the surface. These animals all prefer dry areas, so the soils they unearth are often sandy and gravelly.