CORVALLIS, Ore.—Soil is alive. Much more than a prop to hold up your plants, healthy soil is a jungle of voracious creatures eating and pooping and reproducing their way toward glorious soil fertility.

A single teaspoon (1 gram) of rich garden soil can hold up to one billion bacteria, several yards of fungal filaments, several thousand protozoa, and scores of nematodes, according to Kathy Merrifield, a retired nematologist at Oregon State University. Most of these creatures are exceedingly small; earthworms and millipedes are giants, in comparison. Each has a role in the secret life of soil.

Bacteria make up the largest group in the soil jungle, and they are as diverse as they are numerous. Some kinds of bacteria are responsible for converting atmospheric nitrogen to plant-available forms, a process known as nitrogen fixation. Actinomycetes, with cells like bacteria and filaments like fungi, are thought to contribute chemicals that give newly tilled soil its earthy aroma.

Mycorrhizae are fungi that form a relationship with plant roots and increase their ability to take up nutrients from the soil. These filaments, along with root hairs and other binding substances produced by bacteria and fungi, help hold soil particles together and keep soil from eroding.

Protozoa are single-celled, mostly motile organisms that feed on bacteria and other tiny organisms as well as each other. There may be thousands of them living in that teaspoon of soil. Protozoa release nitrogen, making it available to plants. As much as 80 percent of the nitrogen in plants can come from bacteria-eating protozoa.

Nematodes, simple roundworms, have evolved several feeding strategies. In temperate soils, some eat bacteria while others eat fungi or soil algae. Some nematodes attack plants, piercing plant cells and sucking out the contents. Some nematodes eat other nematodes or other small invertebrates.

Earthworms, giants of the soil jungle, mix and aggregate soil particles, creating deep channels that help aerate the soil and provide channels for growing roots. They shred and bury plant residue that stimulates microbial activity and increases the soil's capacity to retain moisture. Earthworms consume tiny soil organisms and excrete even more microorganisms in their castings.

The base of the soil food web is organic matter, material derived from living stuff that provides a source of energy stored as fixed carbon. Nutrients are "served" along with fixed carbon as carbon is converted to energy. Chemical fertilizers supply specific nutrients directly to plants, but they do not replace the other kinds of food that bacteria and fungi need. Soils with more organic matter tend to have more life. Mulching with compost, cover cropping and no-till farming practices tend to increase organic matter and thus increase the number and diversity of microorganisms in soil.

"All these things that live in the soil may seem unimportant," says Merrifield, "but they work together in a system that is truly the foundation of life."