http://extension.oregonstate.edu/news/story.php?S_No=344&storyType=garden
CORVALLIS - 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 1 billion bacteria, several yards of fungal filaments, several thousand protozoa, and scores of nematodes, according to Kathy Merrifield, nematologist at Oregon State University. Most of those creatures are exceedingly small. Compared to these Lilliputians, earthworms and millipedes are giants. 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 attach to plant roots and increase their ability to take up nutrients from the soil. These filaments, along with root hairs and other binding substances help hold soil particles together prevent erosion. Protozoa feed on bacteria and each othe