There are between 100 billion and 3 trillion organisms in very kilogram of fertile soil, most of it bacteria. The soil is a thin layer of perpetually decaying organic matter that covers much of the Earth. Within the soil, organisms break down dead organisms into their constituent elements, ready to be reabsorbed by plants. Without soil life, new generations of plants would be unable to recycle the biomass of the last generation, and life on Earth as a whole would cease.
Soil life is generally classified by its size. At the top of the food chain are the megafauna, greater than 20 mm in size: moles, rabbits, and rodents. Below them are the macrofauna, ranging in size from 2-20 mm: woodlice, earthworms, centipedes, snails, beetles, slugs, ants, and harvestmen. Then are the mesofauna, ranging in size from 100 microns-2 mm: tardigrades, mites and springtails. The smallest are microfauna and microflora, with a size range between 1-100 micrometers: yeasts, bacteria, fungi, protozoa, roundworms, and rotifers. Even below that are many trillions of viruses, although there is disagreement on whether these mobile genetic elements are truly life.
Over many millions of years, bacterial soil life has formed a deep symbiotic relationship with the plants known as legumes (beans, peanuts, alfalfa), growing in their roots and "fixing" atmospheric nitrogen, which is a gas, into solid organic compounds such as ammonia. Nitrogen-fixing bacteria also live independently in the soil in large quantities. This nitrogen fixation is a function crucial to all life which, until the development of the Haber process, only bacteria could perform. Nitrifying bacteria specializes in converting ammonia, the first product, into nitrates, which can actually be used by plants for nutrition.
Depending on the severity of the environmental conditions, soil life may be more or less abundant. When conditions are harsh, such as in Antarctica, the large organisms are the first to go. In what little there is of Antarctic soil, megafauna and macrofauna are absent, but mesofauna is present in the form of springtails.
If you want to see soil life go to work right in your backyard, throw a ripe fruit there and wait a few days. The effects should be noticeable shortly.
Michael is a longtime WiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.