You may look down your nose at dirt as a filthy mess, but the Earth's soil ecosystem is crucial to life on this planet. The soil is inhabited by a range of microorganisms -- as many as 10 million living things per gram, primarily bacteria and fungi. In fact, there are more living organisms in a teaspoon of dirt than there are people living on Earth, which is 7.3 billion people and counting.
The list of soil's denizens is long and includes bacteria, fungi, algae, viruses, nematodes and protozoa. These organisms are tasked with processing organic matter and turning it into dark, rich, fertile soil suitable for sustaining all kinds of life. And that's not even taking into account the other critters living there, such as microscopic insects, earthworms, beetles, ants and mites.
More about soil: