I always cringe when I hear people refer to soil as "dirt." Soil is much more, loaded with all manner of organisms. A gram of ordinary soil, a pinch, is a clump that includes quartz grains (sand) laced with decaying organic matter and free nutrients and about 10 billion bacteria. How many species? Take one-millionth of the pinch - spread it evenly over culture dish. Norwegian microbiologists Jostein Goksoyr and Bigdis Torsvik found between 4,000 and 5,000 bacterial spp in a single gram of beech-forest soil. A similar number, with little or no overlap of species, was found in gram of sediment from shallow water off coast. There are also lots of fungi, viruses, insects and crustaceans, spiders, and of course earthworms and other types of worms. They all have a function in the complex soil ecology. Soil is really amazing stuff and of course our lives depend upon it because most of our food starts there!