"What these low-lying creatures lack in size, they make up for in numbers. Consider bacteria, the soil microbes with the highest numbers, for example. You can fit 40 million of them on the end of one pin. In fact, there are more soil microorganisms (microbes for short) in a teaspoonful of soil than there are people on the earth. These microbes, which make up only one-half of one percent of the total soil mass, are the yeasts, algae, protozoa, bacteria, nematodes, and fungi that process soil into rich, dark, stable humus."
Source: U.S. Department of Agriculture, Natural Resources Conservation Service, “Healthy Soils Are: Full of Life,” 1, accessed July 30, 2014, http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1193147.pdf.