Soil is the most abundant ecosystem on Earth, but the vast majority of organisms in soil are microbes, a great many of which have not been described. There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely. Estimates range from 50,000 per gram to over a million species per gram of soil of soil. The total number of organisms and species can vary widely according to soil type, location, and depth.