An estimate of the number of species found in soil which have been more intensely studied is given in the table opposite. However, these estimates are still preliminary and much lower than the estimated total number of species within each group. For example, the described number of soil dwelling fungal species ranges from 18-35,000, while the projected number may be >100,000 (Hawksworth, 1991). Other organisms expected to be much more species-rich are the nematodes and mites, with perhaps only 3 and 5%, respectively, of the total species presently described (Walter and Proctor, 1999; Hawksworth and Mound, 1991). The estimates for bacteria and archaea species are particularly problematic because of the differences in opinion as to what criteria should be used to define a species, and the present unculturability of many of these organisms (Hawksworth and Kalin-Arroyo, 1995). Molecular biology comparing DNA sequences are beginning to answer some of these questions and remove many of the problem associated with the unculturability of many microbes (Spiegelman et al., 2005).

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