Soil is an extremely complex habitat for microbes because of the heterogeneity of soil particulates, the constantly changing water and gas regimes, the discontinuity between microhabitats, the intermittent supply of nutrients, etc. The complexity of soil is reflected in the heterogeneity of its microbial inhabitants and in their fluctuations. Although the addition to soil of surface-active particulates may result in changes in microbial numbers and activities, this is not proof that surface interactions between the added particulates and the microbiota have occurred, as the effects of the added particulates could be, and usually are, the result of modifications in the physicochemical characteristics of the habitants of the microbes. Because of the present inability to distinguish between direct and indirect effects in soil directly, model systems are often used. However, these model systems are too often unrealistic, both in their simplicity and in their assumptions, and their relevance to what really occurs in soil in situ must be constantly and critically questioned. Furthermore, more attempts must be made to test and verify the results obtained from model systems in actual soil systems. (Copyright (c) 1986 Soil Science Society of America.)