A distinct microbial community is established in a given environment as a consequence of the community evolution of individual component microorganisms. According to the "Community Theory", microorganisms proliferate not as individual species but as a community in response to changes in the environment, evolving into a biological network that is most suitable for their survival and/or further growth. During this process, individual microorganisms interact and communicate with each other by sharing their limited genetic information in order to exploit available resources. Consequently, a functional community comprising many different species is formed. Such microbial communities are comparable to multi-cellular organisms in many ways, but often too complex to be characterized. Nutrient enrichment is one approach to the study of the functions of these inextricably linked biological networks. For soil microbial communities, amendment with organic matter can be employed to apply selective environmental advantages in order to transfer microbial communities to newly evolved biological consortia with distinctive functions. Many studies have reported that the incorporation of composts or green manures rendered soils suppressive to various soilborne pathogens. Among the pathogens that are controlled effectively are Pythium, Phytophthora, and Rhizoctonia. Soil enrichment with organic matter may be the most fundamental and sustainable approach to the biological control of soilborne diseases.