The book is divided into three parts that are logically connected. The first part defines the principal characteristics of the subterranean world and describes the microorganisms that live there as well as the environmental constraints they are subjected to. The second part shows how the action of the microorganisms can modify the physico-chemical surroundings, the microbiological equilibria and the growth of plants. The third part dwells on a few methods of intervention that would help in limiting the proliferation of harmful microorganisms and how to make the best use of the activity of auxiliary microorganisms.