The aim of this book is to contribute to the understanding of the laws of formation and function of microbial associations in natural and agricultural soils, and to build a scientific basis for the control of soil biological processes. The book highlights the possibility of utilizing certain microorganisms and microbial processes to increase soil fertility and protect the environment. The results are compared with and supplemented by those from the literature after a critical evaluation. The authors do not guarantee that the list of publications is complete. They are aware that such a multidisciplinary subject as soil microbiology must gather knowledge from various other disciplines, and thus it is most probable that some significant aspects or important data have been omitted.