This book investigates soil ecology and biodiversity for its ability to maintain a balance of beneficial organisms to support plant growth. This subject is discussed by a group of international authors in natural, agricultural and urban systems. The importance of biodiversity per se and, specifically, the feedbacks between the plant and soil biota in mediating soil function are emphasized. Examples are selected from allelopathy and invasive plant species along with the, hitherto overlooked, role of viruses in soil. The book is intended to provide a framework for a holistic understanding of the essential role of soil organisms in promoting plant growth.