This book discusses the association that exists between plants and their most important dietary component, nitrogen. The author combines ecological, physiological and biochemical approaches to provide the reader with an overall view of nitrogen in the biosphere and a specific view of nitrogen processing in plants. The processes which make up the nitrogen cycle, including mineralization, immobilization by microbes and nitrification, are discussed and the losses and gains of combined nitrogen from and to the cycle. The part which plants play in this cycling, by their processing of inorganic nitrogen into compounds which are required by plants and animals alike, and the chemistry and production of those compounds, is also covered. Transport of nitrogen compounds within the plant, and the fate of these compounds, is discussed. The final chapter considers the part which humans play in the cycling of nitrogen, with special reference to the nitrogen fertilizers used in agriculture.