This book summarises the up-to-date knowledge on different fractions of amino acids in soil, input of amino acids to soil from both dead and living plant material, different extraction procedures done in order to release amino acids from soil, and the distribution of amino acids into soil profiles in regards to quality and quantity. In addition, this book provides detailed information on the quality and quantity of differently bounded fractions of amino acids in soil, land use and its effect on availability of amino acids and the description of proteolysis in soil and the release of amino acids by these processes.