Despite the large number of papers and books published on soil organic matter (humus), our knowledge of the subject is still very limited, as is our knowledge of humic acid. The author of this book began to study humus at the end of the 1940s and continued until 1984 when he retired from Nagoya University. With the intention of establishing a systematic understanding of soil organic matter, he has compiled facts and a discussion of humus based on his extensive experimental results during the past 40 years.In this book, humic acids are classified into A, B, Rp and P types, based on their optical properties. The elementary composition and other chemical properties of humic acid types are shown to be regularly different from each other. A new method for humus composition analysis applied to various kinds of soils in Japan and several other countries indicates that the diversity of humus compositions of soils is systematically understandable. These findings lead the author to novel theories on the chemical configuration and formation of humic acids and humic substances. Diagenesis of humus under terrestrial conditions is illustrated as to the buried humic horizons of Black soil (Andosol).The book will be useful not only to soil scientists and agronomists but also to geochemists, oceanographers, limnologists, water scientists, biologists and chemists who are dealing with organic matter in terrestrial, aquatic, and sedimentary environments.