This book presents a systematic and analyzed available - own and published data on the content, composition and stocks of organic matter of the main soil types in Bulgaria. It is investigated variability of soil humus systems and their comparative characteristics by statistical methods of analysis and established relationships between some parameters associated with the processes of humus formation and equations are derived describing the relationships established. It is summarized available information of organic carbon composition; stocks in the soils of Bulgaria defined layers 0-25 cm, and the whole soil profile (0-100 cm), based on the data of the organic carbon content and bulk density, counting the soil surface of each soil type occupies on the territory of the country. Soils in Bulgaria are grouped according to the composition of soil organic matter and humus stability system.Details of this work can be used: -by university and PhD students; researchers working on SOM; -intheoretical developments concerning processes of humus formation; -for for applicable purposes in drawing up models for the development of agriculture of the country and sustainable land management.