The chapter explains how soil organic matter, or humus, is formed mainly through biological processes involving plants, animals, and microorganisms in the soil. It discusses how decomposition and synthesis contribute to the formation of soil organic matter. Microorganisms break down plant and animal residues, using some of it to build their own bodies, which then become part of the soil organic matter. The synthesized material has similar elements as the source materials but can differ in physical and chemical characteristics. The microbial portion of soil organic matter is significant, even though it may not be easily distinguishable.