Abstract: The components, structure, and function of soil organic matter are not yet fully understood due to its extraordinary complexity. The enduring fertility of Amazonian terra preta, developed over thousands of years by ancient indigenous peoples, and the results from two decades of biochar amendment studies both suggest that biochar is a critical matrix for fertile soil. Biochar can be considered a pyrogenic, particulate, humus-like substance. Its structural and functional similarities to soil humic matter—the most important component of soil organic matter—support this classification. Most amended biochar becomes free particulate organic matter, while a small portion may be incorporated into the heavy fraction of soil organic matter, either by combining directly with clay minerals or by transforming into humic materials. Converting plant biomass into biochar for use as a soil amendment may be a highly feasible approach to achieving low C or even negative C agriculture. Key scientific questions that require further investigation include the following: (1) How can a desirable root-zone soil structure be permanently established through biochar amendment to support crop root development? (2) How does biochar interact with soil clay minerals? (3) How does biochar interact with soil humic materials? (4) How does biochar drive the soil microbial C pump? (5) How does biochar mediate organic matter mineralization?