While it is recognized that farming alternatively is inherently knowledge intensive, in the United States, farmer knowledge has been widely overlooked and under-documented within the scientific literature. Farmer knowledge of soil in particular is understudied in the US, especially given that healthy soils have been identified as the basis for resilient agriculture. Applying an exploratory, case study approach, we interviewed 13 organic farmers based in Yolo County, California to understand how organic farmers in this region acquire knowledge about their soils, to document what organic farmers in this region know about their soils, and to share key management practices organic farmers use to build soil health in the region. We found the organic farmers in this study acquire knowledge about their farming systems primarily through direct observation, personal experience, experimentation, and inherited wisdom. To evaluate soil health, farmers in this study cited using a range of indicators, including soil structure, crop health, growth habits of weeds, and soil biology. We found that these organic farmers possess extensive place-based knowledge of their local farming systems, and that this knowledge base represents an important source for innovation and adaptive management in scientific and policy-making contexts.