The surface horizon of the soil profile, typically the top 2 to 8 inches, where the highest concentration of organic matter, microorganisms, and plant roots exists. It holds the primary nutrient reserves and moisture required for crop growth. Preserving this layer from wind and water erosion is the central goal of conservation agriculture.
Heard on the job
We lost half an inch of topsoil during that flash flood because the field was left bare after harvest.