In 1865 Franz Ivanovich (Franz Joseph) Ruprecht, botanist and member of the St. Petersburg Academy of Sciences, published a groundbreaking investigation of the famous «black earth» soil. Whereas previous studies treated black soil as a geological formation, Ruprecht argued convincingly for its origin in the steppe flora. This article analyzes Ruprecht's way to the discovery, the social and intellectual contexts of his hypothesis, his new interpretation of microscopic analyses and the dating of samples. Although not all of his geological reconstructions were correct, his hypothesis laid the basis for the subsequent founding of soil science by V. V. Dokuchaev and the works by later generations of botanical geographers.