The identification of quantitative fertility indicators for evaluating the sustainability of cropping and farming systems has become a major issue. This question has been extensively studied by the German agronomist Albrecht Daniel Thaer at the beginning of the 19h century. In this paper Thaer's work is set in its historical background, from the end of the 16th century (Palissy, 1580) to the middle of the 19th century (Liebig, 1840). Then the paper focuses on Thaer's quantitative and complex fertility scale (expressed in “fertility degrees”), which was based on soil properties, on the requirement of nutrients by plants, and on the cropping system (including crop rotation). Thaer expressed soil fertility and economic results as a function of rye production in “scheffel of rye per journal” (ca. 200 kg per hectare). He also proposed a scale to describe the intrinsic fertility of soil. Thaer used this approach to assess the effect of major German cropping systems on soil fertility. He applied it to eight theoretical systems and nine existing systems in a true modeling approach. Thaer completed the fertility evaluation for the nine existing systems with a detailed economical analysis commenting the limits and potentialities of each system. Thaer's approach was used with success during half a century as it combined numerous empirical findings on soils and fertilization with organic substances in a sophisticated model. Unfortunately and despite effective practical applications, the scientific foundations of Thaer's “Humus Theory” proved definitively false as soon as 1840 when Sprengel and Liebig published on mineral nutrition of plants. Thaer's work deserves to be rediscovered since it approaches the modern issue of the sustainability of cropping and farming systems.