Due to their buffer capacity, soils and ecological systems usually react gradually to different land use or climatic changes. Hence, effects are often quantifiable only after many decades. Long-term field experiments make it possible to recognize such long-term effects, which are caused e.g. by different fertilization treatments, for the respective location. This knowledge is essential for developing sustainable and ecologically desirable management concepts. With the “eternal rye” experiment, laid out by Julius Kühn in 1878, the Institute of Agricultural and Nutritional Sciences of the Martin Luther University Halle-Wittenberg has the second oldest long-term fertilization trial of the world after Rothamstedt (UK). In addition, four more long-term fertilization experiments as well as one soil development trial exist in Halle, all founded by Karl Schmalfuß in 1948/49. Wolfgang Merbach and Annette Deubel summarize the most important results and draw conclusions for the continuation of these internationally important experiments.