Long-term agricultural field experiments are essential for quantifying changes in soil properties and associated crop productivity that occur slowly but continue over long periods. Initiated in 1894 in the South of Denmark, the Askov long-term experiment (LTE) is among the few LTEs in the world that continued for more than 125 years. The experiment compares different rates of nitrogen, phosphorus, and potassium applied in mineral fertilizers or animal manure with abundant treatment replicates. With well-managed treatments, detailed yield records, and a soil archive dating back to 1923, today the Askov-LTE represents a unique platform for research not envisioned at the start of experiments. Here, we provide a short description of site characteristics and experimental layout, examples of historic crop yields and changes in soil carbon (C) content. We provide short overviews of some of the studies that have used the Askov-LTE as a research platform. These include crop yield-related research; models simulating changes in soil C; availability of soil P reserves; indicators of biological, chemical, and physical soil quality; and studies related to prehistoric archaeology. Finally, we offer some reflections on long-term field experimentation.