This research sheds new light on the much-debated link between agricultural productivity and development. We do so by estimating the causal impact of a large shock to agricultural productivity—the introduction of the heavy plow in the Middle Ages—on long run development. We build on the work of Lynn White, Jr. (1962), who argued that it was impossible to take proper advantage of the fertile clay soils of Northern Europe prior to the invention and widespread adoption of the heavy plow. We implement the test in a difference-in-difference set-up by exploiting regional variation in the presence of fertile clay soils. Using a high quality dataset for Denmark, we find that historical counties with relatively more fertile clay soil experienced higher urbanization after the heavy plow had its breakthrough, which was around AD 1000. We obtain a similar result, when we extend the test to European regions. Our findings substantiate that agricultural productivity can be an important driver of long-run development.