The development of European agriculture has always depended heavily on nitrogen supply. Until the middle of the 20th century nitrogen was probably the most important single factor. The paper deals with the introduction of clover in Europe, particularly in Denmark, where clover became a major crop between the 1740s and the beginning of the 19th century. The reasons for and the consequences of the disappearence of clover after 1945 are discussed. The author believes that, industrial agriculture being unsustainable, the 21st century will bring back a more sustainable clover-based agriculture.