The Morrow Plots, which were established in 1876 at Urbana, Illinois [USA], are the oldest experimental plots in America, and they contain the longest continuous corn (Zea mays L.) plot in the world. Changes in the content of N and organic C in the dark-colored soil on the Morrow Plots during 70 yr, 1904-1973, under different cropping systems and soil treatments, are reported. In comparison with continuous corn culture, crop rotations retarded the decline in soil N and organic C. However, crop rotations without soil treatment did not maintain the soils in a high state of productivity. Crop rotation plus appropriate fertilization produced the highest crop yields and also maintained soil N and organic C at the highest levels.