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The Morrow Plots, which were established in 1876 at Urbana, Illinois, are the oldest experimental plots in America, and they contain the longest continuous corn (Zea mays L.) plot in the world. This paper reports changes in the content of nitrogen and organic carbon in the dark-colored soil on the Morrow Plots during 70 years, 1904 to 1973, under different cropping systems and soil treatments. In comparison with continuous corn culture, crop rotations retarded the decline in soil nitrogen and organic carbon. 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 nitrogen and organic carbon at the highest levels.
Odell et al. (Thu,) studied this question.