ABSTRACT
Crop yields in developed countries have increased dramatically since World War II. In the 1960s, food production increased dramatically through the Green Revolution, which was based on high-yielding varieties of wheat and rice that responded to high inputs of nitrogenous fertilizers and irrigation. Farmers and agricultural scientists rarely consider how the amounts of fertilizer they use affect pests, diseases, or weeds. In low-input systems of crop production a number of component techniques in addition to the main inputs are used: rotations, machinery inputs, and organic matter inputs. New crop varieties that respond to high levels of nitrogen are a major reason for the increased crop yields produced currently in developed countries. Sustainable agricultural systems depend upon suitable manipulation of the components and on a better awareness of how these components can reduce chemical inputs. These systems also depend upon a much better understanding of how the major and other components interact with each other.
