Approximately 25% of Iowa's annual 12.5 million acres of corn is grown where corn was grown the previous year. With adequate N fertilizer, corn following corn usually yields less than corn following soybeans or some other crop. The magnitude of this yield reduction usually is about 10%, but it varies between fields, locations and years. During 1988 in southeastern Iowa, farmers and researchers reported yield differences as great as 100 bufacre between the yield of corn after soybeans and corn after corn. There are many possible explanations for the rotation effect such as differences in residual soil moisture, soil nitrogen, soil compaction or structure, soil microflora, corn root worm and diseases. Sometimes the yield difference can be related to one or more of these explanations, but not with any consistency. It becomes impossible to characterize the interaction of these various factors with each other in highly variable field environments. Many studies have been conducted attempting to relate substances left by corn which is toxic to the following corn crop or stimulatory substances left by soybeans; this phenomenon of one crop affecting another crop by substances left in the environment is called allelopathy (Anderson et al. 1988).