Studies were conducted to acquire a better understanding of the effects of fertilizer nitrogen and cropping systems on the availability of nitrogen for corn (Zea mays L.) production in Iowa. The availability of nitrogen for corn production was assessed by analyzing soil samples collected when corn plants were 15 to 30 cm tall. The distributions of nitrate found in the soil profiles at this time were surprisingly similar in that the highest concentrations of nitrate always occurred in the top 15-cm layers of soils, and concentrations gradually decreased with increasing depth below the surface. This finding supports recent ideas that nitrate movement in soils is strongly influenced by preferential movement of water through soil macropores.;Amounts of nitrate found in the surface 30-cm layers of soils at this time were highly correlated with corn yields. This finding indicates that a late-spring soil test for nitrate has great potential as a practical tool to determine fertilizer nitrogen needs at sidedressing.;Differences in yields observed between corn after corn and corn after soybean (Glycine max L.) were largely explained by nitrogen effects. When expressed in fertilizer equivalents, the amounts of nitrogen carried over from soybean to corn exceeded the amounts indicated by the late-spring soil nitrate test. The differences in yield could be explained by considering the amounts of nitrogen required to decompose corn residues and the amounts released during decomposition of soybean residues. These observations suggest that continuous corn and corn in rotation with legumes need to be treated as separate categories in the late-spring soil test.;Analysis of soils from long-term rotation-fertility experiments showed that cropping systems and fertilizer nitrogen treatments caused significant differences in the organic nitrogen contents of soils during the past thirty years. The results emphasize a need for greater consideration of changes in soil organic nitrogen content when evaluating the efficiencies and environmental impacts of various cropping systems and fertilization practices.