Curves used to describe observed relationships between incremental differences in rates of fertilization and yields of corn grain are commonly called crop response models. Such models are widely used to identify optimal rates of nitrogen (N) fertilization for corps. A major problem is that relationships between rates of N fertilization and yields vary greatly with many different factors that vary from site to site. Marked variability in these site factors makes it difficult to use observations made in the past to predict rates needed in the future;Data from 70 different response trials were used to explore the possible benefits of modeling crop responses to N by modeling relationships between concentrations of soil nitrate in late spring and concentrations of nitrate in cornstalks at the end of the season. The overall hypothesis was that relationships between indexes of N availability in soils and indexes of N sufficiency in crops should be less influenced by site conditions and, therefore, more useful in modeling crop responses to N;Nonlinear relationships between observed soil and stalk nitrate concentrations were identified as a major problem during data analysis and presentation. This problem was solved by developing a formula for transforming stalk nitrate concentrations to index values that tended to linearly relate to soil nitrate concentrations. These index values also tended to linearly relate to rates of N fertilization and were found to be very useful for identifying optimal rates of N fertilization. The concentration of nitrate required to attain the optimal N-sufficiency level in the crop was surprisingly stable across sites;Analyses showed that relationships between the index values and soil nitrate concentrations or rates of fertilization offer great potential as a tool to aid in explanation of data collected in N response trials or in interpretation of stalk nitrate concentrations in cornfields. More studies are needed to fully explore the potential and limitations of the new index.