Soil and cornstalk testing has been used to evaluate nitrogen (N) management practices used during corn (Zea mays L.) production, but there is need to search for more practical methods. New yield monitoring technologies are rapidly being accepted by producers and offer an alternative method to evaluate N management practices. The objective of this thesis was to explore the potential and limitations of applying extra N in reference strips during the growing season and measure yield responses to evaluate the performance of N management practices used by producers who apply all N before or at planting. Field studies were conducted at 66 sites where extra (i.e., in addition to that normally applied by producers) fertilizer N was applied in replicated strips going the length of the field and yield increases were measured by using yield-monitoring combines. The late-spring test for soil nitrate and the end-of-season test for stalk nitrate were used to help explain why yield increases were, or were not, observed. Amounts of spring rainfall indicated that losses of N were near long-term means. Mean yield increases to the extra fertilizer N were not great enough to justify the expense of the fertilizer and application. This finding indicates that application of extra N is not likely to be profitable unless responsive sites can be predicted before fertilization. The soil and stalk tests showed agreement that some sites were deficient of N even though yield increases to the extra N were not observed. This finding suggests that the extra fertilizer N did not become available to the plants at some sites. This observation offers a new explanation for why the soil test often does not accurately predict yield responses to applied N and suggests that simultaneous use of soil nitrate testing in late spring, cornstalk testing at the end of the season, and measuring yield responses to N applied in strips during the growing season may help identify in-season fertilizer practices that are most efficient.