Fertilizer nitrogen (N) usually is applied to Iowa cornfields before planting in late April or early May. Although it is well established that some of this N can escape from fields before the corn (Zea mays L.) begins rapid growth in June, the practical importance of these early season losses of N has not been established. This dissertation focused on learning more about the importance of early season N losses during corn production in Iowa. Three studies were conducted. The first involved a survey-type approach using the late-spring test for soil nitrate and the end-of-season test for cornstalk nitrate in hundreds of fields managed by farmers. The tests were performed over an eight-year period, and both tests showed a strong negative correlation between mean annual nitrate concentrations and March--May rainfall. The second involved coordinated field trials and laboratory incubations to characterize effects of the organic and inorganic N fractions of animal manure on N supplies for crop growth. The results indicate that early season losses of inorganic N should be considered a major factor determining the effects of the manure. The third study involved use of precision farming technologies in field-scale trials to evaluate the importance of N losses when farmers apply N only in the fall. The importance was evaluated by applying extra N in strips and measuring crop responses to this N. Large responses to the extra N were detected, and these responses suggest that losses of fall-applied N resulted in severe N deficiencies and costly yield reductions. The results of each study provided compelling evidence that early season losses of N are important during corn production in Iowa and that these losses could be substantially reduced by delaying N fertilization until plants are growing. The benefits of this change would include greater profits for producers and less environmental degradation associated with losses of N from fields. The results suggest that these benefits could be obtained if farmers used the new soil and tissue tests in organized programs designed to evaluate and improve the outcomes of N management on their fields.