Corn yield response to nitrogen at multiple in-field locations

Improving N management for corn (Zea mays L.) production with precision agriculture technologies requires that spatial N recommendations adequately represent in-field variability in N availability. Our objective was to evaluate corn response to increasing N rates in several in-field locations that represented the range of soil organic matter (OM) content in the field. In a 2-yr study, three center pivot-irrigated fields were selected in south-central Kansas and south-central Nebraska. Four or five locations were selected within each field. At each location, five or six N treatments (0-336 kg N ha^-1) were surface-applied early in the growing season. The minimum N rate to achieve maximum yield varied by as much as 130 kg N ha^-1 among in-field locations at three site-years. The least amount of N to achieve maximum yield did not coincide with locations representing greater soil OM. Yield response at two site-years was the same among in-field locations; however, mean yield among in-field locations varied by as much as 4.2 Mg ha^-1, representing potential for improvement in N use efficiency. Leaf tissue N was below the critical threshold for 60 to 100% of observations at three different in-field locations but below the critical threshold for <35% of the observations at all other in-field locations. The reason for the discrepancy in N availability among in-field locations was not conclusively identified but was not only related to soil OM content. Variable N recommendations based only on soil OM is too simplistic to reflect variability in N availability within a field.