Comparison of Urea-N15 Uptake in Corn Applied to Foliage and Soil Surface.

Furrow-irrigated corn (Zea mays L.) production systems use raised beds, which allows water to flow downslope between rows to provide crop water needs not provided via in-season precipitation. Corn producers will often apply sidedress and pretassel N as a surface-broadcast application in the form of urea. In the absence of rainfall, the top portion of the beds does not receive adequate moisture through wicking to dissolve urea rendering it unavailable to the corn crop and more prone to loss. The objective of this research was to compare the uptake of N¹⁵-labeled urea applied in the corn whorl to that applied to the soil surface in the field and a controlled environment (greenhouse). Four N rates (0, 56, 112 and 156 kg N ha^-1) were applied within the whorl and directly to the soil and allowed to be taken up for 11 days. There was a significant difference in fertilizer N uptake efficiency (FNUE) across N rates and environments. Under field conditions <2% of the N applied to the soil surface was assimilated by the corn plant, but in the greenhouse ~7-11% was taken up. For urea-N applied directly into the whorl there were no significant differences across N rates under field conditions (7.0% FNUE). Whereas, within the greenhouse trial, there was a rate effect that indicated higher FNUE at higher N rates with 12.5, 16.2 and 16.6% for the 56, 112 and 168 kg N ha^-1 rates, respectively. This data indicates the importance of fertilizer placement on N availability and resultant FNUE in furrow-irrigated corn production. In the absence of rainfall, a significant portion of surface-applied urea which falls on the top of the bed remains unavailable to the corn crop potentially resulting in yield loss and reduced profitability.