Nitrogen Nutrition Indicators in Corn Fertilized with Different Urea‐Nitrogen Forms

Plant‐ or soil‐based N nutrition indicators have been tested with conventional N fertilizers, but little has been done with enhanced‐efficiency urea‐N fertilizers applied only at planting. This study aimed at analyzing the performance of in‐season plant‐ and soil‐based indicators in assessing the N nutrition status of corn ( Zea mays L.). The N nutrition index (NNI) based on whole‐plant N concentration, the uppermost collared leaf N concentration, chlorophyll meter (CM) readings, soil NO 3 ‒ –N fluxes on exchange membranes, and soil NO 3 ‒ –N content were measured during the growing season in corn fertilized at planting with polymer‐coated urea, nitrification inhibitor urea, and granular dry urea at 0, 75, and 150 kg N ha ‒1 . The NNI values at the V12 stage explained a significant portion of the variations in grain yield due to urea forms and rates ( R 2 = 0.83). Leaf N concentration and relative CM readings from the V8 to V12 stages were significantly related to NNI ( R 2 = 0.47 – 0.90), indicating the sensitivity of those measurements to changes in soil N availability due to urea forms. Soil NO 3 ‒ –N content at the V12 stage was poorly related to NNI, but soil NO 3 ‒ –N fluxes at the V6 and V12 stages were related to NNI at the V12 stage ( R 2 = 0.42–0.61). Our results confirm that, with three urea sources applied at planting, NNI at the V12 stage as well as leaf N concentration and relative CM readings are reliable indicators of the level of N nutrition during the corn growing season. Core Ideas Plant‐based nutrition indicators were sensitive to soil N availability changes due to urea forms. The NNI at the V12 corn stage largely explained variations in grain yield. Leaf N concentration and relative CM readings at the V8 to V12 stages were related to the NNI. Plant‐based nutrition indicators are reliable indicators of the level of corn N nutrition.