Plant Tissue Tests and Use of Sensors for Nitrogen Recommendations for Corn.

Chemical analysis or spectral properties of plant tissues may be used to evaluate the sufficiency of the soil + fertilizer N supply, and to estimate the amount of additional N needed.  These evaluations are often more accurate than similar evaluations based on soil analysis, yield goal, or mass balance.  However, plant-based approaches can only support in-season N application and rate decisions.  This is at odds with current practice among the vast majority of producers in the Corn Belt, but can fit well with practices in the southeastern U.S.  In-season N applications can minimize the risk of N loss and accompanying yield loss, as occurred across wide swaths of the Midwest in 2008, 2009, 2010, and 2011.  However, they bring risk associated with limitations in the height of application equipment, and perceived risk of lost yield potential due to early-season N stress.  Real-time variable-rate N applications based on canopy reflectance sensors are among the most promising approaches for managing documented spatial variability in how much N fertilizer is needed.  The need for a high-N reference area is an obstacle to using this approach.