Determining nitrogen(N) requirements to maximize corn yields without wasting money can be a complex task.

Variables that influence N availability change yearly, highlighting the interdependence of soil types and health, climatic conditions, previous crops and input management, says Josh McGrath.

“If nitrogen went straight to the crop, then life would be grand,” says the soil management specialist from the University of Kentucky Extension. “But nitrogen has to pass through the nitrogen-cycle gauntlet, which has a lot of microbially mediated processes.” 

Determining the right N rate requires answers to some tough questions: What are the size of the soil N pools? What is the rate and timing of transformations between those pools? How quickly will organic N applied with manure transform to nitrate and ammonium?

These are particularly tough questions in humid regions like the rain-fed Corn Belt where, McGrath says, “soil testing and tissue testing aren’t particularly useful for N decisions.”

Precision’s Role

There are tools available to help bring more precision to N application. Active sensors that use Normalized Differential Vegetative Index (NDVI) technology correlate very well to total plant biomass early in the corn-growing season, McGrath says.

Through local research, equations can be developed to relate sensor readings to crop health and N requirements. In turn, the sensors can be used to guide variable-rate N applications (VRN) in crops like corn and wheat, varying rates across fields and across years in response to N need, he says.

MAXIMIZING NITROGEN. Josh McGrath has demonstrated that an economic optimum nitrogen…