In the drive to manage nitrogen (N) more effectively, active crop-canopy sensors may become an option in your toolbox. But John Sawyer, Iowa State University agronomy professor, warns that they still have limitations, and more research is needed.

“Active canopy sensors measure emit light reflected from the crop canopy back to the sensor,” says Sawyer. “They don’t know why the intensity of reflected light is at a certain level; that is, why they get a particular reading. They don’t know if plants are N-deficient, have low biomass, if there’s a low population, poor soil on a hillside or other nutrient deficiencies.”

Speaking at an ISU Extension Crop Advantage program recently, Sawyer listed some issues he's found in research that looked at two types of active crop canopy sensors, the Greenseeker 505 and Crop Circle ACS. (Sawyer's research did not look at AgLeader sensors.)

  • Slight N deficiencies are hard for sensor systems to differentiate from non-N stress. As stress level increases, they’re better able to distinguish N stress.
  • Sensors can’t indicate if there is excess N available to the crop.
  • Sensors look at both the canopy biomass and N status as they pass through a field. And when a lot of biomass is present, they don’t do as well distinguishing N stress.
  • To be effective, sensing must occur within a limited time window: after the crop has had enough time to react to an N shortage, but when there’s still time remaining for it to respond to additional N application.
  • Calibration might look different for different sensors and different indexes. For growers, it’s essential to make sure the equipment and calibration are matched correctly. Without proper calibration, the system won’t interpret sensor readings accurately.
  • Sensing indexes are calculated from sensor measurements and are then converted to relative values in order to produce a meaningful assessment of N stress as part of the calibration process, but some indexes calibrate better with N fertilization needs than others. Better determination of relative indexes is needed, according to Sawyer, who says further efforts to refine the protocol for referencing non-N factors that limit growth are a “must.”

Do crop sensing and in-season N application work? he asks. “We know the plants can respond with a yield increase to more N, and our data shows a good relationship to N response [with sensor monitoring] when the plants are really N-deficient.

“What we are after these days is ‘enhancing N-use efficiency.’ We’re constantly looking for tools to improve efficiency, and there are multiple ways of thinking about that,” Sawyer says.

One thing his research did not achieve, however, was savings on total N use. Instead, sensor-guided total N application in-season in Sawyer’s test plots tracked with preplant-N application rates.

Growers using crop canopy sensors will still need to apply their knowledge of nutrient management, “and they will need a good user manual for these tools,” says Sawyer. “There are lots of thing you need to know – how to plug in the equipment, how high above the crop canopy the sensors should be placed, and how to deal with N-sufficient areas. It’s more complicated than just hooking on the tractor and going.”