Contrary to what’s been considered the gospel for years, in regards to nitrogen (N) rate recommendations, Purdue University agronomists believe that the optimum N rate is strongly related to the soils’ capacity to supply N.

The soils’ ability to supply N is dependent on the amount of organic matter, drainage capability, rainfall, soil temperature, mineralization potential, leaching potential and denitrification potential, explains Jim Camberato, Purdue Extension soil fertility and plant nutrition specialist.

Camberato and Bob Nielsen, Purdue Extension corn management specialist, conducted trials over a two-year time span at seven Purdue research farms, 39 sites with a corn after soybean rotation and 18 sites with a corn after corn rotation.

“After conducting N rate trials in 2006 and 2007, we found the optimum N fertilizer rate is not strongly related to yield potential,” Nielsen says. “Or it could be said that higher-yielding fields don’t necessarily require higher N fertilizer rates.

“There was however, an excellent relationship between relative yield in each field and the total N available to the crop, which is the N the soil supplies plus the fertilizer. In some cases more than half of the N supplied from the crop originated from the soil itself.”

Camberato says when the new data is plotted out, yield plateaus at about 275 lbs./acre of soil plus fertilizer N.

“This means if we know how much N the soil supplies, we can subtract that amount from 275 and know how much additional N the plant actually needs,” Camberato says. “But predicting the amount of N supplied by the soil is difficult.”

The fact that the N cycle is dynamic, mineralization rates are hard to predict, rates of N loss are hard to predict and required weather data is hard or costly to obtain, all make it hard for a successful model to be developed, Camberato says.

Despite the challenges, Purdue University agronomist Brad Joern is working to develop a model that will estimate up-to-date mineralization and N loss variables.

Until then, optical reflectance sensors can be used to measure light reflectance from leafy crop canopies, which can be used to estimate the N status of plants and ultimately estimate how much additional N needs to be applied. Healthy, large plants reflect light differently than struggling, smaller plants and plants with adequate N reflect light differently than N-deficient plants.

“Optical sensors helps us recognize and quantify differences in the N content of plants in areas of a field,” Camberato says. “The N rate can be controlled manually or electronically to change application rates based on reflectance differences in a field.”

There are two primary commercialized sensors in use in the U.S, Crop Circle and GreenSeeker units, Nielsen says. Both units emit near infrared rays and visible light wavelengths.

Nielsen and Camberato say there are three rules to remember when working with optical reflectance sensors.

The first being optical sensor measurements and N content of corn relate best once the crop is well into its rapid growth phase – the V8 stage and beyond where approximately 60% of N uptake occurs. The uptake of N during the rapid growth phase is dramatic and differences in plant N levels become more evident to the human eye and especially to optical sensors.

“The down side of waiting to apply N after the V8 growth stage is that the corn plants are much taller, which limits the technology to operations that can apply fertilizer through irrigation water or with high-clearance applicators,” Camberato says.

Second, growers need to include a high-N reference strip for each hybrid in each field. These strips are used to help drive the predictive formulas for the sensors. Because hybrids vary naturally in their greenness, the sensor will need to be recalibrated for each one, Camberato explains.

Third, not all low-reflectance areas in a field are simply N deficient. Consequently, low-reflectance areas within a field need to be carefully interpreted by the operator or the sidedress applicator if the intent is to vary N rates on a site-specific basis. It’s important to keep in mind that not all low-reflectance areas will be no-brainers, like drowned out spots, Nielsen says.