By Ricardo Costa
Nitrogen (N) is the most commonly applied nutrient and one of the most expensive inputs in corn production, averaging from 13 to 18% of the variable costs depending on the type of crop rotation. On the other hand, the nitrogen cost-to-benefit ratio usually exceeds that of other fertilizer inputs.
Nitrogen can be lost in several ways such as through volatilization, denitrification and leaching. It is best to apply nitrogen when corn has its greatest need for this nutrient (in general, 30 to 45 days after emergence) to avoid unnecessary nitrogen losses. For example, 30% of all your broadcast applied urea followed by six days without rain can be lost (these losses can be even higher in sandy soils). For more information on corn nitrogen guidelines, see these Michigan State University Extension articles by MSU soil fertility specialist Kurt Steinke: “Corn nitrogen guidelines and focusing on the right rate
” and “Targeting corn nitrogen strategies for improved resilience.”
To minimize losses, corn producers can apply a smaller amount of nitrogen at or before planting and come later with a sidedress application. Since nitrogen levels can change drastically after the first application, producers can rely on a variety of methods such as the presidedress nitrate test (PSNT) and the chlorophyll meter
readings among others to help determine how much nitrogen to apply for a maximum return to nitrogen application. The goal is to make sure crop yield is not limited by lack of nitrogen throughout the life cycle.
A crop sensor can detect wavelengths of reflected light from the crop canopy and produce a normalized difference vegetation index value called NDVI that is correlated with leaf chlorophyll. Based on this information, sidedress nitrogen rates that are aligned with site-specific crop needs can be prescribed.
Establish a high-nitrogen reference area early in the season in each field to be tested (a small area where more-than-adequate nitrogen was supplied for the entire season). Readings at the high-nitrogen reference area can be taken after corn reaches the V6 stage, and the sensor should be held 24 to 48 inches above the crop. To obtain a representative reading, walk with the sensor while keeping the trigger engaged and maintain a consistent height above the target. The display updates continuously, providing an average when the trigger is released.
After measuring the high-nitrogen reference area, don’t forget to record the readings (values range from 0 to 1). You will need to measure the non-reference area as well following the same steps discussed above.
Use the chart provided by Trimble
to estimate the nitrogen fertilizer rate. Locate the curve closest to your high-nitrogen value using the legend in the normalized rate graph (A); find the number that is closest to your non-reference area on the bottom X-axis (B); find where the colored line meets the bottom number and trace back to the Y-axis and record the number found (C).
On the maximum yield chart, select your crop and the expected yield in bushels per acre (bu/ac). Find where the row and the column meet and multiply the found number by the number you found previously on Y-axis of the graph. The result will be the amount of nitrogen in pounds per acre you will need to sidedress to achieve your yield goal.
Among the three techniques discussed during the 2019 MSU Ag Innovation day (PSNT, cholorophyll meter and the GreenSeeker), the GreenSeeker is the most user-friendly since it doesn't require taking soil samples or focusing on a specific corn leaf.