Fine Tuning N Application In Corn

Jun 10, 2015

Nitrogen (N) management in Ontario corn tends to rely on pre-plant N applications broadcast on tilled soil and incorporated before planting. While this practice "gets the job done", it poses issues with efficient N use and doesn't offer much flexibility in managing N requirements. When all the N is applied early in the season, the rate selected is based on past experience with the field. This doesn't provide any opportunity to tailor the rates to the conditions of the season.

Some corn producers have long applied N in split applications. A small portion of the N is applied pre-plant or through the planter, and the remainder is applied as a side-dress when the corn is in the 4-10 leaf stage. This allows the early "starter" N to carry the crop into June when the spring weather, planting conditions, anticipated conditions for the remainder of the season, and yield expectations allow the producer to estimate the amount of N needed to finish the crop. This offers significant opportunities for N optimization compared to the pre-plant system.

While many side dressers estimate N rates by looking at the crop and the weather, some are using other tools to optimized in-season N rates. These tools include:

  • the Pre Side-Dress Nitrogen Test (PSNT), and
  • more recently, optical sensors.


The PSNT gives an estimate of the N available by taking a 30 cm (12 in) composite soil sample in early- to mid-June. Given the spring weather, varying amounts of available N will be mineralized from the organic N pool present. The N amount in any year is a function of soil type, cropping history, tillage, seasonal temperature and moisture. The limitation of the PSNT has been the amount of test variability within a field and between fields. Research is continuing to determine how and when N mineralizes and what conditions (ie rainfall) impacts its availability. Follow the new GFO supported Sentinel Nitrogen Project tracking soil nitrate levels at 8 Ontario Corn Performance Trial locations.

Optical Sensors

Sensor technology for N status in plants has been available for some time. The biggest issue is that they measure only the plant condition and not the soil. Sensing the plant N status at a moment in time does not necessarily relate to the amount of mineralized and available (or soon to be available) N in the soil. Just because the corn looks N deficient doesn't mean that the amount of N available in the soil is low. Other possibilities might be limiting the plants ability to access the N, or organic N is not yet mineralized and available. Using an early installed N-rich strip for sensor calibration does not provide any indication of the background organic mineralized N from the organic pool.

The other limitation is that the optical sensor technology relies on reading the reflectance of light from plants and then interpretation. Sensing has to occur relatively late so that there is enough plant material in the field to provide reflectance. The sensors do not collect data from light reflectance from bare soil. Sensing optimization has usually occurred beyond the crop stage of corn growth when additional N can be applied without damaging the crop. (Additional information is available at The introduction of technologies like Y-drops and high clearance applicators with soil injectors has made this less of a problem, but research supporting this practice remains scarce. Dr. Peter Scharf, University of Missouri, suggests that response to late N is a function of overall N stress ( and His research suggests that If earlier N applications were sufficient to meet crop needs, the response of late N will be negligible and normal side-dress N gives the best yields.

Greg Stewart and Ben Rosser wrote a good article on use of optical sensors, specifically the Green Seeker ( Other interesting articles can easily be found with a quick web search.

Use In Precision Ag

Dr. Raj Khosla (Colorado State University) is well known for his research on precision agriculture. At the South-West Agricultural Conference, he discussed the integration of optical sensors into the spatial management of crops. ( While his research on spatial management has shown benefit, the use of management zones is only half the picture. Spatial management of the crop within zones really addresses the soil. Both the soil and the crop must be accounted for equally in the data gathering and decision making. The soil is not currently addressed in optical sensor use in Ontario. A combination of crop sensing + soil sensing is required to determine input rates to economically optimize the system.

Figure 1. NDVI readings collected with an optical sensor driven across a field and N rate selected based on whole field management decision support methodology (Raj Khosla, Colorado State U, SW Ag Conference, 2015).

In Figure 1, the Normalized Difference Vegetation Index (NDVI) reading, the measurement unit used in current optical sensors, was the same at 3 locations in the field. The NDVI readings indicated, starting from the bottom of the picture, are from high, mid and low elevation positions in the field. Using a whole field approach to N rate decisions, the rate that would be selected using the equation suggest an optimum broadcast rate of 96 lbs/ac across the field.

If we take into account the field variability, the N rate decision is significantly different. Figure 2 overlays the management zone map on this field. Despite the same NDVI reading being recorded at 3 locations, they are actually within 3 separate management zones.

  • The "Low" response management zone is in a higher elevation position and on a slope. This area is likely to run out of water before other parts of the field. It has lower anticipated yield potential, so its ability to utilize additional N is not as high, Therefore the optimal rate of N is only 37 lbs/ac.
  • The "Medium" zone at the low slope position is where water will flow to and where there is likely higher organic matter providing higher amounts of natural mineralized N to the crop. Although the yield potential of this area is high, its response to added inputs is lower since it naturally is better able to meet the crop needs. The N rate in this zone is optimized at 92 lbs/ac, because the soil in this zone will supply a significant amount of N from natural pools or organic matter.
  • The "High" response management zone is where there is the most potential for the crop to respond to added inputs. It doesn't have quite the water and natural N as the medium response area, so needs higher input rates. A 144 lb/ac N application rate is suggested.

Figure 2. Identification of Management Zones by response potential to added N, and N Rate by Zone for same NDVI's when Site Specific Management Used (adapted from Raj Khosla, Colorado State U, SW Ag Conference, 2015).

Figure 2 illustrates how N rates can be optimized by combining plant sensing (optical) with soil sensing (precision ag spatial management zones). Areas with high responsiveness to added inputs get more, and those with low potential to take advantage of added nutrients get less.

Source: OMAFRA

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