Risk and Evaluation of Nitrogen Loss
Figure 1. Illustration of factors that can influence risk of N loss.Due to being highly mobile and having a higher risk of soil loss, corn plants often experience nitrogen (N) deficiencies more often than other nutrient deficiencies. When managed properly, appropriate N levels can result in substantial potential economic returns. Limited opportunities to apply N in the fall and spring often result in re-evaluating management plans in-season. If unable to apply a preemergence N application or if N loss is suspected, there are multiple in-season options that may help minimize potential yield loss. Determining how much plant available N is readily available or still needs to be applied can be difficult to determine. Not all N applied will be readily available to the plant depending on the N form applied, the application method, the application timing and any environmental-driven N loss factors, such as temperature or precipitation.
Figure 2. Excessive spring rainfall areas in 2013 that were more prone to N loss. Source: Midwestern Regional Climate Center.Commonly Used Methods to Identify Nitrogen Needs
Late spring or pre-sidedress soil nitrate test: Soil nitrate levels increase as soil temperatures warm and mineralization rates increase. Soil nitrate mineralization peaks in late May/early June (around V5-V6) making a soil nitrate test at this time from the top 12 inches of soil a good indicator of N needs. Soil test results of >25 ppm are less likely to need additional N.
Crop sensors: Using leaf light reflectance as an indicator for additional N needs has had success. Examples of tools to use include:
N models: Modelling software such as Adapt-N utilizes soil and live weather data in combination with prior crop management practices to estimate if additional N is needed.
Application Timing and Use
N demand of corn prior to V8 is relatively low – less than 25 lbs. N/acre. With roughly 10% of the total N typically being taken up by mid-June, most early N needs of corn can be met through mineralization of soil organic matter and prior season soybean residue in corn following soybeans rotations. It is more important to ensure at least a portion of N (around 40 lbs. N/acre) is applied before or soon after planting when planting corn following corn. N demand increases dramatically after corn begins to grow rapidly at the V6-V8 growth stages making it important to have most supplemental N applied prior to this timing. By silking, plants will have absorbed near 70% of the total N needed. Yield potential can decline rapidly if N isn’t available during this critical timing. Corn continues to use significant amounts of N throughout grain fill. Split applications of N can help minimize in-season N loss and improve N use efficiency if applied timely. Split applications can provide even more value in sandy soils that are more prone to leaching and have less opportunity to mineralize N from organic matter. To maximize yield potential and enhance plant nutrient use efficiency, time N application prior to peak uptake
Nitrogen Source Considerations
Anhydrous ammonia:
One of the most inexpensive forms of N available, anhydrous ammonia can be applied late fall, early spring or sidedressed in-season. Delaying fall applications until soils begin to cool (<50° F) is important to minimize loss. Adding a nitrification inhibitor may also greatly reduce loss if excessive spring rainfall occurs as soil temperatures begin to warm. Nitrification inhibitors are not needed when sidedressing in-season, however, poor closure of knife applicators can result in ammonia loss and leaf burning.
Urea (dry):
This form of N can be broadcast pre-plant or over emerged corn with a high clearance applicator at higher speeds to cover more acres per day. Consider using a urease inhibitor to minimize volatilization loss if it is not incorporated into the soil. Timing applications of urea prior to rain events or cultivation after application can help minimize N loss. Anticipate minor leaf burn from urea trapped within the whorl of emerged corn.
UAN (liquid):
This form of N can be broadcast sprayed, fertigated, injected into the soil or dribble-applied between rows. Apply UAN as close to the timing of crop uptake as possible since a portion of it is in the nitrate form which may be rapidly lost if excess rainfall occurs. Broadcast applications can cause leaf burn, therefore, applying UAN to small corn (<V4) and at lower rates is advised unless able to sidedress. Leaf burn usually disappears quickly with good growing conditions. Sidedressing UAN to emerged corn with coulters or Y-drop applicators can reduce crop damage risk and achieve higher N use efficiency.
Product performance assumes disease presence.
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