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Spring Nitrogen Application Options

After a long winter and a delayed start to fieldwork, you may questioning which nitrogen (N) application method will be the most efficient this spring. You aren't alone.

“Questions are already coming about options for N fertilization this spring, and the usual question: Should time be taken to get N applied or plant corn and apply N later?” says John Sawyer, Iowa State Extension soil fertility specialist. 
Preplant applications
Planned preplant applications, which can be part of split applications or weed-and-feed systems, should be made if they can be completed without delaying planting. There are a few things to remember when making preplant N applications.

“Materials such as urea or UAN solution (urea-ammonium nitrate 28% or 32% solution) or polymer-coated urea can be broadcast and incorporated with normal tillage before planting,” says Sawyer. “Incorporate rather than leave the fertilizer on the soil surface to avoid volatile N loss from dry urea or urea in UAN. If time is critical and UAN application is to be made with preemerge herbicides, then surface application is an option, although more risky due to potential volatile loss and the applied N remaining on the soil surface (especially in no-till) if there is not sufficient rain to move it into the root zone.”

For UAN, if the field receives at least 0.25 to 0.50 inch of rain within two days of application, the chance of volatile loss will be eliminated.
“Another fertilizer option is polymer-coated urea, designed to delay urea release until soils warm,” says Sawyer. “To avoid runoff loss, incorporate into the soil. Surface broadcast options, especially fertilizers for no-tillage that generally do not have volatile loss concern, are ammonium nitrate and ammonium sulfate.”

Broadcast applications are additional options if you’re in a no-till system. However, there’s a higher potential for N loss or immobilization of N. 
If applying anhydrous ammonia in the spring, remember there is no exact safe waiting period before planting, as there’s potential to burn seedlings and roots. Anhydrous ammonia is a preferable N fertilizer for soils with a greater potential for N loss because it nitrifies more slowly than other products such as urea or UAN solution, says Sawyer.

“The risk of ammonia injury depends on many factors, with several that are not controllable,” says Sawyer. “For example, risk increases if application is made when soils are wet and then dry (ammonia moving up the injection track), with higher application rates, when soils with high clay content are wet (sidewall smearing of the injection track and ammonia moving toward the soil surface during application), and when soils are very dry and coarse textured (larger ammonia band).”

To reduce your risk, you can:

  • Wait to make applications until soil conditions are close to ideal.
  • Inject at a deep depth of 6 to 7 inches or more.
  • Wait several days after the application to begin planting.
  • If the injection placement relative to future rows can’t be controlled, apply at an angle.
  • If the injection track can be controlled with GPS guidance positioning technology, then split future corn rows; with this guided system, no waiting period is needed.
  • Anhydrous ammonia nitrifies more slowly than products like urea or UAN solution, so it's a preferable N fertilizer for soils with greater potential for losses in wet conditions.

Sidedress applications
If you are sidedressing, either because of a compressed spring or by plan, there are still a few things to check. Rates can be adjusted after completing a late-spring soil nitrate test. 

“Sidedress injection can begin immediately after planting if corn rows are visible or GPS guidance positioning equipment is used,” says Sawyer. “Be careful so that soil moved during injection does not cover seeded rows or small corn plants.”

Injecting in the middle of the row is the easiest, and there is no advantage to attempting to place the band close to the row, says Sawyer.

“Corn roots will reach the row middle at a small growth stage. Injected N can also be applied between every-other-row. That technique will provide equivalent response as when placed between every row,” says Sawyer.

If broadcasting urea, ammonium sulfate, or ammonium nitrate across growing corn, remember that leaf spotting or edge browning where fertilizer granules fall into the corn whorl may occur. “The chances of this happening increases with larger corn and higher application rate,” says Sawyer. “As long as the fertilizer distribution is good and not concentrated over plants, the leaf damage should only be cosmetic.”
“Broadcast application of UAN solution across growing corn has the potential to cause leaf burn and reduced early growth,” says Sawyer. “Depending upon the severity of damage, reduced plant growth may be visible for several weeks after application.”

Mid- to late vegetative-stage applications
Once the corn plants are too tall for normal sidedress equipment, high-clearance equipment can be used to make an application.

“The N source typically will be UAN solution, with equipment available to either dribble the solution onto the soil surface with drop tubes or shallow inject with coulter-shank bars (coulter-disk injected), and dry urea, which can be broadcast spread across the top of corn,” says Sawyer.  
“If attempts to get N applied preplant or early sidedress have failed, or there are concerns about N supply from early fertilizer or manure applications, then mid- to late vegetative-stage application can be a helpful rescue,” says Sawyer.

Sawyer recommends keeping several non-N limiting (approximately 50% more than normal rate) reference strips or areas in the field for comparisons. “These areas can be used to visually determine if corn would respond to additional N, or as a check to see if earlier N applications or carryover N is not sufficient,” says Sawyer.

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