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Answers to 5 Spring Nitrogen Questions

Consider these factors as you manage spring N during this crazy spring.

After crazy weather in early spring, it’s finally time to get out in the fields. University of Minnesota (U of M) soil fertility specialists and Extension educators have fielded plenty of questions lately about spring nitrogen (N). Here are five questions and answers about spring N from Fabian Fernandez, U of M nutrient management specialist; Jeff Vetsch, U of M soil scientist and Brad Carlson, U of M Extension educator.

1. If I applied spring anhydrous, how long after application should I wait to plant?

As long as there is separation between the anhydrous band and the seed, you can plant right after application.

Anhydrous ammonia is a gas at the injection point. Immediately after injection into soil, the gas expands and creates a high ammonia and ammonium zone in a round shape roughly 1 to 5 inches in diameter. The size depends on factors like:

  • Soil moisture
  • Texture
  • Rate of application

In general, though the vertical movement is about two inches towards the soil surface. Anhydrous ammonia applied 7 inches deep typically has sufficient separation between the ammonia retention zone and where the seed is placed. Another option when using RTK is to apply anhydrous parallel to the future crop-rows but not directly under it.

The biggest risk for ammonia injury--when planting right after application--is in coarse-textured soils. The retention zone tends to be larger, dry soils (not a problem this year), or excessively wet soils where a poor seal results or when the knife smears the soil surface.

2. Can I top-dress all of my nitrogen after the crop is planted?

Yes. The U of M specialists recommend top dressing for coarse textured soils. For medium and fine textured soils, consider these factors.

  • Applying a small amount of N as starter provides risk management in case farmers cannot apply it as soon as planned.
  • Farmers who have prepaid for ammonia may find it difficult to find equipment to apply after planting. With urea, farmers need a urease inhibitor. This adds cost. The same is true for UAN unless it’s injected.
  • The U of M specialists only recommend topdress for corn after soybeans. Corn after corn ideally needs one-third of the total N at planting. If only a starter rate of N (30 pounds per acre or less) is applied for corn after corn, begin applying top-dress as soon as you can row corn.

3. Should I be concerned about nitrogen tie-up when topdressing N under high-residue systems?

This is possible, particularly under cool weather. This process is called “immobilization,” and occurs when decomposing microbes take N out of the soil to “consume” residue. As decomposition advances, the N will release back into the soil to be used by the plants. The key is to avoid significant nitrogen deficiency stress if this happens.

This is another instance where applying a small amount of N (30 pounds per acre) as a starter would help avoid problems. Volatilization loss is a greater concern for urea than is immobilization, UAN has greater potential for immobilization than urea.

4. What is the best source of nitrogen to topdress if I have to apply all my N after planting?

Any form of readily available N can be used. You can inject either UAN or anhydrous ammonia between rows to reduce plant injury. There is no advantage in trying to apply N close to the row, since roots will grow into the row centers by the 4th leaf stage.

It is also possible to apply N in every other row instead of every row without negatively impacting yield. That’s because every row will have N applied at one side or the other. You can also apply urea with a urease inhibitor (NBPT) for UAN dribbled on the surface. UAN can be applied in a broadcast application when the crop is small to reduce the chance of leaf injury.

5. Should I consider an inhibitor?

First, consider there are two inhibitor types.

Nitrification inhibitors delay the process of converting N in the ammonium form to nitrate. This is significant, as the nitrate form is subject to loss via denitrification (gassing off into the atmosphere), or leaching into groundwater. As the season progresses, the likelihood of loss decreases because the amount of time between application and plant use is shorter. This makes the instances of needing an inhibitor less likely.

Additionally, as the soil warms, the inhibitor will not keep its effect as long in the soil as compared to a fall or early spring application. There is nothing wrong environmentally with using a nitrification inhibitor at this time, but the likelihood of it paying for itself decreases by the day.

The other type of inhibitors are urease inhibitors. These are intended to prevent urea from volatilizing and disappearing into the air. The risk of volatilization happening is much greater when urea is top-dressed without incorporation. Use a urease inhibitor in this situation, unless you are certain that there will be a significant rainfall (at least 0.2 inches) in the 48 hours following an application.

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