By Fabian Fernandez, Soil Scientist

Although we are not quite done with the growing season, it is not too early to start making plans for the next growing season.

For some farmers, nitrogen (N) application in the fall is one of the many important decisions to make. Nitrogen is not only a large input in most corn farming operations, but it also represents an important input in terms of the environment. The soil is not a very good place to store N as this nutrient can easily end up in air or water where it can cause environmental degradation. For these reasons, I would like to review important guidelines developed through years of unbiased research in hopes that this will help you make the best decisions for N management.

Where to Apply?

Fall N applications compared to pre-plant or sidedress applications often bear greater risk of N loss that can translate into reduced profitability and environmental concerns. Still, if the decision is made to go ahead with a fall application, N should be applied in the fall only in those soils and environments with the lowest potential for N loss. Places where fall N applications should not be done are soils with high potential for nitrate leaching in the fall or early spring (sandy soils or those with excessive drainage) or soils that are very poorly drained.

What to Apply?

The goal is to use a N source that will stay in the ammonium form as long as possible because once it transforms to nitrate, that is when N becomes susceptible to loss. For fields where a fall application may be appropriate, anhydrous ammonia (NH3) is the best option. This source eliminates the nitrifying bacteria at the point of application, and the activity of nitrifying bacteria within the ammonia retention zone is also inhibited for a couple of weeks because of the temporary high pH that develops as ammonia reacts with soil water to form ammonium. 

To lengthen the period of bacterial inhibition, it is best to include a nitrification inhibitor with the application of anhydrous ammonia. Nitrification inhibitors, such as N-serve, can protect fall N against loss, but the greatest potential for loss occurs in the spring; by that time the inhibitor is less effective due to its breakdown over time. Nitrogen sources containing nitrate should never be used for fall applications.

When to Apply?

The activity of nitrifying bacteria slows down substantially once soil temperature where anhydrous ammonia is placed (typically at 6-inch depth) is 50 F and getting colder. Using a nitrification inhibitor is no excuse to apply N in warmer soils. In fact, warmer temperatures accelerate the breakdown of the nitrification inhibitor making it less effective. 

Keep in mind that nitrifying bacteria remain active until 32 F when the soil freezes. For this reason, it is better waiting as long as possible after the soil reaches 50 F and is getting colder. So don't go by calendar date but rather keep a close watch on soil temperatures.

Recently the Minnesota Department of Agriculture had a news release explaining when a soil temperature of 50 F is typically reached in Minnesota. The release also provides a useful link to obtain real-time soil temperature from 21 different stations throughout much of the state. However, these values should be used as a reference. It is always best to monitor temperature of individual fields before N application because soil temperature can be influenced by a number of factors such as residue cover, soil color and drainage.

How to Apply?

Nitrogen volatilization during application is a big concern with anhydrous ammonia not only because of the unpleasant odor but because N that escapes to the atmosphere represents a loss on the investment that will not be recovered by the crop. This often happens when soils are either too dry or too wet because the knife tracks do not fully seal. Increasing the depth of application or reducing application rates can sometimes help with this problem in dry soils, but when the soil is wet, little can be done to minimize loss through volatilization.

Also, apply anhydrous ammonia with caution. Anhydrous ammonia is pressurized  in the nurse tank, and when released it reacts quickly with water. If ammonia comes in contact with your skin, eyes, or mucous membranes, it will cause dehydration and burns, so please use extreme caution when handling it.

How Much to Apply?

To make the most profitable decision the corn N rate calculator provides the economically optimal N rate at various corn and N prices. The data used for this calculator came from trials where N loss was minimal, but it does not account for carryover N that might not have been used by the previous crop. Also, if you applied manure or the soil has high potential for N mineralization (as in the case of a field coming off of alfalfa), you will need to adjust the values derived from the calculator to reflect what will be available next year.

Once you know how much N you need, remember that you don’t have to apply the entire amount in the fall. If you don’t like taking risks, but a fall application makes sense, then apply some N in the fall and the rest in the spring. A small portion of the total N requirement applied in the fall can provide all the N the corn crop will need to get started in the spring. Applying the remainder closer to when the crop will need the most N can increase N use efficiency because there is less chance for leaching or denitrification.

For more detailed information on how to manage N please read "Fertilizer Guidelines for Agronomic Crops in Minnesota"  and the "Best Management Practices for Nitrogen" publication series