Some Cornbelt cornfields received the full complement of nitrogen this year, others may not have received any. And some farmers who normally apply one form of nitrogen, may have been forced by the weather and soil conditions to turn to one or more alternatives. While experimentation is always good, forced experimentation is not pleasant, and some of your yield may have been lost because of weather interruption of your fertility program. So what do you do this fall, given a late harvest, potentially wet soils, and a narrow window of application?

To begin on a positive note, you’ll be paying considerably less for nitrogen this fall than you did last year. High-fives everyone! Now, back to reality.

Nitrogen application this year is like any year; hold back until temperatures are below 50ºF, or apply it below 60ºF with the help of an inhibitor to ensure against nitrification. That is one of the primary messages from IL fertility specialist Fabian Fernandez in a recent newsletter. He says some forms of N are more susceptible to loss than others, and ammonium can either stay in colder soils, or be converted to a nitrate by soil organisms active in warmer soils. Even if applied properly in the fall, do not apply N if there is a potential for early spring leaching as sandy soils warm up prior to planting. He recommends getting a soil thermometer and taking regular readings at the 4 inch levels at the warmest part of the day.

Anhydrous ammonia is the preferred source of N because it will quickly react with soil moisture and remain in the ammonium form, either with the help of products such as N-Serve, or because cool soil temperatures retard bacterial activity. Fernandez reminds farmers to avoid volatilization loss by ensuring the soil closes behind the applicator knife and the ammonia is inserted deep enough to ensure the ammonia gas does not escape.

The stability of anhydrous ammonia is not present with other forms of N which are in nitrate form, such as ammonium nitrate or urea ammonium nitrate (UAN). Since they are nitrates, they are more susceptible to leaching and environmental loss and should not be applied in the fall. Fernandez says the lower efficiency of urea is due to the greater risk of nitrate loss before the corn is ready to use it.

Urea that is coated with polymer, which is also known as slow release or PCU are designed to retard that loss of nitrate. Fernandez says the jury is still out on the success of the product when used in the fall. At issue is the thickness of the polymer coating, its integrity, and handling issues, all of which are important for stability, but then you also want it to degrade at the right time so the corn can use the nitrogen.

There has been some renewed interest in the use of manure, poultry litter, and other organic fertilizers to provide N, as well as P & K. Fernandez says their application should be incorporated to avoid loss by volatilization. He says the N will be in an ammonium form that will quickly convert to a nitrate and be loss to leaching if soil temperatures are warm enough. But he says don’t apply manure to frozen soils, either, and be careful against an overapplication of phosphorus. That means, have your soil test results handy, and know the analysis of the manure.

So, how much do you apply? Just because it is cheaper this year does not mean you should apply more than last year. Use the N rate calculator to determine the rate that will return the most profit per pound of N. Since the calculator does not account for any carryover N, think about how much may already be in the soil. High yields will have taken out more. Low yields will have taken out less, but in the case of the latter, the wet 2009 spring may have consumed N before the crop began growing, so there may not be any excess. But remember, it is not necessary to make your entire N application before winter sets in.

And when do you apply it? Fernandez says the best time to apply N is in the spring when the corn needs it and there is little chance for it to leach or denitrify. He says if a full rate pre-plant application is not an option, apply a portion this fall, and the balance as a side dress application. You may, (or may not) have more time in the fall to apply nitrogen, and sometimes there are price considerations.

Summary:
Wait until soil temperatures at the 4-inch depth are below 50°F, or below 60°F if you are using a nitrification inhibitor. Do not apply N, or N with a nitrification inhibitor, if soils are prone to leaching. Use a nitrification inhibitor with anhydrous ammonia applications. Do not apply urea or nitrate-containing fertilizers in the fall. If using animal manure, make sure it is incorporated into the soil, and follow the time of application guidelines discussed for commercial N management. Apply the appropriate rate, taking into account leftover N when applicable, and consider applying only a portion of the total N needed in the fall and the rest in the spring. Consider the risks and benefits of fall N application. If fall application is appropriate, follow the recommendations here to help increase the efficiency.