A common question from farmers is "should we apply nitrogen at all in the fall -- even anhydrous ammonia. Are we better off waiting until spring?" Clarke McGrath, ISU Extension field agronomist at Harlan in western Iowa, answers that question the following way. McGrath, who writes the Corn & Soybean Insight column each month in Wallaces Farmer magazine, covers the fall vs. spring N topic in his October column.
McGrath says, "As a former retail agronomy manager, I contend that fall application allows us more flexibility with equipment, generally lower nitrogen prices, more consistent supply logistics, less compaction, less seedling burn and more time for planting in the spring. Fall vs. spring pricing is a little bit murky at the moment, working with your local dealer will give good insight on a local level though.
"As an ISU agronomist, I also have to say that fall application has to be managed correctly to even come close to the nitrogen use efficiency and economics of spring application. Also, if you are going to apply N in the fall, keep in mind that anhydrous ammonia is the only form of N recommended for fall application by ISU agronomists."
Wait until soils are at least 50 degrees F and trending downward before applying anhydrous ammonia in the fall
The single most important factor affecting good management of fall application is ensuring soils are the right temperature before applying NH3. Wait until soils are at least 50 degrees and trending downward. "That's usually around the first week of November although it looks like we may be there this week if the weather forecast is close," says McGrath. "Soil temperatures for every Iowa county, including three-day histories and yearly trends, can be found here."~~~PAGE_BREAK_HERE~~~
He adds, "I am a proponent of using N-Serve nitrification inhibitor in many fall applied NH3 situations, especially as N prices rise. But one place that it doesn't help is using it to apply earlier in the fall -- make sure those soil temps are 50 degrees F and falling, N-Serve or not. Nitrification inhibitors like N-Serve are an interesting and in-depth discussion. ISU has a great information resource on this topic, publication NCH 55, "Nitrification Inhibitors for Corn Production". It is an older publication, but the chemistry and principles really haven't changed for fall NH3 applications since the publication was revised."
Some other things you can do to help lower the risk of loss with fall-applied N
In addition to waiting until soils cool down to 50 degrees or below and using a nitrification inhibitor, McGrath says there are other things you need to do to help lower the risk of loss of fall-applied N. First, avoid applying nitrogen in the fall on soils prone to denitrification or excess leaching; second, only use anhydrous ammonia for fall nitrogen applications, as other nitrogen sources are highly prone to losses; third, be sure anhydrous applicators are properly calibrated; fourth, run the knives at least 6 inches deep. Depending on your applicator, a few can run a little shallower, he says.
How much N should you apply per acre? For nitrogen application rate information, ISU has developed a Corn Nitrogen Rate Calculator. It can guide you in deciding the proper rate to apply based on your particular situation, says McGrath. You plug in the nitrogen price, corn price and your crop rotation.