It is Too Early for Fall Anhydrous Ammonia Applications for Corn

Published: October 15, 2008

The best approach to insure fertilizer nitrogen is available to a corn crop is to apply it as close as possible to peak plant nitrogen demand in late spring. However, logistics and other factors lead many farmers to consider fall application of nitrogen for corn.

Applying nitrogen too early in fall increases the risk that the nitrogen will be lost from the soil. I do not recommend fall nitrogen applications until soil temperature at six inches drops below 40o F. This typically occurs in mid-November in northern Missouri.

Average six-inch soil temperature in northern Missouri as of October 12 when this article was written were still in the mid to low 60's. You can look up current daily six-inch soil temperatures at the University of Missouri website: http://agebb.missouri.edu/weather/reports/soysoil6.asp. You can also sign up for a periodic weather e-mail that includes a graph of six-inch soil temperature through the University of Missouri service HorizonPoint (http://agebb.missouri.edu/horizonpoint/).

Manure nitrogen and anhydrous nitrogen tend to stay in the ammonium form that resists over-winter losses when injected into cold soil. If you apply either nitrogen source too early in fall to a warm soil the nitrogen will convert to nitrate, a highly mobile form of nitrogen prone to losses.

In southern Missouri, fall and winter soil temperatures rarely remain cold enough to delay nitrate conversion so fall applications are not recommended. North of I-70 fall applications can work if you take steps to insure fall-applied nitrogen is not lost.

Products can be added to nitrogen fertilizers to delay the conversion of nitrogen to nitrate. These products reduce but do not eliminate the risk of nitrogen losses from nitrogen applications. N-serve (a nitrification inhibitor) can be added to anhydrous ammonia. High fertilizer prices make the economics of inhibitors more favorable.

There is no strategy to guarantee that fall-applied nitrogen will successfully make it through fall, winter and spring and be available for your corn crop in June and July. Injecting nitrogen to nearly frozen soils and using inhibitors will make it more likely that fall-applied nitrogen will be there in the spring. The later you wait in the fall and the colder the soil is at the time of application the more likely the nitrogen will make it through the winter.

Be patient! It is still too early for fall anhydrous ammonia applications for next year's corn crop!