Applying nitrogen in the fall is a well-established practice in central and northern Illinois. It helps to speed up planting in the spring; however, it’s important to balance the benefits of fall applied N with the risks.

University of Illinois Extension agronomist Emerson Nafziger explains, “With the trend toward earlier planting dates, it’s an advantage to not have to worry about pre-plant application.”

Nafziger adds that because soils are typically drier in the fall, meaning less compaction and better dispersal of anhydrous ammonia in the soil profile. “Fall applications often work better, but you have to weigh that against increased chances for loss,” he adds.

The obvious drawback to fall-applied N? It’s applied a long time before the crop actually needs it. However, as long as it stays in the ammonium form, it should overwinter without significant loss. Once the weather cools into the 30s, the biological process that converts ammonium to nitrate is very slow, explains Nafziger. He adds that in a typical year, most of the ammonium that’s present in the soil in December will still be there in March.

Temperature determines timing

With harvest winding down, farmers may be eager to start applying fall N, but it’s important to base your fall N anhydrous applications on something more important than your schedule: soil temperature.

As Nafziger explains, “It’s important to wait until soil temperatures consistently are below 50 degrees before applying fall N. For most of the northern half of Illinois, that means waiting until November 1. In southern Illinois, it stays warm later in the fall and warms up earlier in the spring, making fall application too risky.” For growers in southern counties with unglaciated soils, typically those south of Highway 16, fall applications of nitrogen are discouraged, according to the Illinois Agronomy Handbook.

Soil temperature information is available from the Water and Atmospheric Resources Monitoring Program (WARM). Use the maximum daily temperature in four-inch bare soil as your guide.

Nitrogen stabilizers are a sound investment.

Nafziger says using a nitrification inhibitor with fall-applied anhydrous ammonia has become a common practice. “Stabilizers slow conversion of ammonium to nitrate,” he says. “Keep as much nitrogen as possible in the ammonium form going into winter when cold soils stop nitrification.”

Jean Payne, president of the Illinois Fertilizer and Chemical Association, says shifting fertilizer and corn prices have helped farmers move away from overapplying nitrogen. “Farmers see nitrogen as a high-value input, so stabilizers are a form of insurance,” Payne says. “Besides, the goal in adding N is to keep it for the crop, not risk losing more to the environment.”

“Because we can’t predict the weather, the best approach is to manage nitrogen as a system.” Nafziger adds, “As in so many things, the moderate path is a wise one. More farmers are using split N applications. Be aware of the potential for loss while applying only as much N as the crop needs. Fall N can be a useful tool to help get this done.”

Adapted from the September issue of Illinois Field & Bean: Time to Apply New Thinking to Fall-Applied N.