Nitrogen (N) management has been a major topic this year due to early-season fertilizer price volatility. Earlier in the year, I discussed several in-season management strategies that can help maximize profitability while reducing N application rates. Over the years, I have also received questions about fall-applied nitrogen, and interest in this practice seems especially high this year.
Fall nitrogen application is common in parts of the western Corn Belt but is rarely used in Ohio. Several reasons explain why some producers consider this practice.
Historically, nitrogen fertilizer demand is lower in the fall, which can lower fertilizer prices. Taking advantage of lower prices may provide significant savings on input costs. Field conditions are often more favorable for fertilizer application after harvest than they are during the busy spring season. In addition, fall application can help spread out workloads. Spring already includes forage harvest, planting, fence repairs and many other time-sensitive tasks. Moving one operation from spring to fall can help relieve some of that pressure.
Given these advantages, it is easy to understand why interest in fall-applied nitrogen exists. However, as with any management practice, there are tradeoffs. Unfortunately, the risks associated with fall nitrogen application in Ohio often outweigh the potential benefits.
Anhydrous ammonia is the only nitrogen source recommended for fall application. When injected into the soil, anhydrous ammonia temporarily suppresses nitrifying bacteria, slowing the conversion of ammonium to nitrate and reducing the potential for nitrogen loss. Other nitrogen sources, such as urea, do not provide this effect and are more susceptible to loss when applied in the fall.
For producers who do not already have access to anhydrous ammonia application equipment or a nearby supplier, adopting this practice can be costly. In addition, nitrification inhibitors are recommended to further protect against nitrogen losses, adding another expense. Even with inhibitors, research has shown that performance can be inconsistent, and their effectiveness may decline within a few weeks under certain conditions.
While fall soil conditions may be favorable for fieldwork, they are not always ideal for preserving nitrogen until the following growing season. Soil temperatures at a 4-inch depth should be 50°F or lower and remain there throughout the winter to minimize nitrification. In northeast Ohio, soil temperatures frequently remain above 50 F well into November. An early warm-up in February or March can also accelerate nitrification, increasing the risk of nitrogen losses before crops can utilize the nutrient.
Fall nitrogen applications are generally not recommended on poorly drained soils. In Trumbull County, this would eliminate a large percentage of tillable acres. Poorly drained soils can limit the movement of anhydrous ammonia deeper into the soil profile, leaving it closer to the surface where temperatures fluctuate more dramatically. These fluctuations can easily exceed the 50 F threshold, increasing the risk of nitrogen conversion and loss.
Considering these risks, Ohio State University Extension recommends delaying nitrogen applications until spring whenever possible. Spring application reduces the potential for leaching and volatilization losses while placing nitrogen in the field closer to the time when crops need it most.
