Making Every Nutrient Count- Fall Fertilizer Decisions for Productivity, Profitability, and Water Quality

By Rachel Curry and Nicole Haverback el.at

Fall fertilizer season brings decisions about when, where, and how much to apply.

In Episode 86 of the Illinois Nutrient Loss Reduction Podcast, University of Illinois Assistant Professor and State Soil Fertility Specialist John Jones shares what farmers should keep in mind when making fall fertilizer decisions. He discusses fall nitrogen timing, nitrification inhibitors, nitrogen rates, and using soil test results to guide phosphorus and potassium applications.

Wait for soils to cool

For fall nitrogen applications, Jones said soil temperature is one of the most important factors to watch. Farmers applying anhydrous ammonia in the fall should wait until soil temperatures at about a 4-inch depth reach 50 F and are continuing to cool. In the northern two-thirds of Illinois, Jones said those conditions often occur in late October into early November.

The goal is to keep fall-applied nitrogen in the ammonium form for as long as possible. Once ammonium converts to nitrate, it becomes more vulnerable to loss.

Nitrification inhibitors can help delay that conversion, but Jones emphasized they should not replace waiting for appropriate soil temperatures. Instead, proper timing and nitrification inhibitors work best when used together.

Use economics to guide nitrogen rates

Jones also recommended using the Maximum Return to Nitrogen approach when determining nitrogen rates. The approach uses results from Illinois nitrogen rate trials along with corn and nitrogen prices to identify the rate that provides the greatest economic return rather than simply targeting maximum yield.

Jones said the nitrogen rate that maximizes profit is often lower than the rate that produces the most bushels, with only a small difference in yield.

When using the nitrogen rate calculator, remember that the recommendation represents total nitrogen applied to the crop. Farmers should account for nitrogen from all sources, including fall-applied fertilizers and other applications made before corn.