(Photo: Iowa Soybean Association / Joclyn Kuboushek)
Break the fertilizer habit
October 1, 2026 | Kriss Nelson
Fall fertilizer decisions can quickly become routine. But Iowa Soybean Association Research Agronomy Lead Alex Schaffer says routine isn’t always the most profitable strategy.
Whether evaluating soil tests, nitrogen stabilizers or application timing, he encourages farmers to make field-by-field decisions based on crop needs rather than habit. Doing so can improve profitability, nutrient efficiency and water quality.
“It’s paying closer attention to the tons of fertilizer being spread every year and just saving where you can,” he says. “If your soil test levels are high or very high, there’s really no point in adding fertilizer until crop removal brings those levels back down.”
Let soil tests drive decisions
For phosphorus and potassium, Schaffer says soil testing should be the starting point.
Fields testing high or very high typically don’t require additional fertilizer. Optimum fields should receive maintenance applications based on crop removal, ideally using variable-rate technology.
“The agronomics haven’t changed,” Schaffer says. “It’s really about making sure you’re replacing what the crop removes instead of automatically fertilizing every acre every year.”
Schaffer says farmers should also consider whether stretching fertilizer applications over multiple years is the most economical approach.
“If you’re waiting two or three years between applications, are you potentially leaving yield on the table?” he asks. “That’s something worth thinking about from a return-on-investment standpoint.”
Applying nutrients closer to crop demand can improve nutrient use efficiency while avoiding unnecessary applications. Because phosphorus moves primarily with eroded soil rather than through the soil profile, applying more than the crop needs increases both upfront costs and the amount of phosphorus vulnerable to loss if erosion occurs.
Fall nitrogen still benefits from stabilizers
Research continues to reinforce the value of using a nitrogen stabilizer with fall anhydrous ammonia applications, Schaffer says.
“You’re really stabilizing your fall ammonia for the spring,” he says.
Nitrogen stabilizers contain nitrapyrin, which slows the conversion of ammonium into nitrate, helping keep nitrogen in a less leachable form until the crop needs it.
Are stabilizers worthwhile during cold winters? That’s exactly when they continue to provide value, Schaffer says.
“Soil microbes also break down the stabilizer,” he explains. “If it’s cold all winter, those microbes aren’t very active, so the stabilizer is still there in the spring protecting that nitrogen.”
Challenging assumptions
Research conducted through ISA trials has consistently shown treated nitrogen converts significantly more slowly than untreated nitrogen, reducing the amount susceptible to loss.
One finding that surprised Schaffer was how long nitrapyrin remains effective under spring conditions.
“There was this thought that if it only lasted a couple weeks in warm weather, why spend the money?” he says.
ISA research found nitrapyrin remained effective for more than 10 weeks in many cases. Even 10 weeks after application, some locations still had nearly 50% of the nitrogen in the ammonium form, resulting in ammonium levels comparable to fall-applied nitrogen by late spring.
Split applications offer flexibility
Schaffer also remains a strong advocate for split nitrogen applications.
Rather than applying the entire nitrogen program in the fall or before planting, split applications place a portion of the fertilizer closer to the time the crop is actively taking up nutrients.
“You can make a management decision during the season,” Schaffer says.
For example, a farmer may apply enough nitrogen before planting to simplify logistics, then use in-season weather, rainfall, mineralization and yield potential to determine whether additional nitrogen is warranted.
“Combining split applications with decision-support tools such as N-FACT allows producers to respond to the growing season as it unfolds rather than relying solely on preseason assumptions,” he says.
Nitrogen remains a moving target
While fall fertilizer decisions often happen months before planting, Schaffer says nitrogen management doesn’t end there.
Even with a sound fertilizer plan, nitrogen management doesn’t stop after application because mineralization, the natural release of nitrogen from organic matter, varies dramatically from year to year.
“That’s a big part of what makes nitrogen a moving target,” he says.
This season has experienced high mineralization, illustrating why fixed nitrogen programs don’t always match crop needs.
To help farmers respond to changing conditions, ISA continues supporting the Iowa Nitrogen Initiative’s N-FACT tool. The online decision-support tool combines weather data, mineralization estimates and field management information to guide in-season nitrogen decisions.
Using current field conditions instead of historical averages, the tool helps farmers make more adaptive nitrogen decisions while improving profitability and protecting water quality.
Written by Kriss Nelson.
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