Iowa State University recommends that fall manure and anhydrous ammonia application not occur until the soil temperature at 4” is fifty and falling. What's changing this year is when that date is likely to arrive.
The odds of a very strong El Niño (ENSO) this fall and winter are above 90%, with a 69% chance that by October through December it reaches a strength not seen in the ENSO record dating back to 1950. This could help push 2026 toward one of the warmest years on record for Iowa.
The last time ICM News flagged a "very strong El Niño" was the fall of 2023-24 when Sept-Nov temperatures were 2.7°F above normal — the 19th warmest fall in 151 years of record. That December, the average temperature was 9.7°F above normal — the 3rd warmest December on record. This year's ENSO event is forecast to be stronger still. That's worth planning around.
What does this mean for nitrogen fertilizer and manure management?
Now is the time for adaptive management — for profitability, productivity, and the environment. Ammonium nitrogen, from manure or synthetic nitrogen, is stable in the soil. Nitrate is not; it is easily lost to water and air when soil moisture is high and there is no growing crop to use it. Nitrification, the microbial process that converts ammonium to nitrate, slows sharply once soil temperatures fall below 50°F, though it doesn't fully stop until soils reach 32°F.
If you apply manure too early into soil that's still warm enough to drive rapid nitrification, you risk losing enough nitrogen that your hauling costs can exceed the manure value. ISU research consistently shows that early fall manure applications can cost farmers more than 40 bushels of corn compared to late fall or spring application.
What can you do?
Watch soil temperatures rather than calendar dates. Central Iowa typically reaches 50 and falling around November 10. This year that window may land 10 to 15 days later. This is also a year to use nitrification inhibitors with ammonia and manure nitrogen sources. More than 2/3 of Iowa farmers already use a nitrification inhibitor with fall ammonia. Consider planting a cover crop, particularly on fields going to soybeans. If manure is applied early, it should be on a field with a cover crop that can reduce nitrate losses to waterways.
The Iowa Nitrogen Initiative’s N-FACT nitrogen management decision support tool shows the value of retaining fall nitrogen. High fall ammonia and nitrate levels can reduce spring nitrogen needs by more than 50 lbs.
Bottom line:
Watch the ground, not the calendar — Iowa State's Iowa Environmental Mesonet posts real-time 4-inch soil temperatures and county-level estimates across the state, and pairing that with the 6- to 10-day forecast is the best way to know when fifty and falling has actually arrived in your fields. Warmer weather associated with the Super El Niño will likely increase nitrate production – and the potential for loss – from soil organic matter, anhydrous ammonia, and manure.
Good adaptive nitrogen and manure management practices are important every year. But with the Super El Niño taking hold this fall, they may be more important and impactful than ever.
Fig 1: NOAA Climate Prediction Center, Monthly Temperature Outlook for September 2026. https://www.cpc.ncep.noaa.gov/
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