A “super” El Niño weather event bringing above normal temperatures to Iowa this fall could push back the window for timely fall nutrient applications, according to Mike Castellano, Iowa State University (ISU) agronomist. 

ISU recommends waiting to apply manure and anhydrous ammonia until the 4-inch soil temperature is at least 50 degrees and falling in order to reduce nutrient losses.

“What’s changing this year is when that date is likely to arrive,” said Castellano. 

Following fall nutrient application best practices will be especially important with the conditions anticipated this fall in order to reduce the chances of a spike in river nitrate levels this winter and next spring.

The odds of a very strong El Niño (ENSO) this fall and winter are above 90%, with a 75% chance of reaching a historic strength not seen since 1950, according to the National Weather Service Climate Prediction Center. That’s expected to keep temperatures throughout Iowa above normal through the end of the year, pushing 2026 toward one of the warmest years on record for Iowa.

The last time ISU agronomists flagged a “very strong El Niño” was the fall of 2023-24 when September-November temperatures were 2.7 degrees above normal — the 19th warmest fall in 151 years of record. 

That December, the average temperature was 9.7 degrees above normal — the third warmest December on record. 

“This year’s event is forecast to be stronger still. That’s worth planning around,” said Castellano. 


PICTURED ABOVE: Castellano


What Can You Do?

Because of the warmer weather, the date when Iowa soil temperatures reach 50 and falling could be 10 to 15 days later than usual, said Castellano. 

For example, soil temperatures in central Iowa typically hit 50 and falling around Nov. 10, but it could be closer to the week of Thanksgiving this year, he said. 

Nitrification, the microbial process that converts ammonium to nitrate, slows sharply once soil temperatures fall below 50 degrees.

Applying anhydrous ammonia or manure too early into soil that’s still warm enough to drive rapid nitrification elevates the risk of losing valuable fertilizer. 

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.  

“Now is the time for adaptive management — for profitability, productivity and the environment,” said Castellano. “Watch soil temperatures rather than calendar dates. This is also a year to use nitrification inhibitors with ammonia and manure nitrogen sources.”

Data from the Iowa Nutrient Research and Education Council (INREC) shows that more than two-thirds of Iowa farmers already use a nitrification inhibitor with fall ammonia. Cover crops can also help sequester nitrogen, Castellano said. 

Farmers can monitor real-time 4-inch soil temperatures and county-level estimates across the state on Iowa State’s Iowa Environmental Mesonet. 

“Pairing that with the six- to 10-day forecast is the best way to know when ‘50 and falling’ has actually arrived in your fields,” said Castellano. 

The Iowa Nitrogen Initiative’s N-FACT nitrogen management decision support tool (n-fact.ag/start) shows the value of retaining fall nitrogen. High fall ammonia and nitrate levels can reduce spring nitrogen needs by more than 50 pounds, noted Castellano. 

INREC and the Iowa Department of Agriculture and Land Stewardship are also offering free fall soil sampling for farmers in 22 counties in the Greater Des Moines watershed and four counties in the Middle Cedar watershed. See INREC’s website at iowanrec.org for more information.

Iowa Learning Farms is hosting a virtual field day at 1 p.m. Sept. 23 with Castellano and Justin Glisan, Iowa’s state climatologist, to discuss nitrogen fertilizer and manure management during very strong El Niño conditions. 

To join, go to www.iowalearningfarms.org/events-1 and click on the “Nitrogen Management in the Face of Super El Niño” virtual field day or call 646-876-9923 or 646-931-3860 (Meeting ID: 914 1198 4892). 

The field day will be recorded and archived on the Iowa Learning Farms website for future viewing.