Quick look
Super El Niño-influenced weather could make Iowa’s fall wetter and warmer than usual. Iowa State University experts say that may pose challenges for fall fertilizer applications.
AMES, Iowa – Timing is crucial for Iowa farmers preparing their post-harvest fields for the coming season, but they don’t work on a date-based schedule. Fertilizing fields in the fall requires closely watching soil temperatures to avoid lost production in next year’s crop and nitrate pollution in waterways.
Looking to the ground instead of the calendar will be especially important this year, as a super El Niño – an unusually strong variant of the cyclical weather pattern driven by heightened warmth in the Pacific Ocean – is expected to bring a wetter and warmer fall.
Two Iowa State University experts in nutrient management – agronomy professor Mike Castellano and agricultural and biosystems engineering professor Dan Andersen – shared what that could mean for Iowa farmers and why timing is so important for fall fertilizer.
’50 and falling’ is focus
The most essential guideline for applying fertilizer in the fall is to wait for “50 and falling,” meaning the temperature of the soil 4 inches below ground is 50 degrees or less and trending colder. Following that advice helps make sure the nitrogen that farmers add to their fields after the growing season is still there in the spring to help crops thrive.
Though it varies by year, central Iowa typically hits the 50-and-falling mark around Nov. 10. The super El Niño’s influence could easily push that back by a couple of weeks, Castellano said. The delay could give farmers less time to fertilize before winter.
The 50-and-falling standard is a reliable and science-backed method agronomists have long recommended for fending off nitrification, a vital stage of natural nutrient recycling that makes the nitrogen in fertilizer vulnerable to leaching into field drainage.
“Microbial activity is directly tied to temperature. The warmer the soil, the more active the microbes. Once soil drops below 50 degrees and is continuing to cool, the process we call nitrification – the microbe-driven transformation of ammonium to nitrate – slows down drastically,” said Castellano, who leads the Iowa Nitrogen Initiative.
Keeping nitrogen sticky
Nitrate is the form of nitrogen preferred by growing corn plants, the crop that accounts for most fertilizer applied in Iowa fields. But without plant roots to absorb it, nitrate by its nature can slip away from soil with ease.
Almost all fall fertilizer in Iowa is ammonia-based, either synthetic anhydrous ammonia or manure. When it contacts ground moisture, ammonia becomes ammonium – a nitrogen compound that clings to soil like a magnet because its positive electrical charge bonds with negatively charged soil, Castellano said.
When soil microbes consume ammonium, the nitrate created has a negative charge. The negative-on-negative clash repels nitrate from soil, making it easy to wash away. Soil nitrate can also be lost as gas through a different microbial process called denitrification.
“If that nitrogen remains ammonium, it’s going to stay relatively stuck to the soil, and those physical processes aren’t going to act on it nearly as much as they do on nitrate,” Castellano said.
Why fall at all?
Given the potential water quality and yield consequences of nitrification, why don’t farmers just wait until spring to fertilize? There are many reasons. For one, the risk is minimal if farmers follow application recommendations, Castellano said.
“When you use best management practices, there is little difference in crop response or nitrate loss when you apply in the fall versus in the spring,” he said.
Applying fall nitrogen helps make effective use of farmers’ time and late-season weather that’s typically favorable, giving them a better chance of avoiding costly planting delays in the spring, Andersen said.
Even if there were significant advantages, shifting fall fertilizing to the spring is logistically impossible in Iowa without a major infrastructure upgrade, Castellano said. Between 30% and 50% of synthetic nitrogen used on Iowa fields is applied in the fall. Pushing that work to the spring would overwhelm the delivery network.
“We simply don’t have the transport or storage facilities to do it,” he said.
Manure pits filling up
Logistical considerations also make fall application a necessity for manure, which accounts for about one-third of Iowa’s nitrogen fertilizer.
Labor is one of those factors. About 70% of the state’s manure fertilizer is liquid swine manure. All synthetic fertilizer and nearly all swine manure is injected into the ground when applied, but commercial applicators are more common for manure. Based on the pool of licensed spreaders, it takes about 75 working days to pump the state’s swine manure from holding pits into fields, Andersen said. That requires multiple application windows.
Storage capacity is the most pressing issue for swine manure in the fall, he said. Hog barn manure pits have a finite amount of space and don’t stop filling up in years when fall is wetter and warmer than usual.
“Swine manure has to get spread on fields in the fall because there’s no place else for it to go,” said Andersen, manure extension specialist for ISU Extension and Outreach.
Most years, Iowa farmers apply about 75% of the year’s manure in the fall. This year, that percentage could drop as low as 55%, Andersen said. Farmers who are used to emptying their pits in the fall may need to be flexible.
“They just need enough capacity to get to the spring window, which probably means planning for April,” he said.
Options for adapting
How else can farmers manage the potential complications of the super El Niño this fall?
One option is to use a nitrification inhibitor with their nitrogen, an additive that temporarily suppresses a bacterium that kick starts the conversion of ammonium to nitrate. Castellano said research has found inhibitors stave off enough nutrient loss to be worth the extra cost. Andersen said one Iowa State study showed adding an inhibitor to swine manure slowed nitrification to a rate expected when soil temperatures are about five degrees lower.
It’s also a good fall to plant cover crops, Andersen said. Having roots in the ground between harvest and planting provides restless nitrate a safe in-field destination.
“If you have to apply early this fall, you need cover crops on those acres. That’s the No. 1 tool we have in that situation,” he said.
Farmers looking for guidance can rely on Iowa State expertise and information. ISU’s Iowa Environmental Mesonet offers a soil-cooling overview, with county-by-county temperature maps sharing daily measurements and a two-week forecast. The integrated crop management team is a source for a variety of insight, including recent blog posts on fall fertilizer and manure. On Oct. 7, Andersen and Castellano joined state climatologist Justin Glisan for a webinar on super El Niño impacts hosted by the Iowa Farm Bureau.
Above all, farmers should be patient and ready to act when the time’s right, Andersen said.
“This is probably going to be a challenging fall,” he said. “But El Niño could end up working in our favor in the spring because a warm, dry winter brings good field conditions earlier.”
