Soil and water conservationists with the U.S. Department of Agriculture (USDA), Iowa State University and the Iowa Department of Agriculture are recommending farmers wait until soil temperatures are at least 50°F and falling before applying manure and anhydrous ammonia this fall.

State Climatologist Dr. Justin Glisan has flagged that a very strong El Niño is shaping up in the Pacific Ocean, which historically means warmer and generally wetter fall weather throughout Iowa creating less-than-ideal conditions this harvest season.

These guidelines are recommended by conservationists every fall as a best practice as warmer soils can increase nitrogen mineralization, leading to leaching. Because of the warmer weather predicted for this fall, the date when Iowa fields reach 50°F and falling could be 10-15 days later than usual. For example, central Iowa typically hits 50°F on about Nov. 10, but it could be closer to the week of Thanksgiving this year.

Iowa State University Extension and Outreach has issued the following guidance: Fifty and Falling: What the Super El Niño Means for Anhydrous Ammonia and Manure Timing this Fall.

Some key takeaways:

  • Monitor soil temperatures rather than calendar dates.
  • Use nitrification inhibitors with ammonia and manure nitrogen sources.
  • 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.
Warm soils can increase nitrogen mineralization. Use precision tools, like fall N soil sampling and N-FACT, to inform and optimize application rates.