Fall nitrogen application: 'Don't go until its 50 degrees or below'

Fall nitrogen application: 'Don't go until its 50 degrees or below'

To prevent loss, sustained soil temperatures below 50 degrees are needed before you apply anhydrous ammonia.

Farmers are being reminded to wait until soil temperatures remain below 50 degrees F before applying anhydrous ammonia (NH3) fertilizer this fall. Harvest is progressing rapidly in many parts of the state and officials with the Iowa Department of Agriculture & Land Stewardship and Iowa State University Extension say waiting to apply can help reduce nitrogen loss and it better protects the environment.

WAIT FOR COLD SOIL: Farmers are reminded to wait until soil temperatures at the 4-inch depth are 50 degrees and falling before applying anhydrous ammonia in the fall. Waiting will prevent loss of nitrogen and help protect water quality.

"It is important that farmers wait for cooler soil temps to apply anhydrous so that there is a better chance the fertilizer stays put and will be available to the crop next spring," says Bill Northey, Iowa Secretary of Agriculture. "Soil temperatures, like air temperatures, can change quickly so it is important that we wait with applications until soils are likely to remain below 50 degrees."

Fertilizer dealers, farmers can use online soil temperature map
ISU Extension agronomists recommend anhydrous ammonia as the only form of N to apply in the fall in Iowa, if you are convinced you must apply N in the fall instead of waiting until spring. And the soil temperature should be measured at the 4-inch depth.

ISU Extension maintains a statewide real-time soil temperature data map on their website that ag retailers and farmers use to determine when fall N applications are appropriate. The website can be found at extension.agron.iastate.edu/NPKnowledge/.

"The reason for waiting to apply anhydrous ammonia until soils are cold is that nitrification, the process of biological conversion of ammonium to nitrate, occurs at a more rapid rate with warm soils. Since ammonium-N does not leach and is not subject to denitrification, it is more stable in the soil than the nitrate form of N," says John Sawyer, ISU Extension specialist in soil fertility and nutrient management.

Also, soil conditions need to be favorable to get a good seal
In addition to waiting for consistently cooler soil temperatures, farmers should also make sure the soil is not too dry, too hard or too wet. Those conditions can cause injection issues and allow ammonia to move to the soil surface and be lost to the air. If conditions are not suitable, then waiting for better conditions is suggested.


Farmers with questions about timing of fertilizer applications can talk to their local ISU Extension field agronomist or their ag retailer for more information.

During the last week or so, the shift to more "fall-like" temperatures has some farmers thinking, "Maybe I can run some fall NH3 while I'm held up waiting on trucks, grain drying, or other typical fall delays."

Be sure the trend is for consistently cooler soil temperature
Having been present for recent conversations that growers were having with their fertilizer dealers ("I want this certain toolbar") and engaged in discussions about N-Serve and how much N to apply, growers are hoping for a good fall application season, notes ISU Extension field agronomist Clarke McGrath. "Knowing that the application window could be tight this fall and again this coming spring given there is little to no room to store additional rainfall it won't take much rainfall to stop NH3 applications this fall, so farmers are anxious to get a jump on it," he observes.

The bottom line is that fall N applications shouldn't necessarily be targeted to start the first day that temperatures reach 50 degrees F. Be sure that the trend is for sustained soil temperatures below 50 degrees F and continued cooling. The dates when soils cool below 50 degrees F vary considerably from late October to late November depending on whether you are in northern Iowa or southern Iowa. "So don't be tripped up by temporary cold spells, especially early in the fall," says McGrath. "Watch the six- to 10-day weather forecast. A forecast for above average temperatures may signal soil warming that could mean we need to wait to start knifing in the nitrogen."

Using a nitrogen stabilizer, still follow 50-degree guideline
McGrath adds, "To help illustrate where we are at, I like to use the ISU NPKnowledge website that shows soil temperatures over the last several days, and has a link to the six- to 10-day temperature forecasts to help dial in the right timing to start NH3 applications extension.agron.iastate.edu/NPKnowledge/. Even if you use a nitrification inhibitor such as N-Serve, we recommend you wait until the soil is 50 degrees and trending cooler before fall-applying anhydrous ammonia."

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