Farmers are encouraged to wait until soil temperatures at the 4-inch depth remain below 50 degrees Fahrenheit before applying anhydrous ammonia (NH3) nitrogen fertilizer this fall.

With the record early harvest this year, farmers are eager to move in and get the fall-applied nitrogen job done. However officials with the Iowa Department of Agriculture & Land Stewardship and Iowa State University Extension say “hold on a minute." They say waiting to apply anhydrous can help reduce nitrogen loss, which means waiting does a better job of protecting the environment.

DON'T GO UNTIL 50 OR BELOW: Farmers are encouraged to wait until soil temperatures reach 50 degrees Fahrenheit and are falling (the weather enters a cooling pattern) before applying anhydrous ammonia fertilizer to fields this fall. That helps reduce nitrogen loss and protects the environment, say ISU agronomists.

“With farmers finishing harvest earlier than normal, it is important that they still wait for cooler soil temperatures 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."

When do soil temperatures generally reach that cooling pattern? “Historically, soil temperatures at a 4-inch depth cool below 50 degrees in the northern third of the state during the first week of November," says Elwynn Taylor, ISU Extension climatologist. “In central and southern Iowa, soil temperatures cool below 50 degrees during the second week and third week of November."

ISU website gives current soil temperature readings for your area of Iowa

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.

Farmers should also be mindful to pay special attention when applying anhydrous ammonia to very dry soil, says ISU Extension soil fertility specialist John Sawyer. Dry soil can hold ammonia, but if fields are cloddy and soil does not seal properly when the anhydrous applicator knives run through, the ammonia can be lost at the time of injection or it can seep through the large pores between clods after application.

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“That's why farmers and applicators should assure proper depth of injection and good soil coverage when applying anhydrous into dry soils," says Sawyer. “If following a round of application in the field the ammonia can still be smelled, then the applicator should make adjustments to get a better sealing of the soil or else wait for better soil conditions before applying." Farmers with questions about timing of fertilizer applications should talk to their local ISU Extension field agronomist or their ag fertilizer dealer for more information.

Should you apply nitrogen in the fall? Would it be better to wait until spring?

 A common question farmers ask is, “Should we even apply nitrogen in the fall?" As a former retail agronomy manager who is now an ISU Extension field agronomist at Harlan in western Iowa, Clarke McGrath says “fall application allows us to have more flexibility with equipment, generally lower nitrogen prices, more consistent supply logistics and less soil compaction compared to applying nitrogen in spring. There's also less corn seedling burn when you apply anhydrous in the fall compared to spring and fall application leaves more time for planting in the spring."

On the other hand, “As an ISU agronomist, I also have to say that fall application has to be managed correctly to even come close to the nitrogen use efficiency and economics of spring application," adds McGrath. “Also, if you are going to apply N in the fall, keep in mind that anhydrous ammonia is the only form of N recommended for fall application by ISU agronomists."

Have patience, wait until soils are at least 50 degrees and trending downward

The single most important factor affecting good management of fall application is ensuring soils are the right temperature before applying NH3, he says. Wait until soils are at least 50 degrees and trending downward, usually around the first week of November. Soil temperatures for every Iowa county, including both three-day histories and three-day forecasts, can be found here.

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There has been some talk about possibly starting NH3 application earlier this fall by using N-Serve to “make early apps safe." Unfortunately, that isn't how it works. Do what is best for your bottomline and the environment and wait until soils hit the “50 degrees and dropping." That's the guideline to follow, he says.

 “I am a proponent of using N-Serve in many fall applied NH3 situations, especially as N prices rise, but one place that it doesn't help is using it to apply earlier in the fall," says McGrath. “Nitrification inhibitors like N-Serve are an interesting and in-depth discussion. ISU has a great resource on this topic, publication NCH 55, Nitrification Inhibitors for Corn Production, which can be found here.

Other steps you can take to help minimize losses from fall applied anhydrous

In addition to waiting until soils cool down to 50 degrees or below and using a nitrification inhibitor, there are other things you need to do to help lower the risk of loss of fall-applied N, says McGrath. Avoid applying in fall on soils prone to denitrification or excess leaching; only use anhydrous for fall nitrogen applications, as other nitrogen sources are highly prone to losses; be sure anhydrous applicators are properly calibrated; and run the knives at least 6 inches deep.

“One of the most common problems I see with anhydrous ammonia applications is going too shallow," says McGrath. “If you are applying it deep enough, the applicator knife will be polished further up than the depth you are running. To check to make sure you are applying the ammonia at the right depth, do some digging in the knife trench (you will be able to find the trench bottom easily) and measure your depth. Make sure it is at least 6 inches deep."

For nitrogen application rate information, ISU has developed a Corn Nitrogen Rate Calculator, which can be found online here. It can guide you in deciding the proper rate to apply based on your particular situation. You plug in the nitrogen price, corn price and your crop rotation.