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Apply fall N by the rules

This crop season wasn’t particularly grower-friendly. We fought early rains, massive flooding and early-season cold, wet weather. Then came the hot, dry weather, and wind and hail damage across much of Iowa. So, we can only hope the law of averages comes around and we have a decent fall harvest season, and favorable weather to apply dry fertilizer and anhydrous ammonia, or NH3.

Should farmers apply nitrogen in the fall? I hear that question asked more and more each year. As a former retail agronomy manager, I argue that fall application allows us more flexibility with equipment, generally lower nitrogen prices, more consistent supply logistics, less compaction, less seedling burn and more time for planting in the spring.

As an ISU agronomist, I also argue that fall application absolutely must be managed correctly to even come close to the nitrogen use efficiency and economics of spring application. Also, if you are going to apply N in the fall, keep in mind that anhydrous ammonia, or NH3, is the only form of N recommended for fall application by ISU agronomists.

Wait until soils cool down 

The single most important factor affecting good management of fall application is ensuring soils are the right temperature before applying NH3.

Wait until soils are at least 50 degrees F 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 at extension.

There has been some talk about possibly starting NH3 applications 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 bottom line — and the environment — and wait until soils hit the “50 degrees and dropping” guideline.

I am a proponent of N-Serve in many fall-applied NH3 situations, especially as N prices rise, but one place that doesn’t help is using it to apply earlier in the fall. 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,” at

Suggestions to avoid loss

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. Avoid applying in fall on soils prone to denitrification or excess leaching; only use NH3 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 NH3 applications is going too shallow. If you are applying it deep enough, the NH3 knife will be polished farther up than the depth you are running. To check to make sure you are applying NH3 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 at
. 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.

Following these suggestions can increase nitrogen use efficiency and corn production, and decrease nitrogen losses, including nitrate movement into surface and groundwaters.

Keep in touch with dealer

Cost is another reason why fall is often preferred for anhydrous ammonia applications: NH3 prices are traditionally less in fall. With that in mind, if soil and weather conditions are decent, we expect another flurry of fall NH3 applications.

Market forces are also putting pressure on supply and transportation of nitrogen. To ensure you are on top of market changes, brace yourself to be asked to pay for things never before associated with NH3, such as “storage” or “carrying costs.”

Also, be sure to communicate often with your local dealers. Good communication can help your dealer keep the product available and moving to your farm. Be patient and work together to understand your options. Our ag retailers are trying to help manage risk, both yours and theirs, in a very volatile market.

McGrath is partnership program manager of ISU’s Corn and Soybean Initiative.

This article published in the October, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.