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Experts: Minimize N Loss from Fall-Applied Anhydrous

by Jim Camberato

Published: Friday, December 2, 2022

The following is from Jim Camberato, professor of agronomy, and Dan Quinn, assistant professor of corn production at Purdue University.

To minimize the loss of nitrogen (N) from fall-applied anhydrous ammonia (AA), seal the application slot, avoid applications on poorly drained (heavy textured and wet) or excessively well-drained soils (sandy soils with low water-holding capacity), wait till soil temperatures approach freezing, and use a nitrification inhibitor.

First things first, it is important to get a good seal on the application slot to avoid direct loss of ammonia to the air. Slot closure can be difficult when soils are either sopping wet or bone dry. If you can smell ammonia, slot closure is likely poor, and N is being lost—this is especially troubling if you can still smell ammonia several days after application.

Ammonia that is trapped in the soil after application can be lost later by two mechanisms. On poorly drained fields, nitrate-N produced from ammonia can be lost to the air when soils are saturated. On excessively well-drained fields, or tile-drained fields, the ammonium-N and nitrate N produced from ammonia can be leached below the rootzone with excess rainfall.

Fields with optimum drainage result in lower N loss than those that are poorly- or excessively well-drained. Nitrogen losses can occur throughout winter, but the greatest losses occur in spring when soils warm up and the ammonium-N is converted to nitrate-N and soils are ponded or tile drains are flowing.

Delaying AA application until soils become cold is important because soil micro-organisms convert ammonia to nitrate-N much more slowly as temperatures approach 32-degrees F., where conversion of ammonia to nitrate-N stops. The less nitrate-N produced, the lower the chance of N loss. The standard advice is to wait till soils at a 4-inch depth are consistently below 50-degress F. and soil temperatures are expected to continue decreasing toward freezing.

In the northern half of Indiana, this could occur anywhere from the last couple weeks of October to the first couple weeks of December. Fall application of AA south of Interstate 70 is discouraged due to warm and fluctuating soil temperatures throughout the winter which can increase N loss risk.

A nitrification inhibitor, such as nitrapyrin, is an N fertilizer additive that slows the activity of microorganisms that convert ammonia to nitrate N, thus reducing the loss of N from fall-applied AA. Consider using a nitrification inhibitor if you fall-apply AA. Guidance on effective nitrification inhibitors can be found in the bulletin titled Nitrogen Extenders and Additives for Field Crops.

The amount of fall-applied N that is lost is hard to predict, due to N loss being affected by both weather and soil properties. Losses of N can Dan Quinn be minimal in some situations or substantial in others—15-30 percent of the applied N or more. Measuring what N remains is just as difficult as predicting loss, because of the unknown distribution of ammonium-N and nitrate-N in the soil which move both vertically and horizontally from the injection zone.

Therefore, intensive sampling is needed to estimate the amount of N remaining. Bottom line is there is quite a bit of uncertainty come spring whether the crop will have the N it needs, and farmers are often faced with the difficult decision of whether to apply more N. This is the reason we recommend moving away from fall N and apply AA in spring or side-dress AA or liquid N, such as 28 percent UAN.

Lastly, whenever you apply AA remember those practices necessary to keep everyone safe when handling, transporting, and applying AA. Fred Whitford gives safety guidance in a recent publication that can be downloaded from the internet or ordered in printed format.

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