Source: Stu Ellis, University of Illinois

Here is a quiz for you. If you can apply anhydrous ammonia when the soil temperature is 50 degrees F and dropping, what should the soil temperature be if you are going to use a nitrate inhibitor, like N-Serve, along with your anhydrous ammonia? If you answered 60 degrees F you were correct, until recently. But recent research has prompted soil fertility specialists to re-think that recommendation. Here's the scoop.

Researchers began changing their recommendations for nitrogen application in the fall of 2009, but very few farmers were listening, since everyone was more concerned about harvesting a wet crop than applying nitrogen to muddy and rutted fields. So if you didn't get the memo, don't worry, you are forgiven.

The bottom line is that 50 degrees F is the highest the soil temperature should be for applying an ammonium fertilizer, regardless whether a nitrification inhibitor is used. No more 60 degrees F. The maximum is 50 degrees F. University of Illinois soil fertility specialist Fabian Fernandez says, "The use of an inhibitor can further help maintain the applied nitrogen in the ammonium form." He says the recommendations published in the Illinois Agronomy Handbook were issued in 2009, but most farmers had other priorities. His recent newsletter says research indicated 42 percent of the applied ammonia remained in the ammonium form through the early part of the growing season when the nitrification inhibitor was used, compared to only 4 percent when it was not used. But he says the benefit of using such products as N-Serve and DCD will vary with "soil condition, time of year, type of soil, geographic location, rate of N application, and prevailing weather conditions between N application and crop uptake." And he adds, "Yield increases of 10 to 30 bushels per acre are possible by using an inhibitor in years with excessive rainfall, but there is often no advantage when soil conditions are not conducive to leaching or denitrification."

Fernandez said research has shown that a nitrification inhibitor can hold 50 percent of the applied ammonia when the soil temperature is at 55 degrees F for about 5 months, but when the soil temperature is at 70 degrees F, it will only hold 50 percent of the ammonia for about 2 months. He says that indicates ammonium retention is directly related to temperature of the soil. The use of nitrification inhibitors will significantly improve the efficiency of the ammonia in soils with higher organic content, but have a poor record of performance in soil with low organic content.

The fertility specialist says the nitrification inhibitors should be viewed as a management tool to reduce N loss. They are likely to increase yields when the N is applied at or below the optimal rate, but such benefits are unlikely when N is applied at higher than required rates, even when the soil has moisture. Fernandez says while the rate of nitrification is significantly reduced below the recommended 50 degrees F starting temperature, microbial activity will continue to convert the ammonium to a nitrate until the soil temperature reaches 32 degrees F. He says the 50 degree temperature is a realistic guideline because applying it earlier risks too much N loss, and waiting until later risks frozen fields.

Fernandez says most of the nitrogen applied in the late fall or early spring is converted to a nitrate by corn planting time because of the long periods of time while the soil temperature lingers between 32 degrees and when the soil is warm enough for corn to be planted. And he says, "In consideration of the date at which nitrate is formed and the conditions that prevail thereafter, the difference in susceptibility to denitrification and leaching loss between late fall and early spring applications of ammonium sources is probably small. Both are, however, more susceptible to loss than is N applied at planting time or as a sidedress application."

Nitrification inhibitors are valuable tools to retain nitrogen in the ammonium form, but their ability is diminished at higher ranges of soil temperatures, along with soil types, and organic content. Fertility specialists are beginning to recommend that 50 degrees F be the top end of the range of soil temperatures when anhydrous ammonia is applied with or without a nitrification inhibitor.