• Even though more fall nitrogen applications occurred in 2019 than 2018, some acres were left without fall nitrogen due to rain and wet soil conditions.
• Nitrification inhibitors prevent conversion of ammonium-nitrogen to nitrate-nitrogen.
• Nitrification inhibitors should be applied when the soil temperature reaches 50⁰F at a 4-inch soil depth, and long-term weather forecast predicts steady to lower temperatures.
• MiField trials conducted from 2016-2018 where a nitrification inhibitor was used with spring applied nitrogen revealed a good return on investment.

Even though more fall nitrogen applications occurred in 2019 compared to 2018, some acres were left without fall nitrogen due to rain and wet soil conditions.  These acres will be the target of spring applications of nitrogen + nitrification inhibitors. In this article, we will review the mechanisms that drive N-loss (time, temperature, soil moisture), and the best practices required for Nitrification inhibitor applications. Additionally, we will take a look at the performance of Instinct II (a.i. nitrapyrin) from Corteva in MiField trials over a period of three years and look at the National Oceanic and Atmospheric Administration (NOAA) weather outlook for the spring of 2020.

More than 90% of the FS fall applied nitrogen is done with N-Serve as a nitrification inhibitor.  N-Serve has the same active ingredient as Instinct II (nitrapyrin).  Nitrification inhibitors prevent the conversion of ammonium-nitrogen to nitrate-nitrogen. In saturated soil conditions, nitrate can be lost through leaching or denitrification.  Best management practices to protect our fall nitrogen investment include waiting for the soil temperature to reach 50⁰F at a 4-inch soil depth and waiting until the long-term weather forecast predicts steady to lower temperatures thereafter.   At this soil temperatures, nitrification slows down but does not completely stop until temperatures reach 32⁰F.  To better understand the soil temperature trend from north to south at a 4-inch depth, on bare soil, using Illinois as an example, view graphs 1 and 2 below for Dixon Springs, IL (southern IL) and Freeport, IL (northern IL) respectively.

Note that in Dixon Springs this winter soil temperatures only surpassed the 50⁰F mark for a short period, between December 27^th and January 3^rd.  Meanwhile in Freeport, soil temperatures never broke the 50⁰F mark.  These temperatures indicate that little to no nitrification has taken place in these areas of Northern and Southern Illinois during this time period.  So, if little nitrogen has converted to nitrate, why are we even using a nitrification inhibitor?  To understand the value of nitrification inhibitors, let’s look at the spring temperatures from February to May 2019, at a 4” soil depth for Dixon Springs and Freeport (Graph 3 & 4).

At both locations, soil temperatures begin to climb steadily above the 50⁰F point around April 1^st.  Note that this climb happens well before corn plants are established to take up nitrogen and put potential loss to an end.  The reality is that nitrification inhibitors provide much of their protection in the timeframe from when the soil temperature surpasses 50⁰F in early April to the V8 corn growth stage, at which nitrogen uptake increases rapidly. 70% of the nitrogen is used up by the corn crop by the time the plant reaches the tassel stage. Nitrogen applied without a nitrification inhibitor during this timeframe is prone to leaching through the soil profile, to tile drains as a water pollutant, or subject to denitrification when soils approach >75% saturation.  Denitrified nitrogen is lost to the atmosphere as a greenhouse gas.  Additionally, keep in mind that today’s hybrids need 30% of their nitrogen in the reproductive stages.  Corn’s roots will not be able to access any nitrogen that has moved below the top foot of soil, where 80% of the corn’s root mass is found.

What do MiField trials tell us about nitrification inhibitors?

 Instinct II contains the same active ingredient as N-Serve (nitrapyrin) only differing in inert ingredients. MiField trials conducted from 2016 to 2018 in 26 locations, evaluated Instinct II in early spring to protect UAN (urea ammonium nitrate solution containing 75% ammonium, 25% nitrate).  In these 26 locations, acres receiving Instinct II yielded 11.8 bu/A more than the grower standard, resulting in a return on investment of $32.05 (Figure 1).  Keep in mind that the 25% nitrate in UAN is a leachable, denitrifiable form of nitrogen making it more difficult to prove an ROI. 

2020 Spring Midwest Weather Outlook

As we have already mentioned, some of the key factors driving nitrogen loss are soil moisture and temperature. So, the potential for nitrogen loss may be predicted by looking at the weather outlooks for this spring. Three-Month Outlooks are available for the United States at the NOAA website.  The current outlook from March thru May 2020 indicates that much of IL, WI, and IA forecasts equal probability of above normal to normal rainfall and above to below normal temperatures. 

Summary

Fall nitrogen applied after the soil temperature reaches 50⁰F rarely experiences much nitrification until spring, usually around the end of March to the beginning of April.  So, N-Serve applied with fall nitrogen provides most of its protection against nitrification ahead of the V8 growth stage in the spring.  Corn uptake of nitrogen doesn’t significantly increase until V8, still leaving 30% of its uptake needs for the reproductive stages.  MiField trial data from 2016-2018 where a nitrification inhibitor was used with spring applied nitrogen revealed a good return on investment.  This spring’s weather outlook appears to have a high probability of wet conditions, making the use of nitrification inhibitors with spring applied nitrogen a good practice to reduce nitrogen loss. 

Graphs:  *Source: Water and AtmosphericResources Monitoring Program. Illinois Climate Network. (2015). Illinois StateWater Survey, 2204 Griffith Drive, Champaign, IL 61820-7495. https://dx.doi.org/10.13012/J8MW2F2Q