Managing nitrogen is one of the most difficult and unpredictable things that grain producers need to do.  In a world where environmental concerns come to the forefront more and more, nitrogen management deserves attention.

Fall application of nitrogen is popular among retailers and farmers in Illinois. One reason is that it spreads the spring workload out. The first caution with fall applied nitrogen is that it is not recommended south of Illinois Route 16. There have even been years when fall applied was a bad idea south of I-72. It is also not recommended when soil temperatures are above 50oF in the fall, and only the ammonium form of nitrogen should be fall applied. This means anhydrous ammonia or ammonium sulfate.

In order to manage nitrogen we need to understand what happens when we fall apply. When the ammonium form of N is applied, NH3 quickly combines with soil moisture and becomes NH4 +. The positively charged ion then attaches to negatively charge clay particles and becomes stable when soil temperatures are below 50o. Nitrosomonas, Nitrosococcus, Nitrobacter and Nitrococcus bacteria all become active when soil temperatures warm up past 50o F and convert the NH4+ to NO3-. This process is called nitrification.

Nitrification can happen in February as easily as April if the temperatures are right. The nitrate form of nitrogen is less stable, but more easily utilized by plants. Nitrate can also be leached in wet weather and broken down by denitrification. When soil is saturated, many bacteria will denitrify the soil by using some of the oxygen on the nitrate as an oxygen source. Nitrate converts to forms that are not plant available when denitrification takes place. When weather warms up, no matter what month, nitrification takes place. In order to slow nitrification, we can treat with an inhibitor like N-Serve®. Inhibitors can help keep your fertilizer nitrogen stable for up to 30 days of warm weather.

So, what can growers do in the future if #plant2019 repeats itself? First, make sure that soil conditions are suitable for application and that your trench is closing properly whenever you apply anhydrous ammonia. Appling ammonia in wet soils with a mole knife shears sidewalls and the slot may not close adequately, allowing some of the ammonia to escape.

Nitrification inhibitors have been mentioned as one of the tools to manage nitrogen. Tile drainage can reduce the amount of saturation in the soil and slow down the denitrification process. With tile drainage, some sort of filtering mechanism may be appropriate instead of discharging directly into surface waters. Bio filters and saturated buffers can be effective. Time of applications can also help.  Consider spring applications, split applications or even sidedress to avoid loses.

Looking at your overall soil health will also provide nitrogen benefits. Consider using no till; it can help decrease some nitrogen losses. Tillage will promote the denitrification bacteria through the breakdown of the organic matter in the soil. The addition of cover crops into a rotation can gain you even more benefits in potential nitrogen management. Cereal grains such as oats and rye work extremely well to uptake the extra nitrogen that will be in the soil. This nitrogen will be immobilized in the plant until the cover crop is terminated and will be released slowly as the residue breaks down.  On the flip side, legume cover crops can be an opportunity to gain nitrogen as well. When considering covers always do your research and determine what your goals are for nitrogen management from the cover crops.

Determining whether you need additional nitrogen fertilizer during the season needs to be more than a guess. Nitrogen modelling can be helpful in figuring out what is going on with your soil N. Climate, Encirca®, AdaptN® and many others are available. Some work better than others, but I offer no endorsement. In-field testing can be useful.

Early season soil testing should include ammonium and nitrate levels to determine if nitrogen is needed or how much nitrogen to sidedress. Yield 360 SOILSCAN® uses a nitrate test to determine what additional N is needed at sidedress. That can also be useful for a later application using a high clearance applicator or Y drops.

Cultivating with a traditional cultivator or sidedress toolbar can add some oxygen to the system and cause re-nitrification to a certain extent.

Using nitrogen on soybeans is a question that comes up from time to time. Some researchers have gotten a response to soybean nitrogen, but often it is not economical. However, some well-known soybean researchers have gotten good yield response to ammonium sulfate. The response is likely more from the sulfur than the nitrogen, but it could be a positive combined effect.

Proper nitrogen management requires some thought and risk assessment. Producers should consider the risk involved in fall nitrogen application and should use a scientific approach to decide on in-season nitrogen applications.

Thanks to Nathan Rahe, NRCS Soil Conservationist in Knoxville, Iowa for assistance with covercrop advice. 

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About the Author: David Rahe

Rahe specializes in soil fertility, site specific fertilizer application, and soil management and is a Certified Professional Soil Scientist, Certified Professional Soil Classifier, and 4R Certified Crop Advisor. Rahe is a graduate of the University of Illinois with a bachelor’s in Agronomy. After a 25-year career with the Natural Resources Conservation Service in Illinois and Missouri, Dave began consulting in 2005. He is currently a partner with RPM Soils LLC and can be reached at