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Trying to Optimize Nitrogen

Mar 15, 2013

Featured Seedsman: Stanley Knepp, Indiana,

What are your options for applying nitrogen fertilizer this spring?

Trying to optimize nitrogen (N) applications to maximize yield potential and profitability is a conversation about risk.  In a perfect world, we could predict the weather and input prices, and have unlimited resources for supplies, labor, and equipment.  However, we all know this is far from a perfect world!  It often makes sense to apply the majority of the crop required N with fall or spring applications and then supplement as needed by side-dressing.  It also helps to remember the pros and cons for various N sources.  So, let’s start with discussing fertilizer methods, timing, and amounts.NitrogenFertilizerImage

Anhydrous Ammonia (NH3)is the most widely used N fertilizer and is 82% N by weight.  It is a compressed gas that must be injected into the soil to minimize loss and can be applied in the fall, spring, or sidedressed.  Applying (NH3) to soils that are too wet or too dry can result in the loss of N into the atmosphere because of poor sealing at the soil surface.  Keep in mind that ammonia is a strong desiccant, which means it can injure living plant or animal tissue by removing water from them. To help reduce seed and seedling injury, wait 3 to 5 days to plant after applying anhydrous ammonia. The majority of the ammonia is converted to ammonium within 8 to 10 days after application. To help minimize seedling injury apply anhydrous at an angle to your corn rows.

Sidedressing, one of the most economical ways to apply nitrogen, should be done near V3 growth stage.  When supplementing previous N applications, sidedressing can be done through the V8 growth stage.

Urea is a combination of anhydrous ammonia and carbon dioxide.  It has a N content of 45% to 46%.  It can be purchased as prills or in the more common form of dry granules.  It is typically broadcast, and should be incorporated.  Urea is converted to ammonia and then to ammonium.  During this conversion process, urea left unincorporated on soil surface is subject to loss due to ammonia volatilization.  Volatilization is promoted by heavy surface residue, warm windy days, and high pH values on the soil surface.  To minimize N loss via volatilization, incorporation should occur within 2 to 3 days after application with tillage or 1/2 inch of rain.  Ammonium applied to the soil via urea will be readily accessible to microbial conversion to NO3-; rendering the compound at risk of leaching or denitrification.  Ammonium applied by anhydrous ammonium will not be entirely converted to nitrate for 4-7 weeks (as long as soil temperature is less than 50° F); whereas, ammonium from urea can be converted to NO3- in 7 days.  

Spring urea applications are the most common.  Over-the-top crop applications can be done; however, there tends to be more risk from non-uniform application, equipment traffic, and difficulty with incorporation.  That being said, it is a convenient form to use when top dressing wheat as it is relatively quick to apply and the cool temperatures at the time of application help minimize the loss.

Urea-Ammonium Nitrate (UAN) is created by dissolving urea and ammonium nitrate in water.  It is 28% to 32% N by weight. Urea-ammonium nitrate solutions contain 50% urea, 25% ammonium, and 25% nitrate, so losses commonly associated with each N form will also apply with UAN solutions. Fall applications are not feasible due to potential N loss through volatilization and leaching. Spring applications are common by either broadcast or injection.  Many preemergence herbicides are applied with UAN as the carrier. UAN can be applied as a side dress through injection or dribbled on. Injection or incorporation via tillage or rainfall is critical to minimize N loss regardless of application timing.

Ammonium Sulfate (AMS) can occur in two forms.  The conventional AMS is a dry product containing 21% N by weight. AMS can also refer to a manufacturing byproduct which is typically a liquid and only about 6% N.    

Conventional AMS has a fit for farmers who are looking for supplemental sulfur or are looking to reduce the pH in alkaline fields. It is generally applied in early spring or preplant. Post applications are not common due to application difficulties and potential crop injury. 

Byproduct AMS is most often applied in the winter or early spring to minimize compaction and facilitate the volume requirements. Applying a product that is only 6% N requires a large number of gallons to reach desired N rate.  

Other sources of N can include the N component of fertilizers such as diammonium phosphate (DAP), and the N credit from legume crops and manure.

Just remember to always consider the benefits of a nitrification inhibitor if temperatures are greater than 50° F.  The bottom line is when trying to maximize yield potential, make sure the crop has adequate N to do its job. Weigh the benefits and risks for different N application timings and sources and do what best fits your operation.

To learn more about optimizing nitrogen in your area contact Stanley Knepp from Indiana  ( or your local Channel Seedsman.

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