Optimizing N application is influenced by minimizing potential N loss.
Ammonium-N is fairly stable and not subject to denitrification or leaching.
Nitrogen stabilizers can be used to reduce the potential for N loss.
In the Midwest, consider using the Corn Nitrogen Rate Calculator, an online tool, to determine the most profitable N application rate.
Optimizing nitrogen (N) application can be influenced by several factors including source of N, rate, application timing, and method of application. Selecting the appropriate form of N fertilizer and application method may be confusing as there are pros and cons to be considered before making a decision. However, the process can be made clearer once we understand how environmental conditions, such as soil type, soil temperature, and form of N (ammonium-N, nitrate-N, urea) influence the potential for N loss throughout the growing season.
N Loss Mechanisms
N can be lost prior to being utilized by corn plants primarily by the processes of denitrification, leaching, or volatilization.
Denitrification is the process by which nitrate-N is converted by bacteria into forms of N gas which move freely up through the soil and can be lost to the atmosphere. The process is initiated by bacteria that are anaerobic, meaning they are active when oxygen levels are low. This means that most denitrification occurs under saturated soil-water conditions. Denitrification is believed to be the main process by which nitrate-N is lost, except on sandy soils, where leaching is the major pathway.1 The amount of nitrate-N loss is influenced by several factors, primarily the length of time soils are saturated and soil temperature. University of Illinois estimates indicate when soils are saturated, daily nitrate-N loss by denitrification can be 1% to 2% at soil temperatures less than 55 °F, 2% to 3% if soil temperatures are between 55 and 65 °F, and 4% to 5% at soil temperatures above 65 °F.1
Leaching occurs when nitrate-N moves downward in the soil profile out of the root zone with excessive precipitation. In sandy soils, nitrates may contaminate groundwater but in heavier-textured soils, leached nitrates typically reach tile lines and may eventually reach surface water.
Volatilization of urea-based fertilizers can occur when they are surface-applied and not incorporated. Urease enzymes in soil and plant residue convert urea to free ammonia gas. On warm, sunny days, up to 15 to 20% urea-based N can volatilize within a week of application.2
Sources of N and Application Methods
Anhydrous Ammonia (NH3) 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 anhydrous ammonia 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. To help reduce seed and seedling injury, wait three to five days to plant after applying anhydrous ammonia. For preplant applications, consider applying anhydrous at an angle to your corn rows to help minimize seedling injury.
After application, ammonia converts to ammonium fairly quickly, which binds to soil particles and is not subject to nitrogen loss. Soil bacteria convert ammonium to nitrates which are subject to loss via denitrification and leaching. Fall application should not occur until soil temperatures at the 4-inch depth are below 50º F and trending downward because bacteria activity is reduced at these temperatures thus resulting in a slower conversion rate of ammonium to nitrates. Nitrification inhibitors, such as N-Serve®, inhibit the activity of bacteria that are responsible for converting the stable ammonium-N to nitrate-N. Some farmers are reluctant to use nitrification inhibitors because of the concern that these products “tie-up” nitrogen. This is not the case as the inhibitors have activity on soil bacteria and do not have any direct activity on ammonium-N or nitrate-N.
Urea is a dry pellet fertilizer that is 46% N by weight. After application, urea converts to ammonia and then to ammonium. During this conversion process, urea left unincorporated on the 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 three to four days after application with tillage or by 1/2 inch of rain.3 Ammonium applied to the soil originating from urea will also be accessible to microbial conversion to nitrate-N rendering the compound at risk for leaching or denitrification. When applied as a band, urea may cause root and seedling damage if placed within close proximity to the seed.4
Urea can be treated with an urease inhibitor such as Agrotain® to reduce N loss by ammonia volatilization. Additional nitrogen stabilizers include ESN®, which is a polymer-coated urea for slow release of N and Super U™, which contains urea, an urease inhibitor, and a nitrification inhibitor.
Urea-Ammonium Nitrate (UAN) solutions generally contain 28% or 32% N by weight. The N composition is 50% urea, 25% ammonium-N, and 25% nitrate-N. UAN should not be broadcast on high residue surfaces, but instead applied via knife or coulter injection, or can be dribbled in a surface band to reduce residue tie-up.4 Treatments such as urease inhibitors to reduce volatilization loss, if surface applied, are the same as with urea.
Timing of application can be influenced by factors such as weather and workload. It is desirable to apply N as close to the period of rapid plant uptake as possible so there is reduced risk of N loss prior to plant use. When application closest to the period of rapid uptake is the goal, sidedress applications are preferred over preplant, and preplant applications are preferred over fall applications. Fall application has a higher risk of N loss, but may be done to take some workload off the busy spring schedule. If applying N in the fall is preferred, anhydrous ammonia is recommended because it has the lowest risk of N loss of any N fertilizer.3,4 Also, for fall application fields, consider applying part of your N in the fall and the remainder close to planting or as sidedress, which would help minimize the potential for N loss in the spring.
Overall, spring pre-plant, sidedress, or split pre-plant/sidedress applications are preferable due to less potential N loss and improved timing in relation to plant uptake. However, spring can also be a time of great potential N loss due to wet soils. If N is to be applied more than two weeks prior to planting, anhydrous ammonia is recommended to reduce risk of loss.3 When considering in-season N applications, the preferred method is injection of ammonia or UAN, followed by UAN dribbled between rows or broadcast urea.4 Broadcast UAN is not a safe in-season option due to potential plant burn.
Nitrogen Rates and Timing
Consider using the Corn Nitrogen Rate Calculator at http://extension.agron.iastate.edu/soilfertility/nrate.aspx to determine a N application rate. The online tool takes into account previous crop, expected corn price, N price, and geography when calculating the Maximum Return to N (MRTN) rate and the Most Profitable N Rate Range. The MRTN rate changes as the price of N and expected selling price of corn fluctuates. The online tool was developed by Midwestern Universities in Illinois, Indiana, Iowa, Ohio, Minnesota, Michigan, and Wisconsin utilizing results from on-farm N studies in their respective states.
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1Fernandez, F.G., Nafziger E.D., Ebelhar S.A., and Hoeft, R.G. 2009. Managing nitrogen, Chapter 9. Illinois Agronomy Handbook. http://extension.cropsci.illinois.edu/handbook/
2Nielsen R.L. 2006. N loss mechanisms and nitrogen use efficiency. 2006 Purdue Nitrogen Management Workshops. www.agry.purdue.edu.
3Scharf, P. and Lory, J. 2006. Best management practices for nitrogen fertilizer in Missouri. IPM 1027. MU Extension, University of Missouri-Columbia. http://extension.missouri.edu/p/IPM1027
4Sawyer, J. and Creswell, J. 2002. Nitrogen application. NMEP 7I. Iowa State University Extension. https://store.extension.iastate.edu/
5Chatterjee, A. 2013. Nitrogen fertilizer additives: Which ones to choose. Crops & Soils Magazine. November-December 2013.
Web sources verified 01/26/15.