In-season nitrogen application is now more common with the emphasis on achieving maximum economic N efficiency and minimizing N loss to surface water. One of the goals of the Iowa Nutrient Reduction Strategy is to reduce N losses to the state’s streams, rivers and lakes.
Applying fertilizer N as a split application — some of it before planting and the rest of the annual rate per acre after corn is up and growing — is an option. It can be used to help avoid N loss from springtime rain events. Split application, sidedress application or applying N in-season can be used to adjust planned application rates, or touch up applications if N losses are suspected.
Now is a good time to consider your corn crop’s N status. Is there enough N available to finish out the crop? Let’s start with what needs to be considered regarding N that has been applied: When was it applied, in what form, how much, and what has happened since?
Anhydrous ammonia (NH3) which quickly converts to ammonium (NH4+), and liquid manure, which has a high proportion of N in the ammonium form, if applied in late fall after the soil is below 50 degrees F, had little chance for the ammonium to be converted to nitrate after application in the remaining fall period or during winter. Nitrate (NO3-) is the form that’s susceptible to leaching and denitrification.
Because the ammonium form does not leach or denitrify, it will not be lost from soil with excess soil wetness or leaching. That will change, however, as soils warm in the spring and ammonium is converted to nitrate. Then excessively wet conditions can result in loss of the applied N.
Some fertilizer and manure applications were made in early fall. That allowed time for conversion to nitrate, and hence some of the applied N was already in the nitrate form by late fall, and could have been lost during the heavy rains in late November and early December. In addition, with more fall conversion to nitrate, those applications would be further subject to loss in the spring.
2016’s early and warm spring allowed anhydrous ammonia to be spring-applied on a considerable number of acres. As for fall ammonia, it takes time to convert the ammonium to nitrate; hence, losses are typically less with spring N applications than fall. Because the conversion takes place over several weeks, soil sampling for nitrate determination can miss some of the applied N when it remains in the ammonium form. This can be an issue with the late-spring soil nitrate test if you are sampling fields that have ammonia or ammonium containing fertilizers applied close to sampling.
Urea-ammonium nitrate (UAN) solutions, either 28% N or 32% N, have half of the N as urea and half as ammonium nitrate. The urea component of this liquid form of N, if left on the soil surface, can have volatile loss after two or three days of either no incorporation or no rainfall to move the urea into the soil. If the UAN was incorporated either by tillage or rain within a few days, it would have been safe from such losses. The nitrate portion (one-quarter of the N in UAN) would be subject to leaching and denitrifying immediately.
Dry urea has the same loss potential with surface application as UAN. The amount of loss potential is greater at a given N rate due to a greater amount of urea. Urea needs to be converted to nitrate before leaching or denitrifying loss can occur. With both UAN and urea, the potential for volatile loss goes up with surface application, moist-to-drying soils, warm temperatures, high amounts of crop residue, higher N application rates, high pH soils, and low cation exchange capacity soils.
Amount of N applied
If you used the Corn N Rate Calculator, cnrc.agron.iastate.edu, to determine your application rate (maximum return to nitrogen or MRTN rate), then the question becomes: Have you lost enough N since application to justify adding more N as a sidedress or in-season application?
John Sawyer, Iowa State University Extension soil fertility specialist, tells us that in most of Iowa, if April, May and June rainfall adds up to 16 inches or more, we may need to add more N. That determination was made assuming an application to be justified, considering the cost, and it would be at least 20 pounds more N.
In southeast Iowa, if March, April, May and June rain adds up to 17 inches or more, we may need to add additional N.
In both cases, using this springtime rainfall method to answer the question about having at least adequate N or potential for needing additional N will give the correct answer about 75% of the time.
For farmers who did not use the Corn N Rate Calculator, and applied higher rates than suggested, the need for additional N in wet springs goes down.
If you applied less-than-suggested rates, or used the low end of the most-profitable-suggested N rates from the Corn N Rate Calculator, the need for additional N in wet springs goes up. No matter the decision process, there are some additional ways you can go about determining adequacy of the crop’s N supply. See accompanying article on Page 21 explaining the late-spring soil nitrate test (LSNT) and the use of chlorophyll meters and crop canopy sensors.
Wrapping it up
With all of the tools for determining corn N status, or potential loss in wet conditions, you still need to figure out an amount to apply. Once you determine a field needs additional N, you could use the guidelines for each method (spring rainfall, LSNT, chlorophyll meter, active canopy sensing). Or you can go by the old rule of thumb: About 50 pounds of additional actual N is adequate. You certainly don’t want to apply too high rates as that can be cost-prohibitive and can result in more nitrate loss to water systems.
Keep in mind we had more warm soils the last 16 days of April this year than normal, which in turn means we had more N mineralized than normal. Warm, moist soil is ideal for mineralization of N from organic matter to ammonium and nitrate, both of which can be taken up by growing plants. Needed fertilization rates for N do go down somewhat in years with warm, moist soil in much of the growing season.
Johnson is the ISU Extension field agronomist in central Iowa. Contact him at firstname.lastname@example.org.
By MARK Johnson
More helpful information
If you haven’t begun to use the Corn N Rate Calculator to help determine nitrogen application rates, see “Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn,” PM 2015, at store.extension.iastate.edu/Product/Concepts-and-Rationale-for-Regional-Nitrogen-Rate-Guidelines-for-Corn.
The calculator is:
• based on N rate response trials
• based on economic profitability
• not derived from yield level but from actual yield response, which pays for the N applied
• directly provides N application rate guidelines for corn following corn and for corn following soybeans
• does not rely on the inconsistent “soybean credit”
Another good information resource for N management that ISU agronomists often recommend is “Nitrogen Use in Iowa Corn Production,” CROP 3073, at store.extension.iastate.edu/Product/Nitrogen-Use-in-Iowa-Corn-Production.
This article published in the June, 2016 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2016.