With wet weather and a late harvest last fall, plans to apply nitrogen fertilizer had to be postponed until this spring for many farmers. At winter meetings, Iowa State University Extension agronomists fielded questions about various options for spring nitrogen application for corn.

Some farmers are planning to apply the entire rate per acre prior to planting. Others are pondering whether to make a split application. Others are thinking about applying it all later as a sidedress after corn has emerged.  

Farmers also ask about types of nitrogen fertilizer, application methods for various nitrogen materials and how to avoid or minimize N loss. They are pondering what the spring weather conditions might be — another wet spring? Some are considering using a different form of nitrogen for that reason. If so, how should these materials and products be applied to avoid N loss and get best performance? 

ISU Extension soil fertility agronomist John Sawyer provides the following guidelines and recommendations to consider for spring nitrogen fertilizer application. 

Preplant N applications 

Urea, urea-ammonium nitrate solution and other nitrogen products are available. Fertilizers such as urea and UAN can be broadcast and incorporated with normal tillage before planting. Incorporating or injecting rather than leaving the fertilizer on the soil surface can prevent volatile N loss from granulated urea or urea in UAN as it converts to ammonium. Incorporating or injecting can also reduce runoff of the fertilizer nitrogen if a rapid rainfall (or snowmelt) occurs. 

If time is critical and UAN application is made with preemergence herbicides, then surface application is an option, although it is more risky, due to potential volatile loss (loss to the air) from the urea remaining on the soil surface (especially in no-till). Rainfall of at least ¼ to ½ inch within approximately two days after application will eliminate volatile loss concern. 

UAN is half ammonium nitrate and half urea, therefore volatile loss potential from UAN is half of that with urea. Applying UAN in a band on the soil surface will also reduce volatile loss to about half what you get with broadcast application. Predicting the amount of volatile N loss is difficult, but the loss increases with high amounts of surface crop residue (especially no-till), moist soils that are drying, warm soil temperatures, many days without rainfall, high soil pH, low soil cation exchange capacity and higher N application rates. 

Options to reduce N loss 

Although it is an added cost to decrease the risk of volatilization loss, using a urease inhibitor to slow urea conversion will provide time for rainfall to occur and move urea into the soil. Preplant or preemerge applications of UAN or urea can be part of a weed-and-feed or split-N system, with a full N application rate or rate to supply part of the total N application needed and the remainder applied sidedress. 

Another fertilizer option is polymer-coated urea, a product designed to delay urea release until soils warm up. To avoid product runoff, you should incorporate polymer-coated urea into the soil. 

Surface broadcast options, especially adapted to no-till, include ammonium nitrate and ammonium sulfate. These granulated products are either not used extensively in Iowa, or not typically used for main N applications, so there may be limited availability.

Nitrification inhibitors, designed to slow conversion of ammonium to nitrate, haven’t shown adequate yield response in Iowa research when spring-applied to justify use with fertilizers containing ammonia, urea or ammonium. 

If disturbing soil is a concern in no-till from injecting N, then broadcast application is an advantage. However, it also has the disadvantage of potential volatile losses from urea, surface runoff or immobilization of N with surface residue, and is not a highly recommended application. 

Anhydrous before planting 

Anhydrous ammonia has specific considerations. It must be injected, and the ammonia band will initially have high pH and considerable free ammonia which can damage corn seedlings and roots. There is no exact “safe” waiting period before planting, and injury to corn can happen even if planting is delayed for a considerable amount of time. The risk of ammonia injury depends on many factors, with several that are not controllable.  

Risk increases if application is made when soils are wet and then they dry (allowing ammonia to move up the injection track after application). Risk of injury to germinating and emerging corn seedlings also increases with higher application rates, and when soils with high clay content are wet (sidewall smearing of the injection track and ammonia moving toward the soil surface during application). Corn injury risk also increases when soils are very dry and coarse — the ammonia movement results in a larger band.  

A few management practices can reduce the risk of ammonia damage. Wait to apply when soil conditions are good, have a deep injection depth (6 to 7 inches or more) and wait several days until planting.

If the injection placement relative to future corn rows can’t be controlled, apply at an angle across the field to reduce entire sections of corn rows from being damaged. Another option, if the injection track can be controlled with GPS, is to offset the ammonia application a few inches from the future corn rows. With this guided system, no waiting period is needed.  

There is a similar free ammonia or salt issue with shallow-banded urea or UAN solution. Anhydrous ammonia nitrifies more slowly than products like urea or UAN solution, so it is a preferable preplant fertilizer for soils with greater potential for losses in wet conditions. 

Split, sidedress applications 

There is a wide time period for getting split or sidedress applications of N applied. Sidedress injection can begin immediately after planting if corn rows are visible or GPS guidance is used. Be careful so that soil moved during injection does not cover planted rows or small corn plants. It is easiest to inject in the middle between rows, and there is no advantage in attempting to place the injected band close to the corn row. Corn roots will reach into the inter-row at an early growth stage.  

Injected N can also be applied between every other row, and this will provide equivalent response as when placed between every row. Compared to preplant application, split or sidedress has the greatest chance of improved response on soils with high leaching potential (sandy soils) and on poorly drained soils prone to excess wetness and ponding. 

For many soils, when planting corn after soybeans in a crop rotation, there can be adequate N in the root zone to meet the needs of small corn plants. In wet and cool springs, where soil nitrate levels and mineralization rates are low, to avoid early N stress, there should be a preplant or planting time N application. This is especially important for corn following corn, where there is a greater likelihood of low soil nitrate and added N is needed for early growth.

Preplant or starter N can help meet the early needs of corn plants and is especially important if sidedressing is delayed significantly or if you are planning a mid- to late-vegetative growth stage application of N to the corn crop. 

With sidedressing, a urease inhibitor with surface applied and non-incorporated urea and UAN could help reduce volatile loss, like that described with preplant applications. A dry soil surface may be more common within the growing season, which will reduce volatile loss potential. The rate of N applied, and the amount of potential N loss, must be large enough to offset the cost of the urease inhibitor. Rainfall will eliminate volatile loss and is needed to move surface applied N into the root zone. 

Broadcasting N fertilizer 

Broadcasting granulated urea, ammonium sulfate or ammonium nitrate across growing corn can cause leaf spotting or edge browning when fertilizer granules fall into the corn whorl. Damage will be greatest with ammonium nitrate, but that product is not readily available in Iowa, and damage from ammonium sulfate is greater than with urea.

The chance of damage increases with larger corn plants and higher application rates. If the broadcast fertilizer distribution is good, not concentrated over the plants, and the application rate is reasonable, the leaf damage should only be cosmetic. 

Broadcast application of UAN solution across growing corn has the potential to cause significant leaf burn and reduced early growth. Depending upon severity of damage, reduced plant growth may be visible for several weeks after application.  

Research in Minnesota indicated that when corn plants were at V3 growth stage (three leaf collars visible), phytotoxic effects were worse at application rates greater than 60 pounds N per acre (rates applied were 0, 60, 90 and 120 pounds N per acre), but damage wasn’t permanent and did not adversely affect the stand or yield. When plants were larger than the V3 stage, plant damage was worse, and some yield depression occurred with the 120 pound N per acre rate.  

Many herbicides are applied using UAN as the carrier to minimize trips across fields. However, this strategy is only recommended prior to crop emergence. Almost all herbicides prohibit application in N solutions after corn has emerged. Check herbicide labels closely. 

If N is to be sidedress applied, application rates can be adjusted based on results of using the late spring soil nitrate test (LSNT). Soil samples, from a zero to 12-inch depth, are collected when corn is 6 to 12 inches tall. The nitrogen application rate adjustment is based on the measured nitrate-N concentration. See ISU Extension publication CROP 3140, Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production.