Cold and snow kept Iowa farmers out of the fields during March and most of April. Those conditions have led to a compressed period for fieldwork before corn planting begins.

Some farmers are switching planned spring preplant anhydrous ammonia application to another nitrogen product such as urea-ammonium nitrate solution (28% or 32% UAN) or granulated urea. There are also discussions about changing from preplant to sidedress N applications.

What should be considered? “Perhaps the most important item is to have a conversation between dealer and farmer to ensure product availability when desired, equipment needed for application, and any associated change in costs,” says John Sawyer, Iowa State University Extension soil fertility specialist.

In the following article, Sawyer explains what farmers should think about before making a switch in how they apply and manage nitrogen this spring. First, he explains the considerations for making preplant N applications; then he looks at making sidedress applications.

Preplant applications
If planned N fertilizer applications can be made without an undue delay in planting, then make the applications.

• Urea, UAN solution and other products. Forms such as urea, UAN liquid solution or dry polymer-coated urea granules can be broadcast and incorporated with normal tillage before planting. Incorporate or inject rather than leave the fertilizer on the soil surface. Incorporating or injecting will avoid volatile N loss from granulated urea or urea in UAN as it converts to ammonium, or runoff if a rapid rainfall (or snowmelt) event occurs.

If time is critical and UAN application is to be made with preemergence herbicides, then surface application is an option. However, it’s riskier due to potential volatile N loss from being sprayed or broadcast and the applied N remaining on the soil surface (especially in no-till) if there is not sufficient rain to move the urea into the soil. A rainfall of at least a quarter to a half 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. Banding UAN on the soil surface will also reduce volatile loss to about half that with broadcast application.

How much volatile N is lost?
Predicting the amount of volatile N loss is difficult, but it increases with high surface crop residue (especially no-till), moist to drying soils, warm soil temperatures, many days without rainfall, high soil pH, low soil cation exchange capacity, and higher N application rates.

Although it adds cost, a urease inhibitor can be added to slow urea conversion, which provides time for rainfall to move urea into the soil and decreases the risk of volatile loss. Preplant or preemerge applications can be part of a weed-and-feed or split-N system, either with a full N rate, or rate to supply part of the total N application need and the remainder applied sidedress.

Another fertilizer option is the polymer-coated urea in dry granular form; the coating is designed to delay urea release until soils warm. To avoid product runoff, you need to incorporate it into the soil. Surface broadcast options, especially adapted to no-till that generally do not have volatile loss concern, are ammonium nitrate and ammonium sulfate. These products aren’t used extensively in Iowa as a primary N material, so they would likely have limited availability.

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

• Applying anhydrous ammonia before planting. Anhydrous ammonia has certain considerations. It must be injected, and the ammonia band will initially have high pH and considerable free ammonia, which can damage (burn) corn seedlings and roots. There is no exact “safe” waiting period before planting, and injury can happen even if planting is delayed for a considerable time period. Risk of ammonia injury depends on many factors, and several of them are not controllable.

Risk increases if application is made: 1) when soils are wet and then dry (ammonia moving up the injection track); 2) with higher application rates; 3) when soils with high clay content are wet (sidewall smearing of the injection track and ammonia moving toward the soil surface during application); and 4) when soils are very dry and coarse-textured (larger ammonia band).

A few management practices can reduce the risk of ammonia damage: Wait and apply when soil conditions are good; have a deep injection depth (6 to 7 inches or more); or wait several days until planting. If the injection placement relative to future corn rows can’t be controlled, apply at an angle to reduce entire sections of corn rows from being damaged.

If the injection track can be controlled with GPS technology, then you can offset a few inches from the future corn rows — with this guided system, no waiting period is needed. There would be a similar free ammonia and/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 fertilizer for soils with greater potential for losses in wet conditions.

• Sidedress applications of nitrogen. The best options for sidedressing, in approximate order from most to least preferable (and depending on crop emergence and size) are:

• injected anhydrous ammonia, UAN or urea
• broadcast granulated ammonium nitrate or ammonium sulfate
• surface-applied urease inhibitor-treated urea or UAN
• surface-dribbled UAN solution
• broadcast urea
• broadcast UAN

There is a wide time period for making sidedress applications. Sidedress injection can begin immediately after planting if corn rows are visible or GPS equipment is used. Be careful so soil moved during injection does not cover seeded rows or small corn plants. It is easiest to inject in the row middle, and there is no advantage in attempting to place the injected band close to the corn row. Corn roots will reach into the row middle at a small growth stage.

Injected N can also be applied between every other row, and this will provide equivalent response as when placed between every row. For many soils, when planting corn after soybeans, there can be adequate N in the root zone to meet the needs of small corn plants. For corn after corn, there is a greater likelihood that additional N is needed for early growth. Preplant or starter N can help meet early plant needs, and is especially important if sidedressing is delayed significantly or there will be a planned mid- to late vegetative growth stage application in either rotation.

Should you use a urease inhibitor?
With sidedressing, a urease inhibitor with surface-applied and nonincorporated urea and UAN could help reduce volatile loss, similar to that described above 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 hence the amount of potential N loss, has to be large enough to offset the inhibitor cost. Rainfall will eliminate volatile loss and is needed to move surface-applied N into the root zone.

Broadcasting granulated urea, ammonium sulfate or ammonium nitrate across growing corn can cause leaf spotting or edge browning where fertilizer granules fall into the corn whorl. Damage will be greatest with ammonium nitrate, but that product is not readily available or used in Iowa, with damage from ammonium sulfate more than urea. The chance of damage increases with larger corn and higher application rate. As long as the fertilizer distribution is good — not concentrated over plants — and the rate reasonable, the leaf damage should only be cosmetic.

Broadcasting liquid UAN can cause leaf burn
Broadcast application of UAN solution across growing corn has the potential to cause leaf burn and reduced early growth. Depending upon the severity of damage, reduced plant growth may be visible for several weeks after application. Research conducted in Minnesota indicated that when corn plants were at the V3 growth stage (vegetative leaf stage defined according to the uppermost leaf with a leaf collar visible — in this case three leaf collars visible), phytotoxic effects were worse at rates greater than 60 pounds N per acre (rates applied were 0, 60, 90 and 120 pounds N per acre), but damage was not permanent and did not adversely affect stand or yield.

When corn plants were larger than the V3 stage, plant damage was worse, and some yield depression occurred with the 120-pounds-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.

Adjusting N application rate
If N is going to be sidedress-applied, then rates can be adjusted from results of the late-spring soil nitrate test (LSNT). Soil samples, zero to 12-inch depth, are collected when corn is 6 to 12 inches tall with rate adjustment based on the measured nitrate-N concentration shown by the LSNT.

If corn becomes too tall for normal sidedress equipment, it is possible to use high-clearance equipment to apply N. The N source often will be UAN solution, with equipment available to either dribble the solution onto the soil surface with drop tubes, or shallow-inject with coulter-shank bars (coulter-disk injected), and urea, which can be broadcast-spread across the top of corn.

Research in Iowa has shown corn can respond to N application at mid- to late vegetative corn growth stages when there is deficient N supply, but there can be loss in yield potential. Reduced yield occurs more frequently when soils are dry at and after application (applied N not getting into the root zone), and with severe N stress. Best responses occur with sufficient rainfall shortly after application to move N into the active root zone.

Apply N later in season?
If attempts to get N applied preplant or early sidedress have failed, or there are concerns about N supply from early fertilizer or manure applications, then mid- to late vegetative stage application can be a helpful rescue. Having some non-N-limiting (about 50% more than normal rate) reference strips or areas in fields is helpful for comparisons.

These areas can be used to determine visually if corn would respond to additional N, as a check to see if earlier N applications are not sufficient and determine if plants are showing growth or coloration symptoms that are not due to N deficiency. These reference areas are also needed for N stress-sensing tools (such as chlorophyll meters, active canopy sensors or satellite images) to help guide application rates and understand N stress across landscapes.

These reference areas should be planned with N applied early in the season, or be field areas known to be non-N-deficient. Plant and canopy sensing can begin when corn is at the V8 to V10 growth stage. If late N application is needed, it should be applied as quickly as possible and not later than the tassel/silking stage.

Summing up
• Fertilize before planting if it doesn’t greatly delay corn planting; otherwise, consider split or sidedress N application.

• If you decide to change planned N applications, make certain needed N fertilizer products and sidedress or high-clearance equipment will be available; or if hiring applications, be sure the dealer or custom applicator can accomplish the applications.

• Consider the N volatilization potential of different N materials when applying without incorporation or injection into the soil.

• Try not to make poor N management decisions just to get applications completed.

• Communication between farmer and dealer is key.