To optimize corn response to applied nitrogen and to break through current yield barriers, we must move toward in-season nitrogen applications.

Hopefully, most corn has already been, or will be planted within the next few weeks. Many growers have applied all of their intended nitrogen fertilizer for the year, while others plan on using additional nitrogen during the growing season.

From an agronomy perspective, we strive to provide best nitrogen-use management practices for a wide spectrum of nitrogen-use preferences.

Nitrogen-use preferences are diverse and based upon economics, agronomics, convenience, supply, transportation, time requirements, equipment capabilities and personal experience. And decisions that affect the best nitrogen management are just as numerous and varied. Application timing, nitrogen fertilizer source, nitrogen fertilizer placement, nitrogen fertilizer rate, rotational cropping sequence, irrigation capabilities, expected yield goal and soil properties, all make the list. So it stands to reason that based upon the complexity of nitrogen within a cropping system, no one agronomic management program is universally superior.

However, I feel that to optimize corn response to applied nitrogen and to break through current yield barriers, we must move toward in-season nitrogen applications. Many high yielding corn producers have been using in-season nitrogen management for years. But, a large majority of corn growers in our region apply 100 percent of their corn nitrogen as a single, preplant application.

Making the split
In-season nitrogen applications can be based upon set rates and timings or be in response to planned soil/plant/environmental monitoring. Rescue in-season nitrogen applications are sometimes warranted, but by the time corn exhibits a nitrogen deficiency, growth and yield have already been sacrificed.

Increased yields are not the only reason to consider using in-season nitrogen applications. Nitrogen is very mobile and has many loss pathways. It is the most elusive of all crop nutrients. This makes fine-tuning optimum nitrogen management very difficult, especially if only one application is made annually. From an economical (improved nitrogen-use efficiency) and environmental (less nitrogen movement to non-targeted areas) viewpoint, applying some nitrogen during the corn growing season is justified.

Every year from mid-May through July, I receive several questions concerning current corn nitrogen status. There are always a few fields that receive too much nitrogen and many fields that receive too little nitrogen, or have lost enough of the applied nitrogen to reduce overall corn yield. Planned in-season nitrogen applications will help eliminate much of this "where has the nitrogen gone" dilemma.

Most corn nitrogen application rate recommendations are based primarily upon a realistic yield goal. Lesser components of most corn recommendation equations include measurable factors such as soil CEC, soil organic matter and previous crop. For example: The base corn nitrogen application rate for many soil test recommendations is 1.2 pounds of nitrogen per bushel of expected corn produced. Using this base as an example, a yield goal of 150 bushels per acre gives a nitrogen recommendation of 180 pounds per acre. The actual nitrogen recommendation may be less when organic nitrogen and other potential nitrogen resources are credited.

A typical environmentally induced yield range for a cornfield that averages 150 bushels per acre per year may be from 80 to 225 bushels per acre. Even when eliminating the lowest and highest yielding years, the realistic range may still lie between 100 and 200 bushels per acre. Corn in the 100 bushels per acre yielding year could be over-fertilized by 60 pounds of nitrogen and corn in the 200 bushels per acre yielding year could be under-fertilized by 60 pounds of nitrogen. This potential 120 pound per acre range explains why predicting an optimum nitrogen rate prior to the growing season is only marginally effective.

Know what is there
Crop/soil environments have some nitrogen buffering capacity. In environments where nitrogen is over-applied, some of the nitrogen may stay in the system as nitrate and ammonium (readily available sources) or organic (potentially available sources) materials, and be available to subsequent crops. In high yielding environments, where nitrogen is under-applied, soil plant systems often have an ability to release greater amounts of organic nitrogen. This available organic nitrogen fraction is often called "labile" nitrogen. Labile nitrogen dynamics partially explain why corn can produce yields that are higher than expected without additional nitrogen inputs.

This labile nitrogen fraction is not permanently stable and can be drastically reduced if a field is constantly "under fertilized." However, if long-term, sound nitrogen management is practiced using realistic yield goals, the overall flexibility and productivity of the systems are improved. By waiting until the growing season, we can adapt to a given year's environment and provide a much better estimate of a corn crop's yield potential for that year. Therefore, not only will the current year corn crop be more optimally treated, but also the labile nitrogen will remain in a better state of "equilibrium."

Soil testing for nitrate-nitrogen and ammonium-nitrogen can be done before planting or during the growing season. Many agronomists use these tests to estimate the soil's nitrogen supplying potential. Soil nitrate and ammonium levels represent a snapshot in time and may not determine the season-long nitrogen supplying capacity. I do not recommend testing for nitrate-nitrogen and ammonium-nitrogen on a routine basis because it is time consuming and may be misleading. I recommend soil nitrogen testing when the potential for a large amount of nitrogen loss has occurred.

Planned in-season nitrogen applications can be based upon soil test nitrogen measurements. However, I believe that planned in-season nitrogen applications based upon current environmental conditions, crop health, season-specific yield potential and good record keeping will offset the need for in-season soil nitrogen testing.

Many mid-season corn nitrogen monitoring questions that I receive concern fall-applied anhydrous ammonia. These questions come from producers or agronomists who would like to test soil nitrogen levels to determine if enough nitrogen from the fall application has remained in the root zone to optimize the remaining corn growth and developmental needs. When this concern arises, I recommend testing both soil nitrate-nitrogen and soil ammonium-nitrogen. By testing both major forms of inorganic nitrogen, we can evaluate what percentage of the ammonium from anhydrous ammonia has already converted to the more loss-susceptible nitrate-nitrogen form. I also recommend collecting two separate samples, with one taken to include the anhydrous bands and the other taken between anhydrous bands. This dual sampling will give an idea of natural versus applied nitrogen. Soil sampling depth needs to be 12 inches to insure reaching the depth of band placement. If band placement is unknown, then a minimum of 25 soil cores per sample are required to insure the sample will include some soil containing the added fertilizer.

Feed toward need
Spring preplant-applied nitrogen is also commonly monitored. Potential nitrogen loss pathways from both spring- and fall-applied preplant nitrogen include leaching, runoff, denitrification, volatilization and immobilization.

Any 100 percent preplant nitrogen application system requires mobile nitrogen to remain in place for several months. Confounding this situation is the fact that a corn crop takes up less than 5 percent of its total nitrogen needs between emergence and the six-leaf growth stage. Between the six-leaf stage and silking, a corn plant will take up greater than 60 percent of its total nitrogen needs. The remaining 30 to 40 percent nitrogen uptake occurs between silking and denting stages. From this uptake pattern, it is obvious that front-end loading a mobile nutrient like nitrogen can lead to reduced corn yields if nitrogen loss conditions exist during the growing season.

When needed, mid-season rescue nitrogen applications up to 1 to 2 weeks post silking can help reduce nitrogen deficiency induced yield losses. However, these late applications will not bring yields up to where they would have been if nitrogen deficiencies had never occurred.

The most consistent nitrogen management system for corn production has been to split nitrogen applications with some nitrogen applied preplant and some nitrogen applied during the growing season. The most common split application programs apply approximately one-third of the total nitrogen preplant and two-thirds of the total nitrogen at the five- to six-leaf corn growth stage. At the six-leaf growth stage, most application equipment can still be used without mechanical injury to the corn crop. Potential nitrogen losses between the six-leaf growth stage and silking are minimal, especially if nitrogen can be incorporated through placement, rainfall or irrigation.

Irrigation provides a means of "spoon-feeding" nitrogen anytime throughout the growing season. Producers using overhead irrigation systems with yield goals exceeding 150 bushels per acre should consider adding nitrogen injection units.

Fertility on the go
New technologies such as satellite imagery and on-the-go crop sensors are discussed as methods for analyzing in-season crop nitrogen needs. I worked with this crop-stress-based technology in the late 1980's while employed at the University of Missouri. Although, these technologies have the ability to document crop stress, they provide non-direct nitrogen deficiency measurements. Therefore, false positives and false negatives can exist. For example, every year I view apparent nitrogen deficiencies in four-leaf to 10-leaf corn that has received adequate amounts of injected nitrogen and has also been exposed to low nitrogen-loss environmental conditions. In many cases, the corn roots have yet to reached the injected nitrogen fertilizer band. Imagery, stress measurement and tissue testing would have misdiagnosed this corn as needing additional nitrogen inputs.

As sensor technology becomes more efficient, it may become the standard for in-season nitrogen management. Until then, the one-third preplant, two-thirds sidedress nitrogen application system provides an efficient nitrogen-use program. This two-pass program can be further improved by basing the midseason nitrogen application rate upon current environmental conditions and the current season corn crop yield potential.

In summary, optimum corn nitrogen management requires flexibility. High yielding corn cannot be consistently produced in the lower Midwest using a single, preplant nitrogen application strategy. Producers planning to produce high yielding corn need to implement planned in-season nitrogen applications as part of their overall management system.