Published at the Chat 'n Chew Cafe, 21 Feb 2001

Optimizing Nitrogen Fertilizer Decisions

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-1150
Email address:

Nitrogen fertilizer will likely remain high-priced and in questionable supply for the upcoming 2001 corn cropping year in Indiana and other parts of the U.S. Midwest. While some farmers may opt for switching a portion of their intended corn acreage to soybean in response to the nitrogen issue, most will likely ‘ride it out’ as best they can.

As with most cropping decisions, there is no single best answer for how to manage these uncertainties of N price and supply. The suggestions offered in this article will help farmers fine-tune their N application rate calculations and maximize the crop’s N use efficiency.

Nitrogen Rate Recommendations

Nitrogen fertilizer rate recommendations are influenced by your yield goal for the corn crop, the timing of fertilizer applications, and any possible N contributions from previous crops or waste applications to the field. The bottom line is that there are opportunities for reducing total N costs by ensuring that the following factors are taken into consideration when making your N rate determinations.

Yield Goal. Nitrogen fertilizer rate recommendations are typically based strongly on the yield goal determined for the field in question. It is imperative that the yield goal be realistic and not ‘pie-in-the-sky’. Given that any year’s crop yield will be determined primarily by the weather, it is not unreasonable to use the average three- to five-year yield for a field as the yield goal for 2001. For the typical corn/soy crop rotation, this obviously requires field crop records for the past six to ten years.

Timing of N Applications. Pre-plant applications of N fertilizer are typically less efficient than sidedress N applications, meaning that there are more opportunities for N loss to occur during the time from pre-plant to crop uptake as compared to the time from sidedress to crop uptake. Consequently, sidedress N fertilizer rates can be decreased in recognition of its greater N use efficiency.

Credits For Existing Nitrogen. Previously grown legume crops (soybean) or applications of organic wastes (manures, biosolids) may contribute nitrogen to the following corn crop. Such contributions are typically taken into account when making N fertilizer rate recommendations.

Putting these factors into practice results in the following nitrogen fertilizer rate recommendations suitable for most corn cropping conditions in Indiana. Remember that nitrogen rate recommendations represent the total of all N fertilizer applications, including that included in any starter fertilizer that is applied.

  1. For corn following soybean, the rate of pre-plant applied N fertilizer (lbs. of actual nitrogen) is simply equal to the realistic yield goal (number of bushels per acre). For example, a realistic yield goal of 150 bu/ac would require 150 total lbs. of actual nitrogen.
  2. For corn following corn, the rate of pre-plant applied N fertilizer (lbs. of actual nitrogen) is equal to the realistic yield goal (number of bushels per acre) multiplied by 1.2. For example, a realistic yield goal of 150 bu/ac would require 180 total lbs. of actual nitrogen (150 multiplied by 1.2).
  3. The rate of sidedress applied N fertilizer (lbs. of actual nitrogen) can be reduced by 10% if more than half of the nitrogen fertilizer will be applied in the sidedress application. For a corn/soybean example, a realistic yield goal of 150 bu/ac would require 135 total lbs. of actual nitrogen (150 minus 10%).

Pre-Sidedress Nitrate Test (PSNT). The PSNT is a soil test procedure that is valid for organic soils (20% OM or greater) or where organic wastes have been applied to the field (manures, biosolids). The results of this test can be used to modify sidedress N rate recommendations based on the predicted availability of mineralized nitrogen from the organic components of the soil. In some situations, the PSNT may indicate no need for additional sidedress N applications.

Nitrogen Use Efficiency

Sound Agronomic Practices. Recognize that a healthy, vigorously growing corn crop is much more efficient in utilizing all crop inputs than a crop that is under some form of stress. So, if you are looking to improve the crop’s nitrogen use efficiency, then manage all aspects of the crop to the best of your ability to minimize stress to the crop throughout the season.

Know Your Nitrogen Sources. Part of the challenge facing corn growers in 2001 will be that their nitrogen source of choice may simply not be available and so will be forced into using a nitrogen source with which they are not familiar. Recognize that the corn crop could care less which fertilizer source of nitrogen is used. The agronomic differences among N fertilizer sources lie mainly in their relative risk for nitrogen losses due to leaching, denitrification and volatilization.

Nitrate-containing fertilizers (UAN liquids, ammonium nitrate) are susceptible to leaching and denitrification nitrate losses from the day they are applied to the field. Urea-based fertilizers (urea, UAN liquids) convert relatively quickly to nitrate forms of nitrogen and are subsequently susceptible to the same N loss mechanisms. Consequently, these forms of fertilizer N are not well suited to early pre-plant applications, but rather to later pre-plant or sidedress applications.

Urea-based fertilizers are also vulnerable to volatilization losses when surface-applied and not incorporated into the soil, especially so in high-residue tillage systems. Part of the conversion of urea to nitrate involves the formation of ammonia, which is very volatile. When this conversion occurs on the surface, quite a bit of the nitrogen may ‘disappear into thin air’ and not be available to the developing crop. Such surface-applied N is also used by microbes for the decomposition of plant residue and may not become available to the crop until later in the season. For these two reasons, urea-based fertilizers should be injected below the surface trash or at least applied in concentrated bands over the surface as opposed to broadcast surface applications in high-residue tillage systems.

Anhydrous ammonia eventually converts to nitrate also, but the process is much lengthier than other forms of N. Consequently, anhydrous ammonia is typically better suited for early pre-plant applications than other forms of N.

For more details about nitrogen fertilizer decision-making for 2001, obtain a copy of the following Purdue publication from your local county Extension educator or from the Web address listed.

Sylvie Brouder, Brad Joern, Tony Vyn, and Bob Nielsen. Feb. 2001. Nitrogen Decision$ 2001: The Soil Fertility Perspective. Purdue University, Agronomy Dept. AGRY-01-01.

You can also read what our colleagues in adjacent states are saying about this year’s nitrogen issues by browsing the following Web articles:

(Missouri) Nitrogen prices - how do they affect optimum N management?

(Kentucky) High Prices and Nitrogen on Wheat

(Illinois) Getting The Most From Your 2001 Nitrogen Dollars

(Michigan) Estimating the most cost effective nitrogen rate for corn

KingCorn.orgFor other information about corn, take a look at the Corn Growers Guidebook on the World Wide Web at

It is the policy of the Purdue Agronomy Department that all persons shall have equal opportunity and access to its programs and facilities without regard to race, color, sex, religion, national origin, age, or disability. Purdue University is an Affirmative Action employer. This material may be available in alternative formats.
© 2001, Purdue University
End of document