G.V. Johnson, W.R. Raun, John Solie and Marvin Stone


Departments of Plant & Soil Sciences and Biosystems and Ag Engineering

Division of Agricultural Sciences & Natural Resources

PT 2002-19                                                         July 2002                                                  Vol.14,   No.19   


Nitrogen fertilizer is one of the largest seasonal variable costs for dryland farmers.  For the last 30 years the “OSU” recommendation has been to determine fertilizer needs from a recent soil test of available nitrate-nitrogen and a realistic crop yield goal.  For wheat, we have assumed a need of 2 lb N per bushel of wheat in the yield goal.  After subtracting soil test N from this yield goal-based requirement, the difference was to be applied as fertilizer, usually preplant.  While this has been a good approach, it was based on the response of wheat to N fertilizer averaged over many years.  The attractive part of this approach was that it relied mainly on knowing the realistic yield goal for the field.


What is wrong with current nitrogen management?

There are four problems with how nitrogen fertilizer is used today, and two of them have to do with what kind of a ‘production’ year it will be.  The four problems are:


1.       We don’t know what the potential yield for a field will be this year.

2.       We don’t know how much non-fertilizer N (soil-N) will be available for the potential yield.

3.       We apply all, or most, of the seasonal nitrogen requirement before we plant the crop.

4.       We don’t use different rates for areas of the field that have different potential yields.


These “faults” of the traditional strategy are a result of assuming it will be an “average” production year and that the field is perfectly uniform.  In dryland farming, to assume it will be an average year is to assume the weather will actually be average for the year.  Everyone that has farmed for a few years knows that average weather conditions seldom occur.  Likewise, when whole fields are viewed from a distance (as from an airplane at 10,000 feet), they seldom appear to be very uniform.


What does research show?

Results of 30 years continuous wheat research from the OSU North Central Research Station at Lahoma show that when we fertilized for the average maximum yield (2 lb N/bu):

·         60 % of the time we will be off the actual yield by at least 10 %.

·         Additionally, because available non-fertilizer N changes from year-to-year,

o        30 % of the time the average rate will be the correct rate.

o        37 % of the time we will be short by at least 20 lb N/acre.

o        33 % of the time we will have applied 20 to 80 lb excess N/acre (average = 38 lb).

Also, compared to applying all the crop needs as preplant N:

·         Topdress N was about 35 % more efficient than preplant N.


How do we overcome traditional strategy faults? 

To improve on the traditional approach we need a strategy, or plan, that gives us a chance to “read nature”, that is, identify each year what nature is providing in the way of potential yield and the non-fertilizer nitrogen to support that yield.   This reading of nature needs to be done during the growing season at a point when we will still have time to add needed nitrogen.  The reading is made possible by reducing, or eliminating, preplant nitrogen except for a strip (spreader width) through the field at a rate that will assure nitrogen will not be limiting.  This “Nitrogen-Rich Strip” can then be read, or compared, to the condition of the rest of the field in February or early March. 


How do we “read” the Nitrogen-Rich Strip? 

Although differences in the Nitrogen-Rich Strip and the rest of the field could be “read” by any measure of crop condition (height, color, tillers, etc.), OSU researchers have used a commercially available (NTech, Inc.), hand-held optical sensor that integrates components of crop health (biomass and functioning chlorophyll). The sensor calculates an index (normalized difference vegetative index, or NDVI) from reflected red and infrared light.  Once the readings (NDVI) have been made, a response index (RI) is calculated by dividing the reading of the Nitrogen-Rich Strip by the reading representing management for the rest of the field.  Using the optical sensor to read the N-Rich Strips results in an unbiased number that is not affected by who does it or when the strip is being read.  The RI tells us how much of a yield response to expect from topdress nitrogen.


What kind of readings should we expect?

OSU research on 10 fields in 2002 found RINDVI values that ranged from 1.1 to 1.6.  These readings mean that we could expect to get 10% to 60% yield increase to topdress nitrogen.   This wide range of values also shows that the response to topdress nitrogen differs a lot from field to field in the same year and, in some instances, in the same general location.  


Do we need a Nitrogen-Rich Strip in every field?

Yes.  Like soil testing, in order to take full advantage of this new strategy/technology, every field that has a different soil, management history, or growing environment, can be expected to respond differently to the needs for N.  With time some fields may be found to respond alike and treated the same based on reading one Nitrogen-Rich Strip.


How do we treat spatial variability in the field?

OSU researchers have found that areas as small as about 6 square feet can be different from each other and require different input of N.  The OSU “Precision Ag Team” has promoted and researched development of technology to identify and treat field areas this small.  The technology is now available to sense and treat every 4 square feet at 15 mph using conventional boom applicators and solution 28.


What’s in it for the farmer/fertilizer dealer?

The bottom line is increased farmer profits.  Estimates using the 30-year data show that if N was applied only as a topdress at rates based on the Nitrogen-Rich Strip there would be an average increased return of about $19/acre/year, compared to 80 lb N/acre preplant for a 40 bushel yield goal. 

Text Box: Strategies:
·          80pp = 80 lb N applied preplant costing $0.15/lb.
·          40pp = 40 lb N preplant costing $0.15/lb, plus topdress N at $0.25/lb based on N-Rich Strip, plus $2/acre application cost.
·          RI-TD = All N applied topdress at $0.25/lb based on N-Rich Strip, plus $2/acre application cost
 Wheat at $3/bu.




With 40 lb N/acre as preplant and additional N topdressed based on the Nitrogen-Rich Strip, the benefit is not as good because of estimated lower efficiency of preplant N and that some years 40 lb is excessive.  The question is how much fertilizer to apply each year.  The answer is given from the Nitrogen-Rich Strip.  Accurate reading of the Nitrogen-Rich Strip is crucial to this new strategy.  This technology (hand-held sensor) will likely be available through fertilizer dealers.  This and the technology for spatial treatment of every 4 square feet will be an added expense to the farmer, paid from profit.


What has been the field experience of this new strategy so far?

Results from 10 field-scale treatments for the 2002 crop showed an advantage of 4 to 9 dollars per acre from using the N-Rich Strip and applying a “flat” rate, in a year of drought and delayed topdressing.  Combined with the spatial treatment of every four square feet the average improvement compared to a “farmer practice” was consistently greater than $12 and acre.



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Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1913, in cooperation with the US Department of Agriculture, Sam E. Curl, Director of Oklahoma Cooperative Extension Service, Oklahoma State University, Stillwater, Oklahoma.  This publication is printed and issued by Oklahoma State University as authorized by the Dean of the Division of Agricultural Sciences and Natural Resources.