Developing and Using Nitrogen-Rich Strips.


Gordon Johnson, Bill Raun, John Solie and Marvin Stone


Departments of Plant & Soil Sciences and Biosystems and Ag Engineering

Division of Agricultural Sciences & Natural Resources

PT 2003-7                                                          April 2003                                                    Vol.15,   No.7   




Nitrogen-Rich Strips replace the use of yield goals as a basis for making fertilizer-N recommendations.


A newly developed nitrogen (N) management strategy addresses two conditions that greatly affect how much fertilizer N a crop will need.  They are

1.      Point-to-point variability within a field (spatial variability);

2.      Year-to-year variability over time (temporal variability).


Spatial variability has been the focus of ‘precision ag’ research for the past decade and has led to development of variable rate fertilizer applicators.  Temporal variability, was largely ignored until scientists evaluated and described it in 2000, for winter wheat production in Oklahoma. 


Grain yield and nitrogen from non-fertilizer sources greatly affect year-to-year fertilizer need.  Most N from non-fertilizer sources is believed to come from release of soil organic-N just before and during the growing season.  This N is also called mineralized-N.  An N-Rich Strip is used during the growing season to estimate potential yield for that field, that year.  The amount of mineralized N supporting the yield potential is gauged by comparing the field, in general, to the condition of the N-Rich Strip. 


Maximum wheat yields vary greatly from year-to-year, but mineralized-N changes even more.  What’s more, the two are unrelated.  That is, when a field has higher than average yield it doesn’t necessarily have higher than average mineralized-N.  Consequently, sometimes mineralized-N accounts for most of the N used by the crop (figure below).  These surprising findings by scientists led them to look for a way to help farmers better manage fertilizer N inputs.





What is an N-Rich Strip?  Use of mineralized-N by the crop can be estimated only if the crop has not already received fertilizer-N to meet its entire N requirement.  Thus, the new strategy is to apply little or no fertilizer-N preplant, or with the seed, except for a spreader width the length of the field that receives enough preplant (or early season) fertilizer-N that the crop will not be limited by lack of N.  This spreader width application is the N-Rich Strip for that field.


Where should the N-Rich Strip be in the field?  The N-Rich Strip should be located through a representative part of the field.  If possible, it is also useful to place the strip where it will be convenient (after appropriate marking) to periodically observe whether it looks different from the rest of the field.  The strip should be re-fertilized each year.


How is the N-Rich Strip compared to the rest of the field?  The N-Rich Strip, and an adjacent, similar part of the field that did not receive as much fertilizer, are each individually “read” using an optical, hand-held GreenSeeker sensor.  The sensor, in the on-position, is held about 38 inches above the crop canopy while walking about 100 paces to collect an average reading.


When is the N-Rich Strip read?  The N-Rich Strip is read just before topdressing and will identify the fertilizer rate to use.  Earlier than “normal” topdressing is indicated when wheat in the N-Rich Strip appears to be in significantly better condition than the rest of the field.  Sensor readings and decisions about early topdressing should then be made.  When the N-Rich Strip looks the same as the rest of the field, and sensor readings confirm there is no difference, later sensor readings may be taken to confirm no fertilizer is needed, or identify that a small amount of fertilizer may be beneficial.


How is wheat for pasture managed differently?  The major difference is that wheat being managed for pasture is more likely to respond to a low rate (20 to 40 lb N/acre) of fertilizer.  An N-Rich Strip should still be established to help determine later topdressing needs.


What exactly do the sensor readings tell us?  Sensor readings, together with the number of days the crop has been growing, are used to estimate grain yield.  The yield without topdress-N, how much N should be topdressed, and the potential yield with topdressing are all calculated. 


What is the value of using the N-Rich Strip for making fertilizer recommendations?  Research shows an average profit of $10 to $20/acre using the N-Rich Strip and sensor readings to determine fertilizer rates compared to using yield goals.  The increased profit is related to the fact that yield goals (averages) are easy to calculate, but are only experienced about 1/3 of the time.  Using the N-Rich Strip and sensor calculations increase the chance of using the correct rate each year, and only apply N when it is needed so nitrogen use efficiency is about doubled.



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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.