Managing Nitrogen Fertilizer for Spatial Variability in Wheat Fields.


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   




Field “average” is easy to calculate, but only occurs about 1/3 of the time.


More than meets the eye.  When nitrogen (N) fertilizer is applied to wheat fields, the same rate is almost always applied over the entire field.  Farmers and fertilizer dealers know that differences in yield potential may exist in a field, based only on visual observation, but until now have not had the technology to do much about it.   Recent research has shown that nutrient availability in fields may be different for about every 4 ft2.  It is often easy to see, and even soil test, areas of several acres in a field that are different because of landscape position such as upland, slope, and bottomland.  Areas as small as 4 ft2, however, are too expensive to soil test and impossible to distinguish by eyesight in a field. 


Testing soil nutrient availability.  Soil tests are most valuable for evaluating fertilizer needs when they are able to mimic, or act like, a plant growing in the soil.  Good soil tests take nutrients out of the soil in a way that is similar to plants extracting the nutrients.  However, even the best soil tests only accurately identify a portion of the soil-N (nitrate) that plants might use during the growing season.  The most reliable ‘tester’ of soil nutrient availability is the plant itself.  Although plants respond to may factors in the growing environment during the season, the most common, and expensive, input for wheat production is nitrogen fertilizer.


Getting better ‘eyes’.  The human eye is a marvel.  It literally sees everything.  But the human eye cannot focus on only one or two colors and shut all the rest out.  Consequently, we have difficulty seeing small changes in crop condition related to yield potential and N nutrition of the crop.  Active optical sensors emit light and then measure the crops reflection of that light in relation to the “healthiness” of the crop.  Recent research has shown that optical sensors can reliably estimate yield potential and N fertilizer requirement from red and near infrared light reflected by young (Feekes growth stage 5) wheat plants mid-season.  Small crop differences (less than 20 %) that we are unable to see are accurately identified by the sensor.


Fertilizing 4 ft2 areas.  Once the N fertilizer requirement has been determined by the sensor, the next step is to be able to fertilize each 4 ft2 area individually.  A newly designed spraying system allows this to be done while traveling at speeds up to 15 miles per hour (Figure 1).





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.