Gordon Johnson and Bill Raun


Departments of Plant & Soil Sciences

Division of Agricultural Sciences & Natural Resources

PT 2003-12                                                         July 2003                                                  Vol.15,   No.12   


WHO CARES?  The largest purchased input for wheat farmers is usually nitrogen (N) fertilizer.  Costs vary, but are about $20 per acre.  An average of 65% of the applied fertilizer is lost by volatilization and leaching, equivalent to about $13/acre per year.  In Oklahoma, this loss is like dedicating 15 gas wells, each producing a million cu ft per day, for anhydrous ammonia production.  Everyone should care.


Applying nitrogen fertilizer based on the relationship of the crop requiring 2 pounds of available N per bushel of yield may be wrong this year.  For about the last 35 years, specialists at Oklahoma State University have been advising farmers and field agronomists to multiply the yield goal by 2, subtract soil test N and apply the difference as fertilizer.  New research, and new thinking, has led to a change in that recommendation.  While the traditional approach has greatly helped farmers improve profit from fertilizer use, results of past fertilizer responses show that there is much room for improvement.


Averages are often calculated but seldom experienced.  The problem of using the traditional approach of 2 lb N/bushel of wheat is that this “rule of thumb” was based on the average response of wheat over many years and several locations.  At the time it was first recommended (about 1969) it was a big improvement over any other information farmers had for figuring out how much fertilizer to apply.  However, a graph of data from the Lahoma Experiment Station long-term trial (502) shows that actual N fertilizer required for maximum yield sometimes varied almost three-fold from one year (1990) to the next (1991).


The graph also shows that for the 31 years of data, 60 % of the time a 2 lb N/bushel N rate would have been wrong by more than 10 %.  Being a little bit wrong is not too bad, but almost one-half of the time (42 %) the error was at least 35 %.


Applying the wrong rate of N fertilizer.  The result of all this is that fields will often receive the wrong rate of fertilizer.  For the Lahoma research data, the yield goal based rate (80 lb N/acre for a 40 bu yield goal) was wrong two-thirds of the time, and half of the years the cost for being wrong was greater than $5/acre (average loss over 31 years was $9/acre/yr).


Understanding the problem.  In order to cope with this problem, we need first to understand it.  A recent PT (Developing and Using Nitrogen-Rich Strips, PT 2003-07) helps explain that the problem is a result of
1.      the yield goal not being a reliable estimation of final yield, and
2.      the crop use of “non-fertilizer N” is significant and different from one year to the next. 


Solution to the problem.  Instead of using a yield goal, which assumes the year will be “average or slightly better”, evaluating the crop yield potential during the season leads to a more accurate fertilizer N rate.  This means N fertilizer is not applied to the field, except for the N-Rich Strip (see PT 2003-07), until after the crop has been growing and the farmer can develop an idea of what kind of year it will be.  Fertilizer is applied during normal topdressing time.


Added benefit.  As a result of applying the N during the season, instead of preplant, the nitrogen use efficiency is improved, leading to decreased loss of N to the environment and improved profitability.  The net economic benefit from in-season estimates of yield and topdressing all fertilizer N for grain production is estimated at $17/acre/year for the 31 years of data from the Lahoma research trial.


Big change.  Not applying preplant N fertilizer, usually anhydrous ammonia, is a change not easy for most farmers to put into practice.  Tradition is the biggest obstacle.  It is often easiest to do things the way they have always been done, even if it is not the most profitable or best for the environment.  Cost of N is another factor.  Preplant N applications are often justified on the basis of lower cost of N as anhydrous ammonia.  This difference has usually been about $0.05 per lb N, but is less of a factor when an extra cultivation is made to apply the anhydrous.


Wheat for pasture is managed differently.  Wheat being managed for pasture is more likely to respond to an early, low rate (20 to 40 lb N/acre) of fertilizer.  An N-Rich Strip should still be established to help determine later topdressing needs.


For related PTs and other information go to: 





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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.