AgriGold

Not Too Heavy, Not Too Light: Finding the Right Nitrogen Rate

by Jacob Bates, CCA, February 24, 2010

If you have spent any time over the past few months watching football there is a good chance you've seen the latest commercials from a prominent beverage company in the United States.  During their commercials they depict two versions of various situations, one showing what happens when a person approaches things too lightly and another showing the same person going overboard or too heavy.  Thus the tag line "Not too Heavy, Not too Light, Just Right" is used when promoting their beverage.  While I have found these advertisements to be very comical, the core premise can carry over into the world of agriculture in regards to nitrogen application.

Chart 1 shows that historically, nitrogen application throughout the United States has hovered around 135 lbs/acre.  Prior to 2000 the amount of nitrogen applied exceeded the historical yield averages for the United States.  Since the early 2000's, however, our average corn yields have been higher than the rate of applied nitrogen.  That is good news for producers who care about getting the most out of their inputs.  Whether this result is due to genetic advancements in hybrids or improved efficiency in application of nitrogen is hard to say.  Most likely it's a combination of both.

Pinpointing the correct nitrogen rate for each farm is no easy task.  Last year we conducted several on-farm studies in AgriGold's western regions.  We utilized the Late Spring Soil Nitrate Test (also known as the Pre-Sidedress Nitrogen Test) and the fall corn stalk nitrate test to better understand our corn crop's need for nitrogen.   This study was born out of the 2008 growing season when heavy rainfalls depleted much of the applied nitrogen.  Many growers who sidedressed N in the spring saw huge yield advantages over those who did not apply additional N.  The main goal of this study was to determine early in the growing season if there was enough nitrogen to feed the crop and then follow up in the fall just before harvest to confirm that the nitrogen was utilized by the plant.

We began soil sampling specific points in about a dozen fields around mid-May and went back to these points on a weekly basis to resample.  We continued this process until early July.  Our goal in soil sampling on a weekly basis was to have a track record of how the nitrogen quantities were moving over time.  We also broke our samples out into 0-12 inch and 12-24 inch profiles.  Taking nitrogen soil samples to a depth of 2 feet is somewhat unconventional, however it can really help you see how nitrogen moves within the soil profile.   For instance, when lab results showed significantly higher parts per million (ppm) in the 12-24 inch profile, that could lead you to believe that some leaching was occurring.  However, if the 0-12 inch profile came back with a higher ppm than the 12-24 inch profile, nitrogen may have been lost to denitrification.   

Most of the test locations in 2009 received moderate rainfall during the spring.  As a result many of the soil nitrate tests came back well above the critical ppm levels as laid out by Farm Journal's agronomist, Ken Ferrie.  He recommends a critical ppm for the 0-12 inch profile of 10 ppm.  For the 12-24 inch profile the recommendation is 20 ppm.  See Chart 2 for an example of one field we tested last summer.  Notice for this particular field we tested 3 different sampling points throughout the growing season.

Chart 2. Bar graphs represent the Soil Nitrate ppm and the line graph represents the Stalk Nitrate ppm.

In Chart 2 you will also notice the red line graph.  This represents our results from the fall stalk nitrate sampling we conducted.  Essentially we collected 8 inch stalk samples from the points in the field we had previously soil sampled.   Iowa State University divides stalk nitrate results into 4 categories:

• - Low, <250 ppm
• - Marginal, 250-700 ppm
• - Optimal, 700-2000 ppm
• - Excess, >2000 ppm

For these 3 sampling points in Chart 2 notice that the stalk nitrate levels were in the optimal or excessive range for parts per million.  In this field there was a good correlation between the soil nitrate results and stalk nitrate results.  This is shown by how the sample point with a lower stalk nitrate result was also the lowest average soil nitrate test.  Same holds true for the other 2 sample points yet this time in the medium and highest ppm ranges. 

This study was very valuable to the individual growers who participated by providing them with key information during the spring about their nitrogen levels.  Chart 3 depicts that 80% of the corn plant's nitrogen has been taken up by the early reproductive stages.  This shows why spring is such a critical time to be sure that the nitrogen you applied is still in the soil profile and is available for plant uptake. 

We are convinced that pinpointing the right amount of nitrogen for corn will be discussed as long as there is farming.  There is no flat rate that will apply for every situation.  Instead, it needs to be evaluated at the individual field level where past history and future goals are taken into consideration.  With tools like the late spring soil nitrate test and fall corn stalk test we can do a better job of assessing what may be working the best in your field.  Just remember our goal in regards to nitrogen application: not too heavy, not too light, just right.

Categories: Nitrogen Application

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