By George Silva, Michigan State University Extension
The tendency for nitrate nitrogen (N) to accumulate in the lower portion of the cornstalk when soil N is available in excess, and conversely for nitrate N to deplete in lower cornstalks when N is deficient, form the basis for the end of season cornstalk nitrate N test. It is a simple and inexpensive diagnostic test to generate a report card on current season N practices and make adjustments for the next year. This test has been calibrated to conditions prevailing in Michigan and the Corn Belt.
The stalk nitrate concentrations are divided into three general categories; LOW (less than 700 ppm N03-N), OPTIMAL (700 to 2,000 ppm N03-N) and EXCESS (greater than 2,000 ppm N03-N). In a normal year, the excess range is associated with over application of N fertilizer or animal manures during the growing season.
A drawback of this test, however, is that when interpreting the results, consideration has to be given to weather conditions that occurred during the growing season. In abnormally dry years such as 2012, higher nitrate levels may accumulate in the stalk because low rainfall limits N losses and decrease grain yields.
Michigan State University Extension sampled 10 corn fields across Michigan's Ingham County to assess the stalk nitrate N concentrations. The farmers who submitted these samples felt their N application rates were conventional and adequate for their respective yield goals based on previous experiences.
The 2012 Extension stalk nitrate data is shown in Figure 1 below. Five samples were in the excess category with concentrations greater than 2,000 ppm. Two of them were above 5,000 ppm. Four samples were in the optimal category and one sample was in the low category. These concentrations reflect all the site specific factors that influenced N availability on their respective fields. We will follow through with the two extremely high sites to ascertain what may have happened on these locations. The overall average stalk nitrate concentration was 2,472 ppm in 2012 compared to 1,350 ppm in 2011, and 1,872 in 1999 in similar surveys. Furthermore, 50% of samples taken in 2012 were in the excess category. This data supports the notion that the test may have to be interpreted with some caution in extremely dry years.
This article was published by Michigan State University Extension. For more information, visit www.msue.msu.edu. To contact an expert in your area, visit expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).
Figure 1. The end of season cornstalk nitrate N distribution on 10 corn fields in 2012.
Note: The optimal concentration (700 to 2,000 ppm) lies between the two blue lines.