Weather conditions play a major role when it comes to managing nitrogen fertilizer application in corn. The Iowa Soybean Association’s On-Farm Network has been gathering research via a corn stalk testing method known as guided stalk sampling (GSS) in order to more closely match corn nutrient needs.

The GSS stalk nitrate testing method incorporates the use of color aerial imagery of the cornfield taken late in the growing season with soil survey information to target four points in the field where samples will be collected for the end-of-the-season stalk nitrate test.

Researchers have been analyzing Iowa stalk nitrate test results for the past seven years and have compared it with data from other Midwestern states.

In a recent issue of the On-Farm Network Advance newsletter, the On-Farm Network reports that growers involved in the program have learned that corn tends to be short of nitrogen in wet years. By comparison, in dry years, stalk nitrate tests suggest there’s usually an excess of nitrate in the stalks. This trend held true for the past seven years in Iowa. The drier years of 2006, 2011 and 2012 produced stalks with excess nitrate levels when compared with the wetter years of 2007, 2008, 2009 and 2010.

The On-Farm Network researchers point out that the stalk nitrate test measures what the plants actually took up, rather than what was available in the soil. Corn plants take up nitrogen along with water from the soil. In very dry conditions, there may be so little moisture in the soil that roots can’t get all of the water the plant needs. At the same time, if fertilizer is applied in a band, much of what is applied may not be absorbed into the moist portion of the soil. Corn roots tend to grow toward the moisture. If the fertilizer band is shallow and between the rows in fairly dry soils, they may not be able to utilize it.

Looking at trends in other states, researchers saw significant differences in Minnesota in 2012. Minnesota had the highest stalk nitrate test levels among all of the Midwestern states where On-Farm Network programs are conducted. While soil moisture was not in excess in Minnesota, there was enough rain during the growing season to get the applied nitrogen into the soil solution. Meanwhile, in other states south and east of Minnesota, tests showed more and more nitrogen deficiencies. This was due to increasingly dry soil conditions. Nitrogen was side-dressed on most of these fields. Yield loss caused by the drought was higher in other states compared to Minnesota and Iowa.

To confirm that additional fertilizer would not have increased the yields significantly, the researchers suggest looking at the results from 11 replicate strip trials conducted in northeastern Indiana. All of the sites were first-year corn. Three nitrogen rates, consisting of 125, 175 and 225 lb./acre, were compared in replicated strips at each site. On average, the lowest nitrogen rate of 125 lb./acre was the most profitable rate last year, with only a 2.9 bu/acre increase from an additional 50 lb. of nitrogen. The researchers found this interesting because the recommended rate of nitrogen application for that part of the state is 219 lb./acre. The highest rate, 225 lb./acre, did not increase yield over the middle rate, and increased stalk nitrate concentration was only about 120 ppm more than the lowest nitrogen application rate. The following table substantiates the concept that it was so dry that plants were unable to take up the nitrogen.

Table 1:

Many growers in Iowa and across the Corn Belt are concerned that the drought may continue into the 2013 growing season. The On-Farm Network says there are some practices that might help make the fertilizer more available for plant uptake in very dry conditions. These practices involve placing nitrogen fertilizer deeper and closer to the plant’s roots.

To learn more about nitrate test results, attend the annual On-Farm Network Conference on Feb. 21 in Ames, IA.  Conference details can be found here.

Read more about the On-Farm Network GSS method and testing results at