We have been fielding a wide variety of questions around soil nitrate and ammonium soil test levels. Many soil nitrogen levels from fall manure applications are indicating the need for supplemental nitrogen. Wet weather, and brief periods of warm weather, have led to nitrogen loss. When manure was applied later in the spring, the results are looking much more positive, often above the 25 ppm nitrate level that is universally considered adequate to produce a corn crop. There are some soil tests near or just below the 25 ppm nitrate threshold and may need to be reevaluated later in the season for a possible late season nitrogen application. This re-evaluation later in the season should allow time to evaluate nitrogen loss due to weather the remainder of the season and provides the opportunity to determine a realistic yield expectation. More information on interpretation of Presidedress Soil Nitrate Testing (PSNT) for Corn can be found on our website.
The traditional PSNT interpretations can be challenging to relate to. Providing that the samples were collected to a depth of 12” there is a simple “rule of thumb” to help make sense of a soil nitrate and ammonium value. Add the ppm of the nitrate and ammonium together and multiply by four. This is a relative nitrogen application rate available in the soil at the time of sampling. For example, sample 1 below would be 20 ppm nitrate + 10 ppm ammonium = 30 ppm x 4 = 120 pounds nitrogen. More on this concept can be found at https://www.agry.purdue.edu/ext/corn/news/timeless/AssessAvailableN.html.
While the traditional PSNT interpretations assume a continuous release of nitrogen to the soil from the mineralization of manure organic materials during the growing season, this calculation helps relate to a nitrogen level in the soil today. The difference between the estimated pounds of nitrogen, and a total nitrogen program for the season, is about the same as the PSNT interpretations would recommend.