The following article is an excerpt from a previous post titled “How Much Nitrogen Should I Apply”, posted February 23, 2018
The corn plants will take up most of its nitrogen between the V8 and VT growth stages. We want to limit nitrogen deficiency at this time to limit potential yield reductions. This is where a Pre Side dress Nitrogen Test (PSNT) soil sample is a good idea. A PSNT sample is taken 7 to 10 days before a side dress application of nitrogen. This method works well if you are considering a side dress application to supplement for previous nitrogen loss or looking to adjust a planned side dress application. University recommendations may differ between states, but most would agree that 25 ppm nitrate in a 12” sample, or approximately 90 lbs. of nitrate per acre is the critical level for corn. If your test result is higher than 25 ppm, no additional nitrogen is recommended. If the ppm nitrate is lower than 25, additional nitrogen fertilizer is recommended, and some universities will provide equations to calculate your needs, others will have a recommended base rate. You probably are wondering why 90 lbs. of nitrate nitrogen would be acceptable, especially when you applied much higher rates of fertilizer. Remember there is constant conversion of nitrogen in different forms and will continue to convert to nitrate throughout the season. The corn plants will have already extracted nitrogen from the soil and accumulated it in the leaves by the time you take a PSNT sample. This is why an accompanying tissue test may also be of value when taking a PSNT soil sample.