Over the last few weeks, we have received many soil samples for nitrate analysis to help determine what rate of nitrogen to sidedress in corn. Several of the samples have generated questions for our agronomy staff regarding unusually high nitrate results. Upon further investigation we have found that many of these fields with high nitrate levels have one thing in common, they had manure applied since the previous crop was harvested. This is the exact scenario that the pre-sidedress soil nitrate test (PSNT) was developed to assess. Each university extension program in the Great Lakes region have developed interpretations for determining what rate of nitrogen to apply based on the soil nitrate level, and they all basically agree that if it is above 25 ppm, no additional nitrogen is needed. This year it has not been unusual to see 50 to 100 ppm.
Where is all this nitrogen coming from? Simply stated, we can thank the fall, winter, and spring weather. Last fall was dry, which allowed for timely manure applications. The winter was relatively dry and cold. This kept the microbial activity to minimum reducing the chance for nitrification and mineralization to occur that could lead to nitrogen losses. The early spring was cold through most of May, again minimizing nitrogen losses. Then early June got hot with some rain, and it kicked the microbes into high gear converting ammonium and organic nitrogen sources into nitrate.
So, what do we do with these high numbers? Most growers who use manure as part of their nitrogen program know that even an aggressive fall manure application will likely run short on nitrogen and generally require a supplemental source to get through the whole season. Even though the university interpretations say there is more than enough, you need to consider each situation on its own. Irrigated ground with a 300+ bushel potential is certainly going to require much more nitrogen than dry ground with 150 bushel potential. Is there an option for late season applications, such as fertigation or a high clearance spreader? If you are in a situation where your current soil test nitrate levels do not justify a sidedress application, your best bet is to monitor the crop. At tasseling, a corn crop should have taken up 60 to 70% of the nitrogen it needs for the season. Collecting a plant tissue sample at this time can potentially identify a nitrogen deficiency before any visual symptoms appear.
If you have any questions or want to discuss your own situation, contact your ALGL representative.