More agronomists are using the “cornstalk nitrate test” late in the season to evaluate their “N” management. This tissue testing procedure was developed at Iowa State University to help evaluate nitrogen management practices in corn as the crop matures. The research shows that when corn has not had enough nitrogen, the nitrate value in the lower portion of the cornstalk will be low. If too much nitrogen was applied, the nitrate level in the lower stalk will test high. This test may be of particular interest to growers who deal with manure or corn grown after alfalfa or those wishing to evaluate their nitrogen fertility program.
The ranges and indexes used to interpret the test are shown below:
Corn Stalk Nitrate Test Interpretations:
Low: (Less than 250 ppm) Likely that nitrogen was deficient and limited yield
Marginal: (250 – 700 ppm) Possible that nitrogen deficiency limited yield
Optimal: (700 – 2000 ppm) Yield was not limited by nitrogen
Excess: (> 2000 ppm) Nitrogen supply was excessive
When To Sample The Corn Stalks:
Corn stalks can be collected from as early as 1 to 3 weeks after black layer stage through immediately after harvest. If sampling after harvest, it is very critical not to wait very long. Collect the sample as soon as possible after harvest, because nitrates can be leached out of the cornstalk by rainfall.
How to Collect Corn Stalk Samples:
15 stalks are required per sample. Each corn stalk should be about 8 inches in length. From the ground level, measure 6 inches up from the ground and cut the stalk, then cut the bottom 8 inches off the stalk and send this portion to be tested for nitrates. Place the stalks into our paper sampling envelopes (do not use plastic or Ziploc bags or the samples will be ruined by mold) and mail them to the lab.