The nitrate nitrogen concentration in the lower portion of the corn stalk just after the black layer formation is a good indicator of the nitrogen (N) status the crop experienced throughout the growing season. As corn approaches maturity, plants stressed for N will move nitrate from the lower cornstalk to the ear resulting in a low stalk nitrate concentration. When corn plants have more than sufficient N for maximum yield, nitrate N accumulates in the stalk. The ability of this test to distinguishing between sufficient and excess N situations makes it unique. Other tests such as the total N content in the ear leaf at silking or the corn grain N content at harvest are capable of showing differences between N rates that were too low and adequate, but not between N rates that were adequate and excess range.
Based on research in Iowa, Pennsylvania and Michigan, stalk nitrate concentrations fall into three general categories
- LOW (less than 700 ppm NO3-N) (Purdue University uses 450 instead of 700 ppm),
- OPTIMAL (700 to 2000 ppm NO3-N), and
- EXCESS (greater than 2000 ppm NO3-N).
The LOW range indicates high probability that greater availability of N would have resulted in higher yields. Visual signs of N deficiency usually are present in this range. The optimal range indicates N availability was within the range needed to maximize profits. The higher end of this range is more appropriate when N fertilizer is relatively cheap and grain prices are relatively high. The lower end of the range is most appropriate when N fertilizer is relatively expensive and grain prices are relatively low. The excess range indicates a high probability that N availability was greater than that required to maximize economic returns. Quite often the excess range is associated with over application of N fertilizer or animal manures during the growing season.
For most conditions prevailing in the Corn Belt, stalk N03 N concentrations between 700 to 2,000 ppm have been accepted as the critical optimal range. The obvious advantage of the test being that it does not require in-field reference strips or check plots for comparisons.
The time for stalk sampling is critical: two to three weeks after physiological maturity or black layers have formed on about 80 percent of the kernels. The portion sampled is the 8-inch segment of stalk between 6 and 14 inches above the soil. Collect plants at random within an area not larger than 10 acres and 12-15 segments per sample. Please, view my “End of Season Cornstalk Nitrate Test (ECNT)” fact sheet for further information about the test procedure. Areas with different soil types or management histories should be sampled separately. Most soil testing laboratories will perform this test with a two to three day turnaround. The MSU Soil Testing Lab charges $12 per sample.
All corn producers should consider using the ECNT on a few fields each year. Those who find their fields test in the optimal range need not make any N fertilizer adjustments. Those who find they consistently exceed 2,000 ppm are usually applying too much N and will find it profitable to reduce N rates. Familiarity with the test for a few years should help producers optimize N fertilizer rates. Those using animal manure should use the ECNT as most producers tend to underestimate the amount of N supplied by manure. Producers who grow corn after alfalfa should also consider using the ECNT to fine tune their N fertilizer practices.
The test has a few limitations. Consideration has to be given to weather conditions that prevailed during the growing season. In abnormally dry years, higher nitrate levels may accumulate in the stalk. Also, deficiencies of N early in the season may sometimes limit grain yield in ways that may not be directly indicated by the stalk test. Under normal conditions, however, the test values will reflect an overall assessment of N fertilizer practices during the growing season.