Evaluating Nitrogen Management Late Season

As the 2024 corn crop is approaching maturity throughout the Great Lakes region, many producers and crop consultants are starting to question whether they have applied enough nitrogen to finish out the season. A corn stalk nitrate test (CSNT) can be a useful tool in assessing the effectiveness of a nitrogen program.  However, the test results often generate more questions than answers. Remember, when dealing with a natural system, the word always never applies. The general interpretation of a CSNT is that if the result is less than 700 ppm, nitrogen may have limited your yield, from 700 to 2,000 ppm, nitrogen use was optimal, and greater than 2,000 ppm indicates excess nitrogen. However, the CSNT data cannot be used on its own to make future management decisions. It must be utilized as a part of a more holistic approach including crop observations, soil test data, weather history, fertilizer application history, etc.

When a CSNT shows high levels, the most obvious explanation is that too much fertilizer was applied. No, this does not indicate that growers are carelessly over applying fertilizer. It simply means that for the current growing season, a lower amount of applied nitrogen would likely have generated the same yield for reasons that are impossible to predict at the time the nitrogen was applied. Drought stress is one of the leading causes of high CSNT results. Most growers are applying about 1 pound of nitrogen per bushel of projected yield. So, if 200 pounds of nitrogen is applied between a starter and sidedress application expecting to harvest 180-200 bushels, but a late summer drought cut that yield down to 100-120 bushels, the excess nitrogen will accumulate in the lower stalk since there is not enough grain to utilize the applied nitrogen. Another potential explanation is another nutrient deficiency. For example, if the crop was supplied with enough nitrogen to grow 200 bushels, but the plants are experiencing a severe sulfur or potassium deficiency reducing the yield, there will be excess nitrogen in the plants.

On the contrary, a low CSNT does not always mean that the nitrogen was lost or that more applied nitrogen would have resulted in additional yield. There are situations during unusually good growing seasons and the crop yield exceeds the early season expectations. The plants show no physiological deficiency symptoms and the CSNT shows less than 200 ppm. This indicates that the crop was able to efficiently utilize all the applied nitrogen and additional nitrogen would likely add little to no additional yield.

For more information about collecting CSNT samples and understanding the results, please visit our Cornstalk Nitrate Test Fact Sheet.