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. This has been very true of this season so far. 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. This season it has been very common to have results greater than 10,000 ppm from all over the Great Lakes region. What can possibly explain the excessively high numbers? Here we will discuss a few possible explanations.
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 cuts 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.
Generally high CSNT results indicate that some other factor than nitrogen reduced the yield. What has been unique this season is that many of these high CSNT’s are coming from fields that did not have excessive nitrogen applied, the overall soil fertility is good, they were not drought stressed, and they are expecting excellent yields. Where is this extra nitrogen coming from? Most likely the soil. There is a huge bank of nitrogen in the soil organic matter that can possibly be mineralized if the soil has adequate moisture, heat, and microbial community. If or when this will happen is nearly impossible to predict. For much of the region, the spring started off fairly cold with descent moisture and then in late June the temperature skyrocketed and we got some sizeable rain events. These conditions likely spurred a rapid microbial release of nitrate more than what the crop needed since the nitrogen fertilizer was likely applied a month prior to these conditions.
For more information about sampling corn stalks for nitrates, please see our fact sheet or contact your ALGL agronomist.