If you’ve been relying on the corn stalk nitrate test (CSNT) to evaluate and adjust your nitrogen (N) program, you may want to think again, one recent study suggests.
Cornell University Cooperative Extension recently released the results of a 4-year study of the CSNT that says the test shows low accuracy for evaluating corn N deficiencies and excesses.
From 2011-14, Cornell conducted 49 replicated strip trials throughout New York and 42 replicated strip trials in northern Iowa to determine the CSNT’s accuracy. The trials involved two N rates — a “high” and a “low” rate — which researchers say resulted in field-scale strips with rate differences ranging from 10-140 pounds per acre.
The trials were distributed across both states under a wide range of weather conditions and included corn for grain and silage, manure applications and no manure applications, and rotations of continuous corn or corn after soybeans.
The theory behind the CSNT, Cornell says, if that if a corn plant receives too much N, it will have high nitrate levels in the lower stalks at the end of the season. If corn suffered a N deficiency, then it will have removed more N from the lower cornstalks during the grain-filling period.
Based on universities and grower associations, corn with a CSNT result of less than 250 ppm nitrate-N (some states 450 or 750 ppm) is considered low; between 250 and 2,000 ppm is generally optimal; and anything above 2,000 is excessive.
Using those ranges, Cornell found the following:
The CSNT often identified deficiencies when there were none. In many cases, CSNT values were below the 250 ppm threshold, but there were no yield losses.
The CSNT often didn’t identify N deficiencies when the corn was deficient. In many cases where there were yield losses, the CSNT tested values above 250 ppm, indicating frequent false negatives for deficiencies.
The CSNT didn’t identify when fertilizer N levels were excessive. In many cases with excess N rates, the test didn’t show values above 2,000 ppm.
Cornell found the only time the test performed well was when it tested values above 2,000 ppm, as the corn usually did have excess N. Less than 10% of readings above 2,000 ppm were false positives.
The researchers concluded the test has, “very limited ability to support management decisions.”
“The test failed to identify over-fertilized crops (30-140 pounds per acre) in about two-thirds of the cases in New York and half the cases in Iowa,” they say. “In other words, a majority of the excessive N cases weren’t identified by the test. Since the test’s primarily utility is related to determining excessive N rates, it appears to perform weakly in serving its main purpose.”
If you’ve been using the CSNT, do these findings surprise you? If you don’t use the test, what method(s) do you use to determine the efficiency of your N program? Tell us your thoughts in the comments below.