The end-of-season stalk nitrate test for corn: A novel approach to evaluating your fertility program

John Heard, Soil Fertility Specialist

Corn has a large requirement for nitrogen to attain high yields. Under-fertilizing with nitrogen (N) jeopardizes yield, whereas over-fertilizing increases costs and environmental risk. Unlike cereals, corn does not lodge at excessive N rates and so over-fertilizing may not be apparent to the grower. One might be suspicious that excessive N is applied to Manitoba corn when one considers the residual soil nitrate-N amounts. Over the past 10 years, the residual soil N following corn was 15-57 lb N/ac greater than following wheat in Manitoba (AgVise Laboratories summary data). Sometimes extra fertilizer N may be applied due to the grower’s lack of confidence in N contributions from previous legume forages crops or manure.

The end-of-season stalk nitrate test was developed for the purpose of indicating when growers had met the N needs of the corn plant. The basis for the test is that plants tend to take up soil N and store it in the lower portion of the stalk. Plants deficient in N will usually mobilize this stored N from the stalk and leaves to the grain, resulting in low levels in the lower stalk. Conversely, plants with access to high soil N will take up and store those amounts in the lower stem, resulting in high concentrations of nitrate.

Based on initial studies in Iowa (Blackmer and Mallarino, 1996), many states have conducted field calibration studies to develop stalk nitrate guidelines as follows:

Low (<250 ppm N), Marginal (250-700 ppm N), Optimal (700-2000 ppm N) and Excess (>2000 ppm N). These critical levels have been validated in recent MAFRI studies in cooperation with the MB Corn Growers Association.

The sampling procedure is as follows:

• Cut and collect the 8” long section of corn stalk between 6-14” above ground level
• Sample within 3 weeks of kernel black layer formation (physiological maturity) since fall rainfall may leach and reduce nitrate levels if sampling is delayed too long.
• Sample 10-20 plants per field
• Submit immediately to a lab for analysis. AgVise Laboratories and A&L Canada offer analysis for Manitoba growers, so contact them for handling instructions and costs.

Generally it will be the crop adviser or consultant that conducts the grower’s soil sampling that offers this additional service.

Our studies in Manitoba have shown:

• When stalk nitrate levels were <600 ppm, yields were less than full potential in 16 of 19 cases.
• When stalk N levels were greater than 600 ppm, yields met full yield potential in 9 of 11 cases.
• In manured fields additional N application did not increase corn yield, but did increase both stalk nitrate and residual soil levels.

We field tested this technique in the MB Corn Growers Association 2008 high corn yield competition across 58 fields. Only 5% of fields tested Low or Marginal, 30% tested Optimal and 65% tested Excessive. This might be expected since growers wishing to win the contest would logically select their most fertile fields where N was not limiting.

Growers may choose to use this test for a number of reasons:

• To assess the appropriateness of the current N rate practices
• To give confidence that previous legume crops and manure are providing sufficient N for the crop
• In diagnosing the cause of low yield in poor areas of the field. These may result from excessive nitrate N losses before or during the growing season due to excess water. Possible corrective strategies may then be the use of controlled release N or side-dress time application.

The test is not for predictive purposes in fertilizing the next crop. A fall soil test remains the best way to establish N rates for future crops, but may be complimented with the knowledge from stalk nitrate tests, particularly when N credits are being used.

A complicating factor this fall will be the stage of the crop when the killing frost occurred. If the crop was near maturity, much of the N movement required for grain filling had probably occurred and results should agree with the book standards. However, if the crop had not matured the nitrate levels in the stalk may be misleadingly high. After a frost, if the leaf material is dead but stalks and roots are alive, nitrates can accumulate in the lower stalk.

This is of particular concern for livestock farmers who are storing corn silage in upright silos, since this nitrate accumulation increases the risk of nitrate toxicity and deadly silo gas. Growers should use precautions prior to entering such silos to level silage or to setup silage unloaders.
 

For further information, contact your GO Representative.