Stalk Nitrate Sampling

Brinkman's Nutrient Management Notes
October 2, 2006

Now is the time to evaluate or get a report card on how you did with your nitrogen management. Taking advantage of the Stalk Nitrate test is an excellent way to determine if you made the correct decisions on nitrogen this year.

  • What was the Availability of my manure source?
  • Did I apply the manure at the correct rate?
  • With the high cost of commercial Nitrogen did I apply enough?
  • Did I apply too much?

These are all questions that producers ask. Let us help answer these questions:

  • Three weeks after 80% of the kernels have formed the black layer (physiological maturity) good information on black layer.
  • Cut an 8-inch segment of stalk between 6 and 14 inches above the ground.
  • Do not sample stalks that are barren or severely damaged by insects or disease.
  • Each sample should consist of 15 segments from a sampling area.
  • Place samples in a paper bag and send to a soil testing lab or Iowa State.

Results will be reported in PPM of Nitrate. Stalk nitrate concentrations can be divided into four categories; low (less than 250 ppm N), marginal (250 to 700) optimal (700 to 2000 ppm N), and excess (greater than 2000 ppm N). The low category indicates high probability that greater availability of N would have resulted in higher yields. It should be noted that concentrations in this range give little indication of the magnitude of yield increase that might be expected from more available N. Visual signs of N deficiency usually are clear when nitrate concentrations are in this range. The marginal category indicates that N availability was very close to the minimal amounts needed.  Although producers should not be concerned when samples test in this range, this range is too close to economic penalties to be the target for good N management under most conditions. 

The optimal category indicates high probability that N availability was within the range needed to maximize profits for the producer. The higher end of this range is more appropriate when fertilizer N is relatively cheap and grain prices are relatively high (compared to prices in Iowa during the past decade). The lower end of the range is most appropriate when fertilizer N is relatively expensive and grain prices are relatively low. Visual signs of N deficiency often are observed in this range.  The excess category indicates high probability that N availability was greater than if fertilizer N had been applied at rates that maximize profits for producers.

Several environmental factors can influence the amount of nitrate in the stalk. Lower than expected results can occur in years with excessive amounts of rainfall. Higher than expected results may happen in droughty years. The test is a tool to be used and after several tests have been taken over a few years a producer can begin to "zero" in on the correct rates of Nitrogen for his management style and nitrogen source. 

Don't make extreme changes after just one year. As said above, the weather may have had an influence. Make minor changes and experiment with what is the best rate.