OMAHA (DTN) -- Before you cut that field of corn, why not pull stalk samples to help you learn better nitrogen management skills? Stalk nitrate tests are a good post-mortem approach to evaluate if your nitrogen rate was too high, too low or on the money.

Tracy Blackmer, head of the Iowa Soybean Association's (ISA) On-Farm Network, said the origin of the concept dates back to the 1980s. His father, Iowa State University agronomist Alfred Blackmer, wanted to study the dynamics of nitrogen in the soil to understand what was used by the crop and what was lost.

"Surprisingly, his research showed that the suggested nitrogen recommendations did not work and applying nitrogen by yield goal worked as good as anything," he said. "What he did learn was that stalk nitrate concentration was the only reliable way to evaluate nitrogen availability and demand during the season."

The stalk nitrate test is simple. Two to three weeks after black layer and before harvest, collect 15 eight-inch stalk segments (make your bottom cut 6 inches above the soil surface) and send them off in a paper bag to the laboratory for nitrate analysis. Cost is about $15 per sample and only one composite sample is needed per field.

Blackmer said that nitrates accumulate in the lower stalk and above the crown roots. The 6-inch base cutting height and 8-inch length is recommended to get a consistent segment of stalk from the same place on every plant and every season.

The results report nitrate concentration in parts-per-million (ppm) in the lower stalks and fall into one of four categories; low (less than 250 ppm), marginal (250 to 700 ppm), optimal (700 to 2,000 ppm) and excess (greater than 2,000 ppm. The ideal is to be in the 1,000 to 2,000 ppm range at the end of the season.

"We have tested several fields that averaged over 20,000 ppm, an extreme that really tells guys they are overfeeding their corn crop. These are non-stressed fields with high yield potential, irrigated and well fed," he said.

"In Iowa we can see a few fields reach 10,000 ppm every year, such as after alfalfa or after a heavy-manure-use year. Generally most fields record an excess value. However, the last several years fields in Iowa have been below optimal because of nitrogen losses due to heavy spring rains." Naturally this will show up as yellow corn.

Blackmer said the test truly tells you if the plant had more than it needed. "Plants might be yellow and tell you there is a true deficiency but nothing else. But the stalk nitrate will tell you if the rate is low, optimal or too high. It will tell you if you are in the ball park and if you're always sky high (greater than 2,000 ppm) you are over applying." If stalk nitrate tests come back too high you can start to reduce nitrogen rates but you need to continue to monitor yearly."

Of course, the natural question becomes, what will one sample per field per year tell you since fields and plants are naturally variable. Like soil testing, the results are only a guide to what is happening in the field. The true value is not in what you learn in one year, but the trends you see over time. Whether the nitrate levels are generally too low and you need to apply more, too high and you need to cut back or if you are doing something right and the nitrate levels stay within the optimal range. However, if you have an appetite for more work and data, you can do the guided stalk nitrate test and use an aerial image to create zones that will guide you where to sample.

"One year of data is not enough to change your management, but it at least tells you if you were right or wrong last year," said Blackmer. "By repeating over multiple years and watching nitrate concentrations while understanding weather conditions, it will help you adjust nitrogen rate. It is worth your time to look at it more closely."

I have used this test for a number of years to assess the validity of the nitrogen rate I apply to corn. I can admit that my test values tend to run on the low end of the optimal range or the high end of the marginal range. This test just confirms that my philosophy of running nitrogen rates on the low end to be more efficient (equivalent to 0.8 to 0.9 pounds per bushel) is probably hurting yield in the years when weather is optimal.

Dan Davidson can be contacted at AskDrDan@telventdtn.com

(CZ/SK)

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