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Corn+Soybean Digest

Cornstalk nitrate test reveals nitrogen application accuracy

Guided cornstalk nitrate testing told Art Gudas Kewanna Ind that his present nitrogen rate of 175 lbsacre was right on the money Aerial imagery identifies key sampling spots for meaningful results
<p>Guided cornstalk nitrate testing told Art Gudas, Kewanna, Ind., that his present nitrogen rate of 175 lbs./acre was right on the money. Aerial imagery identifies key sampling spots for <span style="font-size: 13px;">meaningful results.</span></p>

Guided cornstalk nitrate testing has been peace of mind, and wallet, for Art Gudas. It told the Indiana grower that his 175 pounds per acre nitrogen (N) rate was right on the money these past two years. He’d considered bumping up his nitrogen rate before two years’ tests told him otherwise.

How many farmers feel that nitrogen application rates are a guessing game, since you don’t know the season’s rainfall forecast? Cornstalk nitrate testing tells you whether you applied enough nitrogen this year, based on cornstalk samples’ nitrate levels at black layer stage. Aerial imagery (NDVI and IR) identifies three typical areas and one area that appears to be nitrogen-deficient.

75% N as sidedress

Gudas applied 15% of his nitrogen as spring preplant, 10% as 10-34-0 starter in-furrow, and 75% as June sidedress.

The Indiana State Department of Agriculture offered this guided cornstalk nitrate test in 2013 for no cost through his county soil and water conservation district. He tested stalks from two fields side by side, comparing conventionally tilled fields with no-till fields and cover crops. “I learned they were both just fine at present nitrogen levels,” Gudas says. (The cover cropped fields yielded 15-20 bushels more, but received identical nitrogen levels.)

Winter grower meetings show farmers’ results anonymously so they can compare their rate with others and consult experts to fine tune test interpretations. Gudas used a lower rate (but sufficient for his crop) than most last year.


Context is everything

Most of his nitrate-N levels were in the optimal range (450-2,000 ppm), ranging from 862 to 1950 ppm. In Pulaski County, one farmer had a high of 6,480 ppm, and the lowest was 51 ppm. The group setting (an Indiana On-Farm Network meeting) provides farmers context for their score. And, agronomists have context for that particular growing season, so that they can tailor future nitrogen recommendations by weather and precipitation based on what they saw this particular year under these particular conditions.

Gudas is one of 265 Indiana growers in the Indiana On-Farm Network’s adaptive nutrient management program, enabling farmers to assess or upgrade their nutrient practices voluntarily. The group aims to reduce its environmental footprint cost-effectively and sustainably with research on their own farms. They receive technical support to evaluate different agronomic practices.

Each test has its role

The cornstalk nitrate test is a snapshot of how close you came to applying the most cost-effective N rate, and no more. The sample sites are guided by aerial imagery, to include three representative areas and one nitrogen-deficient one.

Because weather largely defines a given year’s corn N needs, guided stalk sampling results vary with a season’s weather, but several years’ results are the best way to find your ideal nitrogen rate, says James Camberato, Purdue agronomy professor and Extension specialist.

Several years’ testing reflect various precipitation scenarios--a general idea of whether you’re close to the right nitrogen amount, says 

James Camberato, Purdue agronomy professor and Extension specialist. “The test is especially useful if you fall-apply manure or anhydrous ammonia, and/or spring-apply manure.” By contrast, “in-season tissue testing is good for identifying/confirming deficiency, but not for better defining the optimum application rate.

“The pre-sidedress nitrogen test is used when manure or perennial legumes are your nitrogen source. Unfortunately, it’s been shown to recommend nitrogen when it's not needed 33% of the time,” Camberato says.

Test details

Aerial imagery identifies nitrogen-deficient areas to test, along with three “typical” field areas that represent your farm for cornstalk nitrate testing. Tissue testing corn stalks at the end of the growing season measures what the plants actually used, rather than what was available in the soil. After a few years of these checkups, patterns emerge to guide future nitrogen-rate decisions.

For the stalk tests to be standardized year to year, stalk samples should be 8 inches long, taken 6-14 inches above ground. Cut 10 individual stalks per selected sampling area. 

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