We all know hindsight is 20/20, especially when it comes to a crop season. This past year was made especially interesting by the excessive early rainfall that caused nitrogen leaching and denitrification. Now that most of the corn has been harvested, it’s a good time to review how we managed nitrogen in 2015.
Cornfields that started to fire up in June and July gave us warning that nitrogen was challenged, but sidedressing wasn’t easy in this wet year. A few growers aerial applied urea, and you can apply enough nitrogen this way to make a difference. However, depending on a foliar nutrition is risky because getting enough nitrogen through the foliage to replace what was lost from the soil is a near-impossible task.
So the question is how much did nitrogen limit yields in 2015? A stalk nitrate test offers some insight, and I’d like to see more growers view it as a tool. I have found it useful for checking my own nitrogen practices at the end of the year. It needs to be done after black layer and before harvest.
The cornstalk nitrate test was pioneered by Alfred Blackmer at Iowa State University. Guidelines recommend cutting an 8-inch segment of stalk beginning 6 inches above the soil line and then removing leaves and sheaths. Collect eight to 10 representative segments to reflect the field or an area of the field. Then submit samples to your preferred soil laboratory for nitrate analysis. You can pull stalk samples from black layer up until harvest.
According to Blackmer’s original publication, nitrogen is deficient if the sample measures less than 250 parts per million nitrate; marginal from 250 to 700 ppm; optimal from 700 to 2,000 ppm and excess if greater than 2,000 ppm. Purdue University has calibrated this test in Indiana and has slightly different ratings.
I used the stalk nitrate test in the past, pulling samples in September and into October. The stalk nitrate, in my opinion, reflects what corn experienced in terms of nitrogen availability through the season. In a season like 2015, your corn crop may have been short of nitrogen without exhibiting any visible yellowing. If corn yields disappointed you but everything else seemed right, a lack of nitrogen is a likely culprit.
Visual nitrogen deficiencies weren’t as serious as I expected them to be considering the number of large rain events during June and July. Cooler temperatures reduced stress on the plant, and plants were probably able to extract more nutrients from the soil. Cooler temperature and ample moisture also released more nitrogen from organic matter, which kept nitrogen shortages to a minimum -- as evidenced by the slight yellow hue that never seemed to worsen.
Growers have become savvy about nitrogen. More nitrogen is applied in the spring, growers are splitting applications and using nitrogen stabilizers to protect their investment. The next opportunity is improving organic matter and soil health to improve soil’s ability to mineralize nitrogen and provide a buffer against excessive rainfall and its potential impact on soil nitrogen availability.
The stalk nitrate test is a practical field tool to assess if your nitrogen practices provided enough nitrogen to your crop or if the excessive rainfall caused nitrogen losses that reduced yield.
If you have a question, e-mail Dr. Daniel Davidson at firstname.lastname@example.org
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