Give It the N Test

Take the Guesswork Out of Adding More Nitrogen

Russ Quinn
By  Russ Quinn , DTN Staff Reporter
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With sidedressing season in full swing, now is the time to pull soil and tissue samples in order to mitigate whatever nitrogen losses occurred this spring and summer. (DTN file photo by Pamela Smith)

OMAHA (DTN) -- Nitrogen deficiencies can kill a yield. Visual evaluations of just how much nitrogen (N) is left are inaccurate enough to hurt the pocketbook.

Soil and tissue tests can determine how much nitrogen is present, how much has been lost and how much should be applied. With sidedressing season in full swing, now is the time to pull those sample in order to mitigate whatever losses occurred this spring and summer.

TWO N TESTS

Farmers have a couple different options to ensure that their corn has enough nitrogen for the entire growing season. Penn State University professor of agronomy Douglas Beegle in a release wrote that some nitrogen could have been lost to leaching or nitrification.

"The pre-sidedress soil nitrate test (PSNT) and the chlorophyll meter tests are available to help improve nitrogen management decisions at sidedressing time," Beegle wrote. "Neither of these tests will give you the absolutely correct answer to your N management decision, but research has shown that both, if used correctly, can significantly improvement nitrogen management."

With the PSNT, the soil nitrate nitrogen level should be above 21 parts per million (ppm) when the corn is 12 inches tall or around the five- to six-leaf stage. If nitrogen levels are below this level, more nitrogen will need to be applied.

Beegle said it is important to wait until the corn is at least 12 inches tall as sampling too early can give misleading results. Because of the narrow window for sidedressing, most soil-testing labs, including Penn State's own lab, will provide a one-day turnaround on samples for the PSNT.

He also recommended not to take samples right after heavy rains. Instead, wait two or three days after the rain events. When doing PSNT, take a 12-inch-deep sample and dry the sample the same day it is collected.

With the chlorophyll meter test, Beegle recommended only taking samples after the corn has reached at least the sixth leaf stage. Take readings on the fifth leaf about 3/4 of the way out on the leaf between the edge of the leaf and the midrib.

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Readings for this test should be taken on at least 30 plants in the field, Beegle said.

It should be noted that the chlorophyll meter test cannot be used if more than 15 pounds of nitrogen fertilizer an acre was applied at planting. Not following this rule could result in a false indication of adequate nitrogen for the crop, he said.

N STILL THERE

Brittany Bolte, agronomist at Rock County Agronomy Service located in Bassett, Nebraska, said her region was certainly wet this spring with plenty of moisture. Despite the heavy rain, when it came to nitrogen in the soil, farmers found a pleasant surprise.

"In doing soil tests on a couple different fields that were ugly looking, our nitrogen was still in the six inches and 12 inches (ranges), which kind of surprised us," Bolte told DTN. "As far as nitrogen left, I'd say we lost some but not as much as we thought."

Bolte said most of the corn acres in her home area of north-central Nebraska will have a sidedressing operation. With much sandy soils found in the area, she estimates her growers never have more than 50 pounds of nitrogen on at or right after planting.

They will start sidedressing now and run through R2 stage thanks to applying nitrogen through the center pivot irrigation systems, also known as fertigation.

She will utilize both the PSNT and the chlorophyll meter test and feels both tests are really accurate. By using the test, growers will know exactly what is going on the soil and how the plant is responding, she said.

WEATHER, HYBRIDS CHANGING

Changing weather as well as changing corn hybrids are pushing farmers into monitoring nitrogen levels more closely during the growing season. In a recent webinar, Jim Schwartz, director of Practical Farm Research and Agronomy for Beck's Hybrids, discussed these factors in depth.

Weather data shows more, larger precipitation events in the last couple of decades than in previous ones, he said.

"There is definitely a trend in heavy precipitation," Schwartz said. "In the Midwest, there has been almost a 50% increase in the frequency of days with a basically a four-inch rainfall."

Corn hybrids are also changing. In studies from both Purdue University and the University of Illinois, newer hybrids are seeing a higher rate of post-flowering nitrogen uptake than in the past.

Old germplasm (years 1940 to 1990) in the Purdue study only took up 43 pounds of nitrogen an acre. But more modern germplasm (1991 to 2011) will now take up 55 pounds of nitrogen an acre, Schwartz said. In the Illinois study, the newer hybrids took up 52 pounds while the old hybrids only took around 37 pounds.

"It has everything to do with root protection and insect protection and better standability," he said. "Bottom line is hybrids are taking up nitrogen later in the growing season."

For more on nitrogen tests, check out these fact sheets from Michigan State University and Perdue University Extension services: http://fieldcrop.msu.edu/…

https://www.agry.purdue.edu/…

Russ Quinn can be reached at russ.quinn@dtn.com

Follow Russ Quinn on Twitter @RussQuinnDTN

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Russ Quinn