Illinois
FarmerToday
12
June 2023
Corn takes up most of its N naturally
![](images/images_06_14_2023/Corn1.jpg)
URBANA, Ill. — Corn growers seeking to
increase the amount of nitrogen taken up by their crop can adjust
many aspects of fertilizer application, but recent studies from the
University of Illinois Urbana-Champaign show those tweaks don’t do
much to improve uptake efficiency from fertilizer.
That’s because, the studies show, corn takes up the
majority of its nitrogen — about 67% on average — from sources
occurring naturally in soil, not from fertilizer, according to a
university news release.
The evidence for soil as corn’s major nitrogen source
came repeatedly over the course of four studies, the first published
in 2019 and the rest more recently.
In all four studies, researchers in the Department of
Natural Resources and Environmental Sciences in the College of
Agricultural, Consumer and Environmental Sciences at University of
Illinois labeled fertilizers with a naturally occurring isotope of
nitrogen, known as 15N, and applied it in the field at different
rates, forms, placements and timings.
After each harvest, the researchers analyzed corn
biomass and grain for its nitrogen content, attributing labeled 15N
to fertilizer and unlabeled nitrogen to soil sources. In all four
studies, which included both poor and fertile soils in Central
Illinois, most of the nitrogen in corn at harvest was unlabeled.
“My hope would be that producers would just realize
the magnitude of these numbers. They’re purchasing this nitrogen and
it’s not all getting into the crop,” said Kelsey Griesheim, who
completed the studies as an NRES graduate student and is now an
assistant professor at North Dakota State University. “It’s
important to make them aware of it, so that when they’re looking at
their bottom line and how much they’re spending on nitrogen, they
realize the situation.”
Griesheim’s 2019 study found only 21% of fertilizer
nitrogen made it into the grain when applied in the fall as
anhydrous ammonia. The result made some sense, as fall-applied
fertilizer lingers in the soil for months before corn is planted,
and then has to last throughout the season to nourish the growing
crop.
Incidentally, the study also found nitrification
inhibitors, often applied with anhydrous to slow transformation from
ammonia to more-leachable nitrate, didn’t help to enhance nitrogen
uptake from fertilizers.
Assuming pre-season and in-season application would
achieve greater uptake than fall-applied nitrogen, Griesheim tried
those tactics in her three more recent studies.
Jumping forward to planting season, Griesheim applied
15N-labeled urea-ammonium- nitrate (UAN) at planting in subsurface
bands using 2 x 3 placement, surface dribble, and drag-chain
applications at 80 pounds per acre.
Reaching up to 46% 15N content in corn biomass,
banded placement was more efficient than broadcast fertilization,
which only reached 34% in the most optimal sites.
“No question, banding is more efficient than
broadcasting nitrogen. That was very clear from the data,” Griesheim
said. “However, whether we applied one band or two bands, or whether
we used the 2 x 3 placement or a drag chain, there weren’t a lot of
differences in efficiency.”
Griesheim also tested fertilizer placement during
in-season growth, or side dressing, applying 200 pounds per acre of
15N-labeled UAN with a Y-drop attachment that delivers liquid
fertilizer at the base of a growing corn stalk. In this case,
Griesheim split the application between planting and the V9 growth
stage. She compared the Y-drop application against subsurface
placement at both growth stages.
“When split between two application times, 15N uptake
was higher at side dressing than at planting, but even when applying
in-season, more nitrogen was derived from soil than fertilizer
(averaging 26% in grain and 31% in biomass from fertilizer),”
Griesheim said.
“We didn’t see a difference between the Y-drop and
subsurface applications for five of the six study years, but under
conditions conducive to volatilization, uptake was greater with
subsurface applications.”
Finally, Griesheim labeled multiple fertilizer forms
— UAN, potassium nitrate and liquid urea — with 15N and applied them
as surface side dress applications with a Y-drop applicator.
Surprisingly, uptake was greatest when fertilizer was applied as
potassium nitrate, followed by UAN, then urea.
“It was interesting that nitrate emerged as the most
efficient of the three sources, despite weather conditions that were
fairly conducive to nitrogen loss by leaching or denitrification,”
said Richard Mulvaney, professor in NRES and co-author on all four
papers. “Laboratory incubation experiments that were part of the
same study showed this was due to ammonia volatilization and
immobilization by soil microbes.”
The full body of work suggests there are things
farmers can do to increase nitrogen uptake from fertilizers —
namely, apply nitrate-based sources in-season while the crop is
actively growing. But the recurring lesson that soil supplies the
greatest amount of nitrogen to corn is an important one that should
lead to changes in nitrogen management, the researchers say.
“If the soil is the main source of nitrogen for crop
uptake, which it almost always will be, we need to take the soil
into account. It’s just that simple. Otherwise, with factors like
timing, rate, placement, and form, we’re tweaking, but probably
won’t find a miraculous increase in efficiency using those
approaches,” Mulvaney said. “We really should go toward adjusting
rate according to the soil and the soil-supplying power, going
towards variable-rate nitrogen.”
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