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 The study used a “tagged” form of ammonia to determine how much
of the nitrogen in corn grain and plant material comes from
fertilizer, versus nitrogen supplied naturally by the soil.
“There have been a number of studies to compare yields with
fall- versus spring-applied ammonia or other treatments. But our
study is different because we are tracing the nitrogen from the
fertilizer ammonia into either the grain or the whole corn plant
above ground. That's what makes this unique,” says Richard
Mulvaney, professor in the Department of Natural Resources and
Environmental Sciences at Illinois.
Mulvaney and his graduate student, Kelsey Grieshiem, used a
stable isotope of nitrogen, 15N, in formulating the tagged
ammonia. They applied it at a typical rate of 200 pounds per
acre in mid- to late November in six Illinois fields in 2016 and
2017.
The fields differed in soil type and crop rotation. Four were
Mollisols, which Mulvaney describes as prairie soils: rich,
black, and productive. The two others were Alfisols, or timber
soils, which are typically poorer in comparison to Mollisols.
Two of the Mollisol fields were cropped to continuous corn,
while the rest were a under a corn-soy rotation.
After the fall ammonia application, the researchers looked for
the 15N isotope in corn plant and grain material at harvest in
the following growing season. Any nitrogen not tagged with the
isotope was assumed to have come from natural nitrogen stores in
the soil, rather than from the fertilizer.
“Just as we expected, the poorest soil showed the highest uptake
efficiency while the richer soils were much lower,” Griesheim
says. “On average, only 21% of the nitrogen applied was
recovered in the grain, ranging from 34% at the poorest Alfisol
to 12% for the richest Mollisol.
“Farmers apply ammonia in the fall thinking they've supplied
nitrogen to their corn crop for the coming year. But based on
our results, most of the fertilizer nitrogen will not be taken
up by the crop.”
If it were up to Mulvaney, he would recommend
nitrogen applied as a side dressing, which delivers the
fertilizer when the plant is actively growing. “You're fighting
time with fall applications,” he says. “You're counting on
keeping the nitrogen in the soil for six months before the next
crop needs to take it up.”
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The study also evaluated the effectiveness of N-serve
(nitrapyrin), a nitrification inhibitor commonly applied in the fall
with anhydrous ammonia. This product is meant to slow down the
microbial conversion from ammonium, which is immobile in the soil,
to nitrate, which, during a wet spring, can leach away or be lost as
a gas.
“Nitrapyrin has long been used in conjunction with fall-applied
ammonia with the motive of increasing uptake efficiency. We didn't
find that to be the case at all. And in fact, in our study, the only
significant effect of the product was a yield decrease on continuous
corn,” Mulvaney says.
Should farmers apply anhydrous ammonia in the fall?
Griesheim says, “Considering the low uptake efficiencies observed in
our study, farmers should think twice before putting their nitrogen
on in the fall. Low fertilizer recoveries mean less return from the
farmer's fertilizer investment and a higher risk of environmental
pollution."
Adds Mulvaney, “To make matters worse, farmers have been taught for
decades that they should follow yield-based recommendations, such
that the soils that generate the highest yields need the most
fertilizer. But that inverts reality. The highest yielding soils
need the least fertilizer. Our research supports that.”
The article, “Nitrogen-15 evaluation of fall-applied anhydrous
ammonia: I. Efficiency of nitrogen uptake by corn,” is published in
the Soil Science Society of America Journal [DOI:
10.2136/sssaj2019.04.0098]. Authors include Kelsey Griesheim,
Richard Mulvaney, Tim Smith, Shelby Henning, and Allen Hertzberger.
The research was supported by BASF.
The Department of Natural Resources and Environmental Sciences is in
the College of Agricultural, Consumer and Environmental Sciences at
the University of Illinois.
[Source: Richard Mulvaney,
News writer: Lauren Quinn] |
