Consider soil in fall-applied ammonia
rates, Illinois study says -
University of Illinois College of
Agricultural, Consumer and Environmental Sciences
Ill. - Fall-applied anhydrous ammonia may not fulfill as
much of corn's nitrogen needs as previously assumed.
According to a new study from the University of
Illinois, the effectiveness of the practice depends on
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.
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.
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.
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.
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.
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.
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."
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."
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.
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.
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."
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."
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.
Department of Natural Resources and Environmental
Sciences is in the College of Agricultural, Consumer and
Environmental Sciences at the University of Illinois.
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