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Fertilize with sulfur selectively

Sulfur fertilizer offers no
yield or protein benefits to spring wheat grown in heavy soils with high
organic matter, according to a recent study done by the University of Minnesota
(U of M).

“For a producer in the Red
River Valley, sulfur (S) applications should not be a high priority for spring
wheat,” says Russ Severson, U of M regional Extension educator.

In 2008 and 2009,
researchers evaluated the results of applying two types of sulfur fertilizers
to spring wheat fields located in the northwestern and southeastern regions of
Minnesota.

“When selecting sites for
the trials, we did not purposely go to areas that were known to be deficient in
sulfur,” says Severson. “We focused rather on soils in the regions where most
of the wheat is grown.”

One question driving the
study centers on yield increases in the wheat-growing regions with high-quality
soil. “We wondered whether the sulfur fertilizer recommendations developed for
a 40-bushel (per acre) wheat yield were still adequate for the 90-bushel yields
actually being produced,” says Severson.

Researchers wondered, too,
if sulfur deficiencies could be contributing to the low protein content in
these bumper wheat crops.

Yet a third reason for doing
the study relates to decreases in the amount of sulfur deposited through
rainfall as acid rain. “With the scrubbing of sulfur from smokestack exhaust,
we’re getting less sulfur from acid rain,” says Severson. “We wondered whether
or not this could be making a difference in wheat yields.”

The two sulfur fertilizers
they studied were spring-incorporated ammonium sulfate and the brand name
fertilizer MicroEssentials MES15. Ammonium sulfate was also applied as a
tillering treatment in season to both the ammonium sulfate and MES sulfur
fertilizer treatments.

In the brand name product,
one half of the sulfur is immediately available as ammonium sulfate. The other
half is in the elemental form of sulfur that must be mineralized by soil
bacteria in order to become available to plants later in the growing season.

The straight ammonium
sulfate was applied at amounts supplying 12.5 pounds per acre of sulfur, 25
pounds per acre, and 37.5 pounds per acre, as was the brand name product. The
tillering in-season application was ammonium sulfate only applied at 0, 12.5,
and 25 pounds per acre. Unfertilized plots were control sites. Glenn was the
wheat variety grown in all plots.

When researchers tested
plants during the growing season, they found that applying sulfur fertilizers
did, in fact, increase sulfur uptake when plants were tested at the soft dough
stage of growth.

“But when we looked at
yields at plant maturity, we saw no increase in yield as a result of the sulfur
fertilizers,” says Severson. “Neither did we see an increase in grain protein
as a result of sulfur fertilization. Over the two-year period across all
locations, the grain protein averaged 14.9%.

“That’s not to say that
there aren’t soils that will benefit from sulfur fertilization,” he adds. “You
might expect to see a response in wheat to sulfur on soils that are sandy and
low in organic matter.”

Soil organic matter contains
sulfur, but it is not available to plants unless it is mineralized.

Crops vary in their need for
sulfur. Canola and alfalfa, for instance, are heavy users of sulfur. Small
grains and corn have a medium demand for sulfur, while soybeans have a low
demand.

For producers with sandy
soils or soils with low organic matter, doing side-by-side plot comparisons of
sulfur-fertilized wheat with nonfertilized spring wheat could provide a
site-specific indicator of the need for sulfur

Learn More

Russ Severson

218/281-8695 |
sever014@umn.edu

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