West Lafayette, Indiana
April 17, 2009
A
Purdue University agronomist said that many farmers over the
last several years have observed a mid-season yellowing of their
soybeans, commonly called "yellow flash," which is caused by a
manganese deficiency.
Tony Vyn, Purdue Extension cropping systems specialist, spent
the last two years studying the causes of manganese deficiency
and various treatments.
"We found that manganese deficiency is much more likely to occur
in soil where there is low inherent availability of manganese,
relatively high pH levels and relatively dry soil conditions,"
Vyn said. "All three of these factors have an impact on the
amount of plant-available manganese."
The worst possible combination, he said, is to have low soil
manganese concentrations and a high pH, and low moisture levels
only exacerbate the problem. Moisture affects the form that
manganese is in and its availability to plants. Dry soils tend
to have more manganese in a form that is unavailable to plants,
and wet soils result in a chemical change of a portion of total
soil manganese -- to a plant-available form, Vyn explained.
"This is partly why manganese deficiency is called 'yellow
flash' because the symptoms can disappear quickly after a rain
or after soybean roots grow into a soil zone with more
plant-available manganese," he said.
Vyn's research team, which included Jim Camberato, Purdue
Extension soil fertility specialist, and graduate student
Yanbing Xia, looked primarily at two treatment methods. One
method was to add manganese like a traditional starter
fertilizer and position it 2 inches beside and 2 inches below
the seed at the time of planting. The second method was foliar
application 7-10 days after a post-emerge glyphosate
application.
Vyn said they are not completely satisfied with their research
because of the amount of soil spatial variability they
encountered even within their 4-acre trials.
"There has been very little yield improvement that we could
prove statistically from the banded application of manganese at
the time of planting," Vyn said.
The foliar application of manganese proved to be more beneficial
in increasing leaf manganese concentration immediately, but the
results were short-lived—one to two weeks. Vyn said that they
are beginning to learn more about the critical level of
manganese needed in the soybean leaves at the top of the soybean
canopy during its vegetative and reproductive growth in order to
obtain optimum yields.
"The traditional thought has been that we need to have
approximately 20 parts per million of manganese in the top fully
expanded trifoliate leaves during vegetative production," Vyn
said. "We are learning that the actual manganese concentration
in soybean plants can range widely, and producers will
definitely see more in the way of manganese deficiency symptoms
when there are only 10 parts per million.
"But we have also observed yield responses when leaf
concentrations of manganese increase to 30-35 parts per
million."
Prior research done at Purdue shows that soybean fields with too
little available manganese can result in a yield loss up to 12
bushels an acre. Vyn said that additional research is needed to
find just how much of a yield increase is possible by
maintaining higher manganese levels, especially during the
reproductive stage of soybean development.
To help prevent manganese deficiency from occurring this year,
Vyn recommended careful liming to keep soil pH from getting too
high. Manganese availability to the soybean plant is reduced as
soon as soil pH climbs above 6.6 or 6.8, he said.
"So as farmers deal with low soil pH problems by applying lime,
they should consider using variable rate lime applications to
avoid raising soil pH above 6.7 in any area of the field," he
said. "Variable rate technology has already improved the soybean
plant's access to micronutrients."
Because measuring micronutrient concentrations is an additional
cost when submitting a soil test, most farmers opt not to have
them measured, Vyn explained. Although there is very little
change in soil micronutrient concentrations from year to year,
he said, they should be tested at least every five years.
Soybean farmers should also keep a close eye on their fields for
visible symptoms of manganese deficiency following a glyphosate
application to make sure the soybean leaves maintain manganese
concentrations above 20 parts per million.
"The 20 parts per million critical level is based on old
research, and critical levels for manganese may have changed
because of modern, higher-yielding cultivars, as well as the
dominant use of foliar applied glyphosate," Vyn pointed out.
If a field is suspected of being manganese deficient, farmers
should randomly collect 30-40 top, most fully-expanded,
trifoliate leaves in the suspected area and send them to a
commercial lab for analysis, Vyn said.
If a manganese deficiency is present, Vyn recommends a foliar
application 7-10 days after the glyphosate application.
He said that farmers should not tank mix manganese with
glyphosate because, when mixed, the efficacy of both -- the
glyphosate on weeds, as well as the manganese for the soybean
plants -- is reduced.
This year, Vyn and his team will look at manganese availability
in glyphosate-resistant corn and glyphosate-resistant soybeans
that are in rotation with and without glyphosate applications.
They will also test two foliar applications versus one
application and no application. Manganese by itself and in
different combinations with other trace minerals such as zinc
will also be evaluated.
This research has been funded by the Indiana Soybean Alliance. |
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