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A Little-Known Nutrient
1/11/2008
By Darrell Smith
Molybdenum—or more precisely the lack of—may be
robbing yields from a surprising number of fields.
Research
conducted by Matt Hagny, who runs Pinnacle Crop Tech (www.agronomypro.com),
a consulting service based in Wichita, Kan., discovered deficiencies of
molybdenum, an essential micronutrient, while searching for the cause
of pale green plants despite adequate nitrogen levels in
clients’ wheat fields.
“Ordering
molybdenum testing was a shot in the dark,” Hagny says.
Ward
Laboratories, an ag testing and consulting service based in Kearney,
Neb., analyzed plant tissue samples from more than 20 soybean fields
and 30 wheat fields in central and north-central Kansas.
“Around half of the soybean fields were acutely molybdenum
deficient,” Hagny says.
Some
samples were below the detectable limit of 0.01 parts per million (ppm)
molybdenum. The deficiencies were found in soils with a pH ranging from
4.7 to 8.0. Deficiencies also showed up in sunflower, field pea and
sorghum fields. (In sunflowers, the symptom is deformed plants, rather
than pale color.)
“Some
areas had only a few deficient fields,” Hagny says.
“But in some townships, counties and soil types, nearly every
field we looked at was deficient. In some cases, the deficiencies were
spotty within a field.”
Hagny
conducted a study using replicated, randomized plots. He found that
soil-applying less than $1.50 per acre of sodium molybdate fertilizer
improved soybean yield by an average of 5.7 bu. per acre in deficient
fields. Foliar-applied rescue applications turned many fields from pale
green to normal dark green in a couple days, he says.
Molybdenum
fertilizer can also be applied to the soil or sprayed on plants at
rates of 0.02 lb. to 0.04 lb. per acre, Hagny says. It also can be
applied to seed as a seed treatment, notes Dave Mengel, a soil
fertility specialist at Kansas State University.
“Crops
most sensitive to low molybdenum include sunflowers, soybeans, peas and
canola,” Hagny says. “But alfalfa, wheat, corn and
sorghum also can be affected.”
A
plant’s requirement for molybdenum is so small, seeds grown
in molybdenum-adequate soil usually contain enough of the micronutrient
for normal crop growth.
“For symptoms to show up, not only the soil but also the seed
must be deficient,” Hagny explains. “Many of the
soybean fields I examined were grown from seed produced at diverse
locations in Illinois. Some of the molybdenum-deficient sunflowers were
grown from seed produced in California and Puerto Rico. That suggests
soils over a wide area may be deficient in molybdenum.”
Mengel
recalls hearing of molybdenum-deficient fields in Indiana when he was
at Purdue University. “Those deficiencies occurred on acid
soils,” he says. “Symptoms disappeared when a seed
treatment containing molybdenum was applied or when fields were limed.
But I also know of situations in Alabama where low molybdenum could not
be cleared up by liming.”
Too
much molybdenum is toxic to plants and animals that consume the forage
or grain. Identify deficient soils before applying molybdenum
fertilizer.
“Handle
samples carefully to avoid contamination,” Hagny says.
“Wear vinyl or latex disposable gloves when handling tissue
samples. Make sure the laboratory you choose is capable of testing for
molybdenum to levels of 0.01 ppm or less.”
Fast
Facts
- Molybdenum
deficiencies were found in a number of soybean and wheat fields in
Kansas.
- For
symptoms to show up, the seed
and the soil must be deficient—suggesting
there are more deficiencies in the states where the seed was grown.
- In
deficient soils, applying molybdenum fertilizer paid off.
- Excess
molybdenum is toxic to crops and the animals that consume the forage or
grain. Test soils before applying fertilizer.
You
can e-mail Darrell Smith at dsmith@farmjournal.com.
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