SHABBONA, Ill. — Researchers continue to search for ways to
unlock the secret to higher soybean yields.
“We’re trying various things, but we haven’t found the right
key for that lock,” said Emerson Nafziger, University of Illinois Extension crop
production specialist and professor. “The drive to put nitrogen on soybeans to
get yield is not the first time in history this question has been asked — it’s
been asked for decades now.”
Since it takes energy for the plant to fix nitrogen,
Nafziger said, “the idea is if we relieve them of that need for energy, there
will be more energy to put into yield and move up to the next level for soybean
However, the professor said, there really isn’t any
“One thing we know for sure is soybeans need a lot of
nitrogen,” he said during a presentation at Agronomy Day held at the U of I
Northern Illinois Agronomy Research Center.
“An 80-bushel crop will take up about 400 pounds of
nitrogen, and that’s a higher requirement than a 220-bushel corn crop,” he
reported. “About two-thirds of that nitrogen is harvested off, and the rest
stays in the plant material.”
There is an unlimited supply of nitrogen in the air.
“About 78 percent of the air we breathe is nitrogen,”
Nafziger said. “However, the nitrogen bond is very strong, and prying it apart
takes a lot of energy. The bacteria in the nodules get energy from the plant to
During the time the soybean plant is filling seeds, the
nitrogen uptake is going to be linear, from five to six pounds of nitrogen per
day, Nafziger said.
“The energy requirements are large during the month of
August,” he added. “But the soybean plant seems to be capable of managing to
find the energy to do that.”
Nitrogen uptake slows by the mid-R6 period, Nafziger said.
“That’s when the leaves are pale yellow, and they’re sending
their nitrogen into the seed,” he said.
Nafziger reminded farmers that soil already contains a fair
amount of nitrogen.
“The corn crop takes up more nitrogen than the fertilizer
rate you ought to be applying,” he explained. “I’ve seen 230-bushel yields from
150 pounds of nitrogen — that means the difference comes from the soil.”
One percent organic matter in the top six to seven inches of
soil will contain about 1,000 pounds of organic nitrogen, Nafziger said.
“If you have 3 percent organic matter, 12 inches deep,
that’s 6,000 pounds of organic nitrogen,” he noted. “We think about 2 percent
becomes available to the crop, or 120 pounds.”
Instead of a soybean nitrogen credit, Nafziger said, it is
really a lack of corn penalty.
“Corn residue ties up nitrogen, and the soybean residue you
leave behind does not,” he said.
Nafziger has seen estimates from 40 percent to 70 percent of
the plant’s nitrogen requirement is met by nitrogen fixation.
“The amount depends on the soil type, the organic matter and
how much nitrogen was left behind from the previous corn crop,” he said.
In soybean trials in Illinois over the past four years at 20
sites, yields have ranged from 40 to 90 bushels per acre.
“We found one time where we got significant yield response
from nitrogen,” Nafziger said. “We do not see a tendency for high-yielding
soybeans to respond more to nitrogen.”
It is possible to have too much nitrogen for the soybean
“We have the danger of tipping soybeans over the edge so
they make a wonderful plant with a poor seed set,” Nafziger said. “In this part
of the state in 2003, the plants were tall, with big leaves and lots of water,
but they yielded poorly.”
At the research center in Monmouth, a study has been ongoing
for 30 years comparing corn following corn and corn following soybeans.
“The rate of nitrogen we applied to the previous corn crop
seemed to have an effect on soybeans,” Nafziger reported. “In one case, there
was a 20-bushel higher soybean yield with 240 pounds of nitrogen applied the
However, he does not recommend farmers apply more nitrogen
than the corn crop needs.
“Economically we don’t think it works,” he said. “And we
don’t want 150 pounds of nitrogen left in the soil at the end of the growing
season for corn.”
Researchers are not giving up on understanding nitrogen
applications and the soybean plant, Nafziger said.
“We hope to get some better answers,” he added.