Emerson Nafziger compares soybean plants from a research trial at the U of I Northern Illinois Agronomy Research Center. Some of the plants in this trial received 250 pounds of nitrogen at planting. “Nodules usually form when the plants are small and the roots are developing,” he explained.
Emerson Nafziger compares soybean plants from a research trial at the U of I Northern Illinois Agronomy Research Center. Some of the plants in this trial received 250 pounds of nitrogen at planting. “Nodules usually form when the plants are small and the roots are developing,” he explained.
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 yields.”

However, the professor said, there really isn’t any proof.

“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.”

Nitrogen Sources

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 do this.”

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.

Yield Ranges

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 crop.

“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 previous year.”

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.