MINOT, North Dakota — Farmers can reduce input costs by not fertilizing soybean fields with nitrogen if the crop has been grown on that ground in the past four years, a soil science expert says.
Research trials conducted across the U.S., including at North Dakota State University sites in north-central and northwest North Dakota, showed that supplemental application of nitrogen fertilizer does not increase soybean yields when nodules are functional, said David Franzen, NDSU soil science specialist.
"If you go out in the field, and you dig up the plant, and it has nodules, you don’t need it," he said.
Franzen was one of several NDSU Extension
researchers who spoke about soybean topics at the annual Western North Dakota Soybean School held Jan. 26 at the KMOT Ag Expo in Minot. Topics also included soybean weeds, insects and carbon markets.
Refraining from fertilizing with nitrogen could save farmers $50 per acre, Franzen estimated.
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Meanwhile, inoculation of soybeans is not necessary if fields have successfully been inoculated within the previous four years, he said.
Another unnecessary expense is fertilizing soybean fields with phosphorus when the results of the Olsen test shows that the level is 6 parts per million or greater, he said. If the phosphorus level is lower than that, broadcast or band seeding is preferred over seed-placed phosphorus, especially if the soil turns dry post seeding, he said.
Inadequate sulfur also is likely not an issue for most soybean farmers, Franzen said. If it appears during the growing season that the soil is deficient in sulfur, the fertilizer can be applied in late August.
Insect threats to soybeans
A potential new insect threat to soybeans that researchers at NDSU and in several other states are monitoring is the soybean gall midge, said Travis Prochaska, an Extension crop management specialist at the North Central Research Extension Center in Minot.
The insect, which was first identified in Nebraska in 2011, has spread into four other states and in 2021 was found in 141 counties, Prochaska said.
“This will be one of the up-and-coming pests,” he said.
Larval feeding of soybean gall midge results in blackened area on plants which can weaken them and result in plant death. The insect can cause from 20% to 100% yield loss, depending on the number of them in fields. Most of the damage to fields is on the edges, and it diminishes toward the center, Prochaska said.
Soybean gall midge, which is similar in size and shape to other midges, has been found in Minnesota and South Dakota, he said. So far, it has not been found in North Dakota, but it was identified in northeast South Dakota, not far from the border of the two states, Prochaska said.
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In 2021, a known nemesis, grasshoppers, were “the insect of the year,” in North Dakota, he said.
“This was a big deal last year. In some parts of the state, infestations were really high,” Prochaska said. The highest numbers in North Dakota were found near Grenora, in Williams County, and in the Ryder, Makoti, Logan and Sawyer areas of Ward County, Prochaska said.
The cold temperatures of this winter have potential to reduce the grasshopper population, but that may not be as effective as it typically is because early snowfall provided the pests with insulation, he said.
A cool, wet spring would be optimal weather conditions for reducing the population, Prochaska said.
