Seed inoculation Helps Soybeans Get Nitrogen

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Written September 26, 1997

Newly developed soybean inoculants can save growers the expense of uneconomical late-season nitrogen applications, says Ohio State agronomist Jim Beuerlein.

Soybeans need nitrogen to reach their yield potential, so when nitrogen deficiencies show up late in the growing season, farmers sometimes pay for aerial nitrogen applications. They may spend as much as $15 to $20 per acre just to raise yields by 1 to 3 bushels per acre, or about enough to break even.

Relatively inexpensive seed inoculants used at planting, however, can increase the nitrogen-fixing potential of the roots, Beuerlein says. Seed inoculants can increase yields by as much as 6 bushels per acre at a cost of $1 to $3 per acre, he says.

"For some reason, the plants prefer nitrogen from this method than from direct applications," Beuerlein says.

Beuerlein says the new seed inoculants, now commercially available, are highly effective in increasing yields when combined with good variety selection and seed fungicide treatments to improve root health.

Seed inoculants promote a naturally occurring process between soil bacteria and the plant. Bacteria infect the root system, and after they consume the plant's sugars and carbohydrates, they produce nitrogen for the plant to use.

Generally, the use of seed inoculants for two consecutive plantings can generate an improved nitrogen-fixing process in the soil. After that, inoculation should not be necessary unless nitrogen deficiencies show up again.

Fields being planted with soybeans for the first time will require special treatment because seed inoculants may not be enough to produce the desired bacterial activity in roots, Beuerlein says.

First-time soybean planters should make a 75-pound-per-acre urea nitrogen application at flowering, and follow with a post-harvest disking. The following spring, they can use seed inoculants again when planting soybeans for a second year.

Yellowed leaves in soybeans can be a symptom of nitrogen deficiency. A leaf analysis by a reputable laboratory can confirm a problem. Nitrogen levels are considered sufficient when leaf nitrogen content is greater than 4.2 percent on a dry-weight basis.

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