University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

March 17, 2006

Soybean inoculation

Also see
Soil and plant mechanisms of soybean inoculation
Note: As soybean continues to be an important crop in Nebraska and is extending into areas where it has not been grown previously, producers should consider whether inoculation with B. japonicum bacteria is advised. Two stories in this week’s CropWatch address 1) plant mechanisms that convert atmospheric nitrogen to a form that can be readily used by the plant, and 2) how to determine whether inoculation is recommended (below).

This material is based on two UNL NebGuides on soybean inoculation which feature recent Nebraska research on the practice. These are to be published this spring and will be announced in CropWatch.

Determining when innoculation is necessary

Deciding whether a soybean field should be inoculated depends on whether the field is “new” or “old.” A “new” soybean field is one that has never produced soybean. An “old” soybean field is one that has produced a soybean crop within the past four to five years.

“New” soybean ground

Research throughout the Midwest indicates that there is no question that new soybean fields should be inoculated with B. japonicum. Unless soil nitrogen levels are very high (more than 60 lbs per acre), a yield increase (1 to 10 bu/ac) should be expected when an inoculant is used. When soybean fields are inoculated for the first time, a soil-applied inoculant (granular or liquid) is recommended because a higher B. japonicum population can be applied. Research also has shown that when an inoculant is applied to the seed, both dry and liquid inoculants perform similarly.

In “old” fields, a yield increase from re-inoculation will likely occur when there are multiple factors deterring bacteria survival.
“Old” soybean ground

Will re-inoculation increase yield in a field that has recently had soybean? Fields that have had an inoculated soybean crop before retain the bacteria and can inoculate the next soybean crop. New inoculant products contain strains of B. japonicum that are said to be more aggressive than previous products. These new inoculant products are said to fix more nitrogen and increase yield compared to soybean inoculated by B. japonicum carried over from previous inoculations. University of Nebraska–Lincoln research conducted from 2001 to 2004 did not show a yield difference between any inoculation product and an untreated control when applied to soybean on “old” ground. In general, research trials across multiple locations in Nebraska have shown no yield advantage to re-inoculation.

Some states in the Midwest have shown yield increases on "old" ground from inoculants. A good soil environment is critical to the development of nodules and the overall productivity of B. japonicum. A soil environment that is not supportive of long-term bacteria survival could be the reason why some see positive yield responses when fields are re-inoculated. Factors that could cause increased bacteria mortality include soil pH, temperature, texture, water content and presence of fungicide. The Great Plains region does not have a consistent yield response to re-inoculation of fields within four to five years of the last soybean crop.

Inoculation products

Within the past 10 years, new inoculants have emerged that have increased potency due to sterile carriers and new packaging techniques. In addition some of these new inoculants are being packaged with “growth promoters.” These promoters can include an array of chemicals, with some of the newest products containing lipochitooligosaccharides (LCO’s) or nutrient solutions. These products are suggested to increase early-season growth, nitrogen fixation and overall plant health.

Overall recommendation

Inoculation of a field is recommended if soybean has not been grown within the past four to five years. If the field has had soybean more recently, use the Soybean Inoculation Decision Worksheet to determine if re-inoculation is necessary. In “old” fields, a yield increase from re-inoculation will likely occur when there are multiple factors deterring bacteria survival.

Lori J. Abendroth
Former Agronomy Research and Extension Associate
Roger W. Elmore
Adjunct Professor of Agronomy, Former Extension Crops Specialist

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Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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