Soybean Inoculation:
Applying the Facts to Your Fields
This NebGuide details research conducted in Nebraska on soybean inoculants and provides a method and worksheet to determine if inoculation is needed in a specific field.
Lori J. Abendroth, Former Agronomy Research and Extension Associate
Roger W. Elmore, Adjunct Professor of Agronomy, Former Extension Crops Specialist
Present Contact: Richard B. Ferguson, Extension Soils Specialist
- What types of inoculant products are available?
- When do I need to inoculate soybean ground?
- “New” soybean ground
- “Old” soybean ground
- Overall recommendation
Why is it that we see soybean yield responses from inoculating with Bradyrhizobia japonicum in some cases but not others? In part one of this two-part series we discussed field conditions that support B. japonicum survival and encourage fixation of nitrogen. Please refer to the publication titled, Soybean Inoculation: Understanding the Soil and Plant Mechanisms Involved (NebGuide G1621) for more information on the basics of soybean inoculation.
This publication explains how to distinguish which fields will likely respond to inoculation with B. japonicum from those that will not. Fields are designated as either “new” or “old” based on how often soybean is grown. This designation is one criterion among many in deciding when to inoculate. In general, research trials across multiple locations in Nebraska have shown no yield advantage to re-inoculation.
B. japonicum are included on two types of carriers — peat and water. Peat (or humus) is used as a carrier in either a granular form, which is applied in-furrow, or in a powder form, which is applied to the seed at planting. Water-based products include liquid inoculants (seed applied or in-furrow) and frozen concentrates. New inoculants, introduced within the last 10 years, have increased potency due to sterile carriers and new packaging techniques. In addition, “growth promoters” now are being packaged with inoculants. These promoters can include an array of chemicals, with some of the newest products containing lipochitooligosaccharides (LCOs) or nutrient solutions. These products are suggested to increase early season growth, nitrogen fixation and overall plant health.
When do I need to inoculate soybean ground?
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. Nebraska fields that have never had soybean are decreasing as soybean production continues to spread westward. An “old” soybean field is one that has produced a soybean crop within the past four to five years. If the field has not been in soybean production for more than four to five years, it is again considered “new.”
There is no question that new soybean fields should be inoculated with B. japonicum; research throughout the Midwest supports this. Unless soil nitrogen levels are very high (more than 60 lbs per acre), a yield increase (1 to 10 bushels per acre) should be expected when an inoculant is used. In south central Nebraska new soils showed a yield increase of up to 2 bushels per acre with inoculants in 2002 and 2003. 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 has also shown that when an inoculant is applied to the seed, both dry and liquid inoculants perform similarly.
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Will Nebraska producers see a yield increase from re-inoculating- a field that has recently had soybean? Fields that have had an inoculated soybean crop before are able to retain the bacteria from year to year. These bacteria are able to inoculate the next soybean crop. New inoculant products contain strains of B. japonicum that are said to be more aggressive than those in 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. Results from this research across south central Nebraska are highlighted in Table I. Nodule counts on roots were also unaffected by inoculant treatment on “old” ground (data not shown).
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Some states in the Midwest have shown yield increases on “old” ground from inoculants (Figure 1). 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 we see some positive yield responses when fields are re-inoculated in certain regions. Factors that could increase mortality of soil bacteria include the soil pH, temperature, texture, water content and presence of fungicide. A yield response to re-inoculation appears to occur more often in the northern and eastern soybean production states within the Midwest. Northern states may see a response due to cold spring temperatures which limit growth and multiplication of B. japonicum. Seed-applied fungicides also are used more extensively in eastern production states and this could be responsible for part of the observed yield response. The Great Plains region does not have a consistent yield response to re-inoculation of “old” fields.
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 decide if re-inoculation is necessary. In “old” fields, a yield increase from re-inoculation will likely be found in situations where there are multiple factors deterring bacteria survival and not due to one lone factor.
Reference
Percent yield response data for Figure 1 was supplied by the following university extension specialists: Robert Henson, North Dakota State University; Robert Hall, South Dakota State University; Palle Pedersen, Iowa State University; Roger Borges, University of Wisconsin–Madison; Emerson Nafziger, University of Illinois; Kurt Thelen, Michigan State University; Shawn Conley, Purdue University; and Jim Beuerlein, The Ohio State University.
Index: Crop Production/Field Crops
Soybean
Issued May 2006
