Inoculating Soybeans Proves Profitable

Particularly on ground new to soybeans, or several years removed from the last bean crop

By Tracy Sayler

If you’re planning to grow soybeans on ground that’s never seen beans before, or is a few years removed from the last bean crop, then it’s recommended to inoculate the seed.

Soybeans have the ability to fix nitrogen from the atmosphere if properly nodulated. Nodulation requires inoculation of the seed prior to planting with certain species of rhizobium bacteria specific for soybean.

Research clearly indicates a positive yield response of soybean to inoculation without previous soybean history, and even in fields with a soybean history, although those gains are more modest.

Ohio State University extension agronomist Jim Beuerlein reviews results of several inoculation studies in his paper, “Soybean Inoculation; its Science, Use and Performance” online at www.abm1st.com/pdf/soybeanInoculation.pdf.

Beuerlein reports that the average yield increase from 64 Ohio trials is nearly two bu/ac, and has produced a profit of about 300% when beans were worth $6.00 per bushel and when the inoculation material costs $3.00/ac. 

North Dakota State University tested 15 to 39 soybean inoculation materials yearly during 1999 through 2005 at the Carrington Research Extension Center.  He points out that the average yield increase from those 155 product evaluations is 2.7 bu/ac, with a yield change due to inoculation ranged from -0.5 bu/ac in 2005 (soybean history) to 8.2 bu/ac in 2003 (no soybean history).

Purdue University has tested inoculation products for 11 years and reported average yield increases up to 2.4 bu/ac in some years and a long-term average yield increase of 1.0 bu/ac.

Over the long term, soybean inoculation appears to be profitable throughout the Midwest, Beuerlein concludes. Although yield increases are small in many fields with a history of soybeans, the cost to inoculate the seed is also small and the practice is profitable over the long term – for most inoculation products, even a one half bushel per acre yield increase is profitable, he points out.

Iowa State University recommends inoculating seed if:
• the field has never been planted to soybeans
• soybeans have not been grown in the field in the past three to five years
• the soil pH is below 6.0
• the soil has a high sand content
• the field has been flooded for more than a week, creating anaerobic conditions

NDSU points out that inoculation of wheat the year before planting soybean on fields where soybeans have never been grown can help prepare fields for soybeans. This has been demonstrated with limited research.

“This strategy has no impact on wheat performance but enhances the ability of soybean, grown on ground without previous soybean history, to produce nodules for nitrogen production,” says Greg Endres, NDSU agronomist, Carrington.  Previously-inoculated wheat seed allows the bacteria to be introduced early and more uniformly distributed in the soil profile instead of very localized introduction with the soybean seed, he says. This aids the development of nodules over a greater portion of the soybean root system, which may improve the soybean plant’s ability to produce nitrogen.

Inoculants come in two forms, dry or liquid, and a wide range of inoculant products are available to growers, from materials that improve production over a wider range of environmental conditions to extenders that allow application to seed 30 days, 60 days or even 90 days prior to planting without loss of productivity. Dry and liquid formulations of a product appear to perform similarly, says Beuerlein.

Take care of precautions with inoculation materials and fungicide seed treatments, notes Beuerlein. When applying a fungicide or using fungicide treated seed, be sure the fungicide has dried before applying inoculation material to the seed. Currently, many inoculation products may NOT be mixed with fungicides and applied to the seed together. One exception is that liquid formulations may be mixed with ApronMaxxRTA fungicide and applied together.

When loading a drill or planter using an auger, liquid or dry inoculation materials should be added to the seed as it enters the auger for thorough application, Beuerlein recommends. When loading a planter or drill from bags, fill the seed box to a depth of three inches and scatter an appropriate amount of inoculation material over the seed and mix thoroughly. Continue to add seed in six-inch layers, treating each layer until the box is filled. With some dry materials it may be necessary to moisten seed to increase adherence.

A few small specks of inoculation material on each seed are adequate, says Beuerlein. At the recommended use rate, there will be up to 1,000,000 bacterial cells on each seed. Excessive application of inoculation material is expensive, will likely not increase yields more than the recommended application rate, and could cause seed metering problems resulting in reduced numbers of seeds planted. Excessively wet or dry periods following planting have decreased performance of inoculation materials.

Inoculation materials retain their activity on treated seed for at least 24 hours, and some survive on untreated seed for up to seven days.  It’s best for most inoculated seed to be planted as soon as possible after treatment (12 hours or less) so the bacterial cells will remain moist and survive long enough to infect soybean roots following germination. However, some inoculation materials include additives which allow application up to 20 days before planting, even on some fungicide treated seed.

See inoculant package label for specific management and application information, or consult with an agronomist.  NDSU soybean production information can be found online at www.ag.ndsu.edu/pubs/legumes.html.

Yield response of soybean to inoculation with or without previous soybean history*

Soybean in rotation	Number of sites	Sites that had a significant yield response to inoculant (%)
Yes	43	7
No	13	93