Nebraska farmers must scrutinize inputs and focus on finding profit for their estimated 5.3 million soybean acres  in 2016. Using products that may increase yields 1-2 bushels per acre might have been worth the gamble when soybeans were $14.40 per bushel in 2012, but become a poor roll of the dice at the $8.85 per bushel set in February for some crop insurance policies.

During soybean planting season we often think of planting date, seeding rate, and seed treatments. In regard to seed treatments, one of the choices being made right now is "Should I inoculate my soybeans?" The risk versus reward of inoculating soybeans has been discussed for decades. The risk is not lower yields, but rather whether the inoculant is going to return a profit,

How Inoculants Work

Nathan Mueller (left), Nebraska Extension cropping systems educator, talks with Market Journal host Jeff Wilkerson about the nitrogen needs of soybeans. Mueller says before growers decide to inoculate their soybeans, they should answer several questions. (Source: Market Journal)

The process of fixing nitrogen in the soil air to ammonia occurs through the symbiotic relationship between soybeans and the Bradyrhizobium species, a bacteria within the nodules of soybean roots. Soybeans get needed nitrogen and the bacteria get some carbohydrates in return. This process provides soybean plants 46%-74% of the N they need; the remaining amount is taken up from the soil (Salvagiotti 2008). A 70-bushel-per-acre soybean crop takes up roughly 330 lb N/acre in the aboveground portion of the plant, roughly equivalent to the nitrogen demand of a 245-bushel-per-acre corn crop.

Your soybean field likely fits into one of three situations:

1. No previous history of soybeans.
2. It has been more than three to five years since soybeans were grown or soybeans have been grown in rotation every two to three years, but an environmental factor (e.g., flood, drought) may have negatively impacted survival of bacteria. This would include documented N deficiency or poor nodulation in a previous soybean crop.
3. Soybeans have been grown in the rotation every two or three years and environmental conditions have been good.

In situations where soybeans have never been grown, yield increases of as much as 49 bushels per acre were measured in Nebraska fields with the addition of inoculant (Elmore 1984). However, more modest yield increases of 1 to 10 bushels per acre are expected. Yield effects are largely dependent on the nitrogen supply from soils and the relative increase in nitrogen fixation. For example, a 2009 Iowa study reported inoculation increased soybean yield 2 bushels per acre on land coming out of pasture (Ruiz Diaz 2009).

Factors Affecting Soybean Nodulation

If your field falls in the second situation, several environmental factors can cause poor nodulation, decrease nitrogen fixation, and reduce survival of these bacteria, including:

• Non-optimal soil pH. Low soil pH can decrease nodulation, nitrogen fixation, and survival of various strains.  Ideal pH is near 6.8. Soil pH less than 6.0 can decrease root hair modification needed for the formation of nodules (Duzan 2004). Therefore, following economic thresholds for lime application when soil pH decreases to 5.6-5.8 also manages soil bacteria.  High soil pH and salinity also decrease nodulation, nitrogen fixation, and survival of these bacteria.
• Floods and droughts. Fields flooded for more than one week decrease nitrogen fixation and can decrease survival of bacteria because bacteria need oxygen. However, two recent research studies suggest not to be overly confident in assuming a yield response to inoculants after floods (Furseth 2011, Wilson 2014). Sandy soils and droughts in years when soybeans are not being grown also can lower bacteria populations.
• Increasing number of years since soybeans were last planted. Most university guidelines suggest inoculating if it has been more than three to five years since soybeans were last grown in the field. These bacteria have been found in soils 30 years after being planted back to perennial grasses, just at a much lower population. Two more common situations in Nebraska where soybeans may benefit from inoculation are when soybeans are planted after continuous corn or after corn that followed four years of alfalfa—alfalfa utilizes a different bacterial species for successful inoculation. The probability of a yield response in fields with a recent history of soybeans is extremely low.  An eight-year study (2000-2008) testing 51 inoculant products in 73 experiments conducted in Wisconsin, Iowa, Indiana, Minnesota, and Nebraska resulted in average yield response of 0 bu/ac (De Bruin 2008).

If your field falls into the third situation described above and environmental factors (bulleted list) were not an issue, some still advocate for inoculating with more effective nitrogen-fixing strains of these bacteria over common indigenous strains, e.g., Bradyrhizobium japonicum Bj123 (Shiro 2013), in the Midwest even when environmental conditions have been conducive for nitrogen fixation. In the 2000 to 2008 study, new inoculant products and strains were used. Of the 10 most widely tested products, yields were not influenced in fields with a recent history of soybean. This is likely due to highly competitive indigenous strains diminishing potential gains from applying more effective strains.

Summary

If you have managed soil pH well, planted soybeans recently, have medium to heavy textured soils, and have not experienced extended droughts or floods, direct your money away from inoculating soybeans and move it to other management options that have an improved chance of return.

Resources

For more information on soybean inoculation, see these Nebraska Extension resources:

• Soybean Inoculation: Understanding the Soil and Plant Mechanisms Involved (NebGuide 1621)
• Soybean Inoculation: Applying the Facts to Your Fields (NebGuide 1622)

References //

De Bruin, J.L., P Pedersen, S.P. Conley, J.M. Gaska, S.L. Naeve, J.E. Kurle, R.W. Elmore, L.J. Giesler, and L.J. Abendroth. 2008. Probability of Yield Response to Inoculants in Fields with a History of Soybean. Crop Science 50:265-272.

Ruiz Diaz, D.A., P. Pedersen, and J.E. Sawyer. 2009. Soybean Response to Inoculation and Nitrogen Following Long-Term Grass Pasture. Crop Science 49:1058-1062. Crop Science 49:1058-1062.

Duzan, H.M., X. Zhou, A. Souleimanov, and D.L. Smith. 2004. Perception of Bradyrhizobium japonicum Nod factor by soybean [Glycine max (L.) Merr.[ root hair abiotic stress conditions. Journal of Experimental Botany 55:402: 2641-2646.

Elmore, R.W. 1984. Soybean Inoculation: When is it Necessary? NebGuide G84-737.

Furseth, B.J., S.P. Conley, and J.M. Ane. 2011. Soybean Response to Rhizobia on Previously Flooded Site in Southern Wisconsin. Agronomy Journal 103:573-576.

Shiro, S., S. Matsuura, Rina Saiki, G.C. Sigue, A. Yamamoto, Y. Umehara, M. Hayashi, and Y. Saeki. 2013. Genetic Diversity and Geographical Distribution of Indigenous Soybean-Nodulating Bradyrhizobia in the United States. Applied and Environmental Biology. 79:12:3610-3618.

Salvagiotti, F., K.G. Cassman. J.E. Specht, D.T. Walters, A. Weiss, and A. Dobermann. 2008. Nitrogen uptake fixation and response to fertilizer N in soybeans: A review. Field Crops Research 108:1-13.

Wilson, J. 2014. Returning Crop Production to Flooded Soils Following the 2011 Missouri River Flood. Nebraska Extension.