Soybean acreage has been projected to increase in 2022 in Louisiana as well as in several other states due, in part, to the high price of fertilizer. Since both corn and cotton require more fertility compared to soybean (does not require N) to maximize yield, many producers will rotate their continuous corn and cotton fields with soybean or plant soybean after soybean.
It is also possible producers will plant into marginal land that has not been profitable in recent years or pasture acres may be converted to row crops.
If the field has not been planted to soybean over the previous three to five years, it is recommended to inoculate the seed with Bradyrhizobium japonicum bacteria. The bacteria and the soybean plants will form a symbiotic relationship to convert atmospheric nitrogen gas (N2) into ammonium (NH4+).
This nitrogen fixation process, that is common among different legumes, converts a form of nitrogen that is unavailable to plants (N2) into plant available nitrogen (NH4+). Further, many commercially available Rhizobia inoculants may have higher nitrogen fixation potential.
Multiple studies conducted at the Red River Research station showed that use of inoculant even in the fields that had soybean grown in the last three years had a 3 – 7% increase in yield. When inoculating the seed of a legume plant, it is important to distinguish between the different types of Rhizobia bacteria that is required by the specific legume.
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For example, Bradyrhizobium japonicum is appropriate for soybean, but Rhizobium leguminosarum biovar trifolii and Sinorhizobium meliloti is appropriate for a clover and alfalfa mix. Therefore, although alfalfa and clover are legumes, the appropriate bacteria may not be present in the soil to form the symbiotic relationship with soybean plants if the clover and alfalfa field has not been planted to soybean in the previous three to five years.
More information on types of inoculants can be found at this Penn State Extension publication.
If the soil has low pH (acidic soils), the nitrogen fixation process can be limited due to a low availability of molybdenum (Mo). A Mo trial was conducted in a Louisiana acidic soil (pH 5.6 to 6.2) in 2018 and 2019. The results of the LSU AgCenter trial indicated applying Mo in acidic soils can significantly increase soybean yield from 6.9 to 17.4%.
If a Mo treatment is required, it is important to remember that Mo should not be combined with Bradyrhizobium japonicum inoculum, unless they are combined immediately prior to planting. The Mo treatment can desiccate and decrease number of viable bacteria. For more information, read “Nitrogen Fixation in Soybeans” in the Louisiana Crops Newsletter Volume 10, Issue 4 – May 2020.