By Dr. Laura Lindsey and Stephanie Karhoff, OSU Extension; also Sunjeong Park, Chunxue Cao, and Brian B. McSpadden Gardener, Department of Plant Pathology, The Ohio State University

Following wet weather conditions and fallow fields, some producers are wondering if they need to inoculate their soybean seed with Rhizobia.

Soybean plants have a symbiotic relationship with bacteria in which the bacteria fix nitrogen from the atmosphere into a plant-available form of nitrogen. In soybean, nitrogen fixation is associated with Bradyrhizobium japonicum (commonly referred to as just Rhizobia).

Stephanie Karhoff, OSU Extension Educator, Williams County

Generally, fields with a history of soybean production have an adequate population density of Bradyrhizobium japonicum. In our research trials, we have measured a yield increase of approximately 1.5 to 2.0 bu/acre when soybean seed is inoculated and the field has a history of soybean production. However, statistically, this is only at the 70% confidence level (e.g., I’m 70% confident there is a 1.5 to 2.0 bu/acre yield increase when soybean seed is inoculated when the field has a history of soybean production.)

What about fields that were flooded? In Wisconsin, researchers examined Rhizobia populations and effect of inoculant following flooded field conditions the previous year. Trials were conducted in three field locations that were flooded for at least three weeks. Soybean yield was not influenced by inoculant (four inoculant products tested). Even following a flood, Rhizobia populations were adequate for crop growth. However, if large amounts of soil or plant residue from an unknown origin were deposited in the field, Rhizobia inoculation may be necessary.

What about fields that were never planted? When fields remain unplanted, there may be a decline in beneficial mycorrhizal fungi, which is commonly referred to as “fallow field syndrome.” Keep in mind Bradyrhizobium japonicum are bacteria, not fungi. There is very limited information on the effect of fallow fields on Rhizobia populations. However, if soybean were planted sometime during the past three years, there should be an adequate Rhizobia population.

If you are concerned about having adequate Rhizobia populations in your fields, inoculant is a relatively cheap insurance.

What about if the soybean production is certified organic? How does that impact Rhizobia inoculation decisions?

Some non-seed inputs can be used in organic agriculture to ensure and/or improve crop productivity. Once a grower is certified, they need to comply with the regulations described in the Organic Foods Production Act and must use products that meet the requirements of USDA’s National Organic Program (NOP).

There is a wide range of available inoculants and soil amendments that can be applied to change soil properties and to improve plant growth. Specific microbial inoculants also are available to improve soil nutrients for plants and to reduce disease pressure.

A listing of commercially available products certified for use in organic agriculture by the Organic Materials Review Institute (OMRI) can be found online. Many additional products currently on the market may be acceptable to individual certifiers. Organic growers in Ohio are encouraged to check with OEFFA’s approved product list as they plan disease management strategies and prepare their organic management plan.

Rhizobia are soil bacteria that form symbiotic relations with plants (mostly with legumes). They belong primarily to the genera Rhizobium, Azorhizobium, Bradyrhizobium, and Sinorhizobium. Rhizobia first invade plant roots and form nodules in which they convert atmospheric nitrogen into plant-accessible forms of nitrogen.

Responses to inoculation are greatest on land during the initial plantings of a particular legume species. Eenhanced yield responses have been seen with regular inoculation as well. Rhizobia and associative nitrogen fixers can provide substantial nitrogen to plants and soil without additional nitrogen fertilizer input.

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