Rhizobial inoculants crucial for soybeans planted into previously flooded fields

Ames, Iowa – March 26, 2009 – Because of severe flooding that inundated millions of acres of Midwestern farmland last season, agronomic experts are predicting reduced rhizobia counts in these soils in 2009. Without inoculating these fields, they say, soybean farmers could be sacrificing significant yield and profit potential. Soybean seed planted in fields that undergo any period of flooding, or extended waterlogged conditions, prior to planting this spring also should be inoculated with fresh rhizobia bacteria to reestablish populations and improve yield potential.

Flooding, or extended periods of soil saturation, result in anaerobic conditions in the soil that can kill native rhizobia. With soybean production projected to reach as much as 80 million acres in 2009, the use of a rhizobial inoculant is especially recommended for adequate nodulation of soybeans planted into fields that have experienced these conditions, the experts say.

Based on our experience, saturation and flooding for as little as two to five days will create an anaerobic soil environment that will kill most rhizobia,” says Jim Beuerlein, professor of agronomy and soybean research and extension specialist for The Ohio State University. Beuerlein has evaluated inoculants in hundreds of field trials for the past 14 growing seasons.

“Over time, these flood-damaged fields will gradually become reinoculated through small amounts of soil moved by wind, water, machinery, insects and animals. Still, I would not advise growers to rely on Mother Nature to solve this problem. My recommendation is that they take matters into their own hands and inoculate their soybeans with one of the highly effective and inexpensive rhizobial products that are available to them,” Beuerlein notes.

Palle Pedersen, Iowa State University Extension soybean agronomist, says it is good insurance to inoculate a field that has been flooded for more than a week. On the Iowa State University Soybean Extension and Research Program Web site, he also recommends inoculation if the field has never been planted to soybeans, if soybeans have not been grown in the field in the past three to five years, if the soil pH is below 6.0 or if the soil has a high sand content.

Charlie Hale, U.S. inoculant product manager for Becker Underwood, which produces VAULT® inoculants, points out university studies and independent trials, on average, have shown a yield benefit from using inoculants of approximately two bushels per acre. But, the yield benefit often is greater in fields where soybeans have not been planted in several years, such as occurs with continuous corn or CRP acres and on soils that have experienced flooding and the accompanying loss of native rhizobia in the soil, he adds.