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When to inoculate bean seed

Considering last fall’s wet weather and the soggy fields this spring, farmers wonder about using inoculants on soybean seed in 2010. Should you use a rhizobium inoculant? Or, is there enough of the nitrogen-fixing bacteria, Bradyrhizobia japonicum, remaining in the soil?

True, rhizobial bacteria don’t survive wet conditions very well. But whether it will pay to use an inoculant depends largely on whether soybeans were grown in the field sometime during the last five years or if the field has been flooded for more than a week, says Aaron Saeugling, Iowa State University Extension field agronomist in southwest Iowa.

The presence of rhizobia in the soil is necessary for a legume such as soybeans to be able to convert nitrogen from the atmosphere into a form usable by the plant, he says. This process, nitrogen fixation, is critical for producing high soybean yields. For nitrogen fixation to occur, the nitrogen-fixing bacteria need to be readily available in the soil or must be applied to the seed.

When soybean seed germinates, the rhizobia bacteria invade the root hairs of the seedling and begin to multiply. Nodules, the structures that house the bacteria, form on the roots. Under field conditions, nodule formation can be seen shortly after emergence, but active nodule fixation of N from the air doesn’t begin until about the V2 to V3 growth stage of beans. After that point, the number of nodules formed and amount of nitrogen fixed increase with time until about midway between the R5 and R6 growth stages; then N fixation decreases sharply.

Beans in most rotations

Applying nitrogen fertilizer for soybeans isn’t recommended because it typically doesn’t increase bean yields in Iowa production systems. The total number of nodules that form decreases with increasing amounts of applied N.

Today, most fields in the Midwest have had soybeans grown on them in the crop rotation, likely increasing population density of Bradyrhizobia bacteria in the soil, notes Saeugling. Improved inoculant technology, coupled with higher commodity prices, ease of application and relatively low-cost inoculant products, have many growers reconsidering the use of inoculants.

Since 2003, ISU agronomists have conducted trials each year to assess new inoculants on the market. Overall, the ISU studies haven’t shown a consistent yield response to use of soybean inoculants in fields in Iowa with a history of having soybeans grown on them.

Current recommendations for Iowa are to inoculate the seed if a field has never been planted to soybeans, or if beans haven’t been grown in the field for the past three to five years. Other situations where it’s recommended to inoculate are if soil pH is below 6.0, or if a field has sandy soil, or if a field has been flooded for more than a week, creating anaerobic conditions.

Deciding to inoculate depends on whether a field has a recent history of healthy-looking soybeans. “Most soils in Iowa have a good population of rhizobium bacteria if the field has grown beans recently,” says Saeugling. “But if a field is new to soybeans or has been out of soybeans for more than three to five years, it’s good insurance to inoculate.”

Rising cost of production and volatile commodity prices are driving the need for soybean growers to improve profits. But the use of inoculants to increase yield may not be the answer, especially if beans have been grown in the field recently. In fact, using inoculants on soybeans without an active on-farm testing program could be costing you money. But if a field hasn’t produced soybeans in the past three to five years or has never produced soybeans, an inoculant is needed for nitrogen fixation to occur.

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This article published in the April, 2010 edition of WALLACES FARMER.

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