Stressful Planting Conditions Increase Need For Soybean Inoculants

Ames, Iowa – Extremely stressful weather conditions throughout much of the country are impacting soybean producers’ fields this spring. Some regions have endured flooding or unusually wet fields; others are experiencing below-average temperatures; still other fields are abnormally dry. As a result, soybean growers’ may experience a silent, but potentially expensive yield-limiting condition — a significant reduction in rhizobia bacteria and the nitrogen-fixing potential of their soybean fields.

Rhizobia are soil bacteria that, after becoming established inside the root nodules, enable soybeans and other legumes to fix nitrogen from the atmosphere. “Waterlogged or flooded fields are particularly vulnerable to rhizobia loss. Flooding, standing water and even ‘super saturation’ of fields can create a soil environment which reduces the oxygen content to the point that most rhizobia can die in as little as two to five days,” says Dr. Jim Beuerlein, professor emeritus in agronomy extension at The Ohio State University and technical advisor to Becker Underwood.

While research is limited on just what the optimum soil moisture level for rhizobia is, research has shown rhizobia survival also drops as soil dries out, especially if soil moisture is below 50 percent of field capacity. “It’s a question of whether there are sufficient numbers of effective rhizobia for root nodule formation and nitrogen-fixation to occur.”

Rhizobia, just like yeast, vaccines and other living organisms, can lose their viability over time says Beuerlein. “Even if rhizobia do manage to survive stressful conditions, that doesn’t mean they’re the best nitrogen producers. Soybean growers really need to make sure the seed is surrounded with fresh, active rhizobia to ensure the best nodulation and nitrogen-fixation potential.”

Spring planting in the eastern and southern areas of the Soybean Belt has been delayed by more than two weeks due to frequent April rains and producers in the northern portion are facing perhaps the latest planting season on record. The North Dakota Agricultural Statistics Service now estimates the state’s spring planting season could be the latest in recorded history due to late season snow and cool temperatures. In the far south and southwestern portions of the U.S. soybean production area, producers are experiencing abnormally dry to moderate drought conditions.

Soybeans can require more than 300 pounds of nitrogen per acre and most of it is needed during the initial flowering through seed-set stages. During pod fill, the rhizobia can be producing three pounds or more of nitrogen per day per acre.

“Cold soils, flooding, drought and low soil pH will stress rhizobia and soybean seedlings and interfere with the plant’s ability to deliver enough nitrogen for high yields,” explains Beuerlein. “The longer soil stays cool, the longer the delay in recognition of both the plant and rhizobia signals. The plants and rhizobia have to produce more signals in order to begin nodulation, which delays the start of nitrogen fixation.”

Once those signals are received, and the nodules develop and are occupied by rhizobia, nitrogen can be produced and provided to the crop. Improved nodulation and nutrient uptake means enhanced plant growth, more vigorous root structure, increased nitrogen-fixing root nodules and greener plant foliage. It also means more nodes, blooms and pods can form as the plant develops. The combination of these factors translates into more yield and profit potential for soybean growers.

“Applying a multi-action growth-enhancing inoculant to soybean seed before planting gives a producer a competitive advantage over one who doesn’t plant inoculated seed.” says Becker Underwood’s Kurt Seevers, field development specialist. Seevers notes that long-term research by Beuerlein and others at Ohio State University shows the use of soybean inoculants can increase soybean yields at least two bushels per acre, across all soil and weather conditions. “While that may not seem like a large percentage increase, with soybean prices at $13 per bushel that’s still a very ‘economically’ significant return on investment to a producer worried about reduced yield potential.”

Producers who don’t inoculate soybean seed, Seevers says, could lose significant yield and profit potential. Inoculating soybean seed with fresh rhizobia bacteria before planting will help ensure the reestablishment of fresh, vigorous rhizobia populations in the soil, “There’s really only one way to immediately overcome the loss of native rhizobia, improve nitrogen fixation and boost potential yields and that’s to use an inoculant with a guaranteed high rhizobia count,” says Seevers.