Poor nitrogen uptake has soybean growers pondering rescue application

Lack of moisture has also meant lack of nodules and poor nitrogen fixation for some soybean fields, leading some to consider a mid-season fertilizer pass

More soybean fields are trying to shake off the impact of a dry spring on nitrogen uptake this year.

Laryssa Stevenson, western production specialist with the Manitoba Pulse and Soybean Growers, told a SMART field day here last week that she has heard of more growers struggling with nitrogen deficiency this summer, which she blames partially on inoculant failure.

She said inoculant may desiccate in dry conditions, and Manitoba’s extremely dry spring this year may have left crops without the bacterial relationship that fosters nodulation and jump-starts nitrogen fixation.

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“From what I’ve seen, it’s been on the fields that haven’t had soybeans in previous years where we’re dealing with maybe five to six nodules on a plant versus 10 to 15 that we would probably feel more safe about,” Stevenson said. “We’re in that kind of unsure zone, even on fields where we double inoculated. I wouldn’t say it’s a complete nodulation failure, but just maybe reduced efficacy of the inoculant.”

Long-standing soybean growers may have a leg up on newcomers who don’t have the buffer of past inoculations.

MPSG agronomists typically advise double inoculation (and a mix of types for extra insurance) if farmers have not grown at least two soybean crops, and one within four years, on a given field. The same guidelines suggest single inoculation for years afterward, assuming there is no flooding or drought to kill the introduced bacteria and crops are nodulating well.

The University of Manitoba’s Don Flaten says he has noted the inoculation problem in one of his soybean trials this year.

The researcher said his team turned to a last-minute nitrogen application after their liquid inoculant failed in dry, sandy soil at one site.

The weather gave a “double whammy” this year between early-season impacts on inoculation and less nitrogen produced from the nodules that did form due to the persistent dry, Stevenson added.

“Drought conditions will reduce nitrogen fixation capacity as well,” she said. “When the plant is stressed from drought conditions, it won’t be photosynthesizing to its full potential, so it will have less energy to contribute to nitrogen fixation. It won’t affect nodulation, probably, but it will affect nitrogen fixation.”

Rescue application

Farmers fighting nitrogen deficiency may be putting their hopes on a later-season shot of the nutrient.

Producers in Manitoba have typically been told to apply 50-100 pounds per acre of nitrogen for a rescue application, advice that was put to the test by Manitoba Agriculture crop nutrition expert John Heard, MPSG and the University of Manitoba in 2014.

The three-site study found a “consistent but slight” yield bump (anywhere from four to eight bushels an acre) from the added nitrogen, although there was little difference between the full 100-pound-per-acre rate and plots that saw half the product.

The same study suggested that lack of rainfall in the 19 days after application might have affected results, since nitrogen might have been stranded on the surface and lost.

MPSG usually opts for the low end of the recommended range, although Stevenson said that already-deficient soil might merit the full 100-pound-per-acre application.

“Our recommendation is to go on with 50 pounds of nitrogen per acre,” she said, “but that’s really assuming that the plant has had enough nitrogen up until that pod-fill stage.”

She cautioned, however, that the benefit of rescue nitrogen might not outweigh the cost unless there are fewer than five nodules per plant. Rescue application cost is worth about five bushels an acre, she estimated, pointing to product cost, equipment and yield loss from tramping down the crop.

“If you’re between five and 10 (nodules,) I’d be hesitant in pulling the trigger just because rescue applications cost a fair bit,” she said. “Our recommendation is to go in with a product that you can direct below the canopy, so usually that means at this point in the season it’s going to be urea being floated on. If you use UAN at this point, the canopy’s already closed and you’re going to cause leaf burn.”

MPSG also advises farmers to pull plants from at least five points in the field when testing for nodulation, since deficiency can be patchy.

Flaten also warned that it might be too late for a rescue application.

Recent heat, “has been moving all the crops along at an amazingly fast rate,” he said, and soybeans might be past the R2 to R3 stage commonly tagged as the window for a late nitrogen pass.

About the author

Reporter

Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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