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Manitoba soybeans not responding to K?

Soybean potassium fertility trials leave researchers scratching their heads

While it has generally been known that Manitoba’s lighter-textured soils are low in potassium, it took soybeans to really bring the issue to light. With soybeans taking up more and more acres, researchers felt it was time to conduct potassium fertility trials. While preliminary results are in, results are mixed. More data will be needed to fully understand what’s going on in the field.

More than any other crop, soybeans remove high levels of potassium from the soil. John Heard, soil fertility extension specialist with Manitoba Agriculture, said each bushel of soybeans produced removes about 1.4 pounds of K2O. “So a 50 bu./ac. crop is removing 70 pounds of K2O/acre/year,” he said. “And with soybeans taking up a great proportion of the rotation, this can mean high removals in short rotations.”

Potassium deficiency

The visual symptoms of potassium deficiency in soybeans are hard to miss. In the early stages of growth the lower leaves will start to yellow before browning along the leaf margins. Symptoms will first appear at the base of the plant and will continue upwards as potassium moves up the plant to support new growth. Later in the season when the plant needs potassium for pod fill those visual symptoms will begin to appear on the upper leaves as well. Eventually, they will brown and fall to the ground.

Heard refers to this late season potassium deficiency as the “canola swath syndrome. “This is observed in late August in many fields with low potassium levels that follow a previous canola crop that was swathed and let sit through rains before combining,” he explained. “During that canola curing phase, rainfall can leach the potassium out of the canola straw into the soil below, so this soil under the swath is marginally more replenished in potassium than the areas between swaths.”

Currently, Manitoba Pulse & Soybean Grower on-farm tests and University of Manitoba studies are evaluating potassium rates and placements for soybeans. Greg Bartley, on-farm specialist with MPSG, shared the preliminary results.

The goal, said Bartley, was set up 16 on-farm trials per year, targeting low and marginally low soil test levels (<100 ppm and <150 ppm soil test K). “We’re trying to get a range of deficient soils to determine the critical soil test level, where soybeans are likely to respond to additional fertilizer,” he said.

Farmers participating in the on-farm trials have the choice of applying two different treatments, depending on their equipment and operations: 60 lb./ac. K2O band (side band, pre-plant band, mid-row band); and 120 ls/ac. K2O (broadcast and incorporated). All trials were spring applied. They began in 2017, and will be repeated in 2018.

In the early stages of soybean growth the lower leaves will start to yellow before browning along the leaf margins.
photo: Greg Bartley, MPSG

Data results

Although Bartley is able to share the results of the plot trials, he wants to make clear that this is preliminary data and he’s not yet ready to make recommendations.

In terms of yield results, of the 14 successful on-farm trials, two responded positively to potash, and two responded negatively to potash. “It’s those negative ones that we’re scratching our heads about,” said Bartley.

During the course of the on-farm trials, in-season data was collected as well. Around mid-season, they evaluated paired soil and plant samples to see potassium uptake. That data is still being analyzed.

The University of Manitoba trials took a more detailed approach, comparing multiple rates and placement in four same small-plot trials located in some of the on-farm trial producer fields. They looking at 30 and 60 lb./ac. sidebanded, and 30, 60 and 120 lb./ac. broadcast and incorporated. “They found no yield response in these trials,” said Bartley. “Yeah, we were scratching our head on that one as well.”

“It’s not what we were expecting at all, especially with these small-plot trials,” Bartley continued. “One of these sites had less than 50 ppm soil test K. We saw visual potassium deficiency symptoms within the trial, but when we harvested the trial, there was no yield difference between treatments.”

While the researchers involved have a few different theories, they look forward to gathering more data in 2018. Once ready, project results will be available at the MPSG’s website.

About the author

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Melanie Epp is a freelance farm writer.

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