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Research Highlights
Evaluating P and K Impact on Minnesota Wheat-Soybean Yields

A terregator applies fertilizer for Minnesota P and K trials.

By Barb Baylor Anderson

Soybean acres are rising in northwestern Minnesota. But the average soybean yield is not. 

Meanwhile, spring wheat yields in the region have climbed nearly 20 bushels per acre during the last decade. Given greater wheat yields and at least the potential for better soybean yields, researchers are looking at whether phosphorus (P) and potassium (K) may be limiting factors for plant growth and development as well as yield in the wheat-soybean crop rotation.

As background for the research, a 2017 AGVISE soil survey in northwest Minnesota suggested more than two-thirds of soil samples for P and more than one-third of samples for K may have fertility levels that limit the production of high-yielding soybeans and wheat. 

With funding from the Minnesota Soybean Research & Promotion Council and Minnesota Wheat Council, principal investigator Melissa Carlson with the Minnesota Wheat On-Farm Research Network and Dave Grafstrom with the University of Minnesota Magnusson Research Farm, have been evaluating enhanced P and K fertility through a combination of small-plot, replicated research and large on-farm trials. They are trying to determine if P and K levels are limiting soybean and wheat yields. They also are confirming whether existing P and K recommendations provide adequate fertility in a high-yield, wheat-soybean crop rotation. 

“We are currently in year three of a four-year project to evaluate elevated P and K levels,” says Grafstrom, who is using 50 units of P and K in the large on-farm trials and up to 100 units of each in the small-plot replicated trials to see how that influences growth and development. 

So far, preliminary results in the small-plot replicated trials suggest a higher wheat and soybean yield response with elevated levels of P and K while results in the large-plot replicated trials show a positive response in about 25 percent of the research locations.

“This suggests that growers should be on the upper end of P and K fertility if their goals are top-end yields. Growers should strive for soil test levels in the medium-high to high ranges for both P and K to maximize plant growth, development and yields,” he says. “It may take several years to build soil test levels, but growers should use regular soil testing to monitor progress.”

Once P and K levels are where they need to be for high-yield soybean and wheat production, Grafstrom says a micronutrient analysis should be the next step to make sure plant nutrition is not the limiting factor in a high-yield wheat and soybean rotation. 

“After year four of this project, we will do a complete soil test analysis to determine interaction, if any, with elevated levels of P and K on the soil micronutrient complex,” he says.        

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.