Research Highlights

Managing Potassium for More Profitable Soybean Production in Minnesota

By Barb Baylor Anderson

Soybeans traditionally remove higher quantities of potassium from the soil into their harvested grain than do other crops. This has become more apparent in Minnesota, where areas that have had greater availability of potassium in the past now show a faster rate of potassium decline in soil tests where soybeans are grown repeatedly. And that can negatively impact yield.

“Potassium is a major macronutrient, so a deficiency significantly affects yield,” confirms Dan Kaiser, University of Minnesota Extension soil fertility specialist and principal investigator of the Minnesota Soybean Research & Promotion Council funded research. “Because of this decline, I have gotten more questions about soybeans and potassium fertility. Farmers have to watch soil tests more closely now to ensure yield reductions are not occurring.”

“Agriculture is a constantly evolving industry and research is constantly needed to ensure American farms remain competitive and sustainable. In this era of change, it seems that every question answered leads to a dozen new questions. Our checkoff investment is crucial in helping researchers tackle problems as they arise and come up with solutions that benefit farmers and the industry partners that support them,” says Glen Groth, soybean farmer from Ridgeway, Minnesota, and director for the Minnesota Soybean Research & Promotion Council.

Kaiser’s ongoing research goals are to evaluate the long-term impact of potassium rate and timing in a corn/wheat–soybean rotation on soybean grain yield and quality and determine if the application of chloride (Cl) in potassium fertilizer has negative effects on soybean grain yield and quality. He also plans to evaluate the impact of macronutrients on distribution of essential amino acids in soybeans. The multi-year research project began in 2019.

“My research currently focuses on the optimal rate of potassium and the optimal timing to apply potassium in rotations containing soybeans,” says Kaiser. “There is interest from growers in applying potassium in front of every crop, so I want to help resolve that question.”

Kaiser additionally is taking a deeper look into decreased yield following potassium applications, including potash, to see how much of an impact it has on yield in northern growing regions. 

“Past research has shown yield decreases in single-year potassium fertility trials, but long-term trials are more advantageous as the impacts of weather can be assessed at each location on the build-up of chloride (Cl) in the soil,” he says. “The chloride issue is interesting to me.”

Kaiser explains that in the southern U.S., greater emphasis is placed on variety tolerance to chloride where chloride excluder varieties are selected to reduce the risk for yield loss where Cl additions are high. In more northern regions, almost all the varieties are considered chloride includers and can be susceptible to potential yield loss in the presence of high levels of chloride.  

“For the majority of soybean growers, this means about half to one bushel per acre reduction in yield if too much Cl is present, which in most cases they won’t notice,” he says. “However, it is a loss in productivity. I have seen greater yield loss, depending on the amount of chloride applied.”

For now, Kaiser encourages farmers to keep potash rates as low as possible if applying ahead of soybeans. He advises no more than 100 lbs. of actual potash per acre, but he hopes to research more into specific rate tolerance in the future. As for timing in a two-year, corn-soybean rotation where corn yield benefits when potassium is applied ahead of corn, a split application may benefit both crops. A two-year application of potash should not be made ahead of soybeans. 

“If a high rate needs to be applied, I would highly suggest applying the bulk ahead of the preceding crop. I am hoping to fine tune these numbers moving forward,” he says.

“When it comes to grain quality, further work needs to study macronutrient phosphorus, potassium and sulfur application impact on distribution of amino acids in the grain,” he adds. “Research has shown sulfur and potassium can impact cysteine and methionine content.”

Published: May 3, 2021