Research Highlights
Research Reveals Potassium Fertilizer Response in Oklahoma Soybeans

Soybeans with potassium deficiency show symptoms of yellowing leaves, which is also a symptom of other issues. Soil testing for nutrients can help to determine possible problems. Photo: Brian Arnall

By Carol Brown

Traditionally, if a farmer tests a crop field and finds low nutrient levels and then applies additional nutrients, the crop should respond favorably. But an Oklahoma research project that looked at nutrient management in soybeans showed this was not always the case.

Brian Arnall, a plant and soil sciences professor at Oklahoma State University, has been studying precision crop management throughout his career. His research with soybean nutrient management has led to several more projects because of the answers he was — or wasn’t — finding.

“In a previous study we were looking at whether farmers were effectively managing their nutrient strategies,” Arnall says. “Overall, in soybeans, potassium was the most limiting nutrient. There was no response in nitrogen or sulfur and very little from phosphorus; it was all potassium.”

In that project, funded by the Oklahoma Soybean Board, Arnall and his research team had added strips of nitrogen, phosphorus, potassium, and sulfur in double-cropped soybean fields on top of producers’ nutrient management to see what responses they would see. 

“Because of the potassium response in this earlier study, we wanted to look at whether Oklahoma state recommended application rates were effective for in-season dry and foliar potassium fertilizer,” he says.

This study, also supported by Oklahoma Soybean Board, is focused on potassium management in full-season soybeans over three years. The project was challenging, says Arnall, as they traversed the state, running soil tests in fields for low potassium levels and establishing research plots. 

“The over-arching problem we found was that we didn’t always get a response from added potassium fertilizer, even when soil tests showed low levels. In nine locations across the state, only three sites responded to K, and based on the soil tests, they should have responded.”

Arnall says one year showed benefits and one year did not, and one year the responses were split between pre-plant and in-season applications. The rationale was rainfall, which was a positive but confounding result. Arnall says that if he could have predicted the rain, he could have also predicted the best K application timings.

“One result from this study revealed that foliar application of potassium was of no value,” says Arnall. “We could never put enough K on through foliar application to impact the crop. Farmers have had a lot of interest in foliar application; so, this was one solid outcome.” 

As in many research studies, outcomes lead to new questions and Arnall’s work was no different. These studies have led him to more closely look at potassium management in soybeans. 

“The first goal is to do an in-depth investigation across farms in the state on how to better predict fields that will be responsive to potassium,” Arnall says. “We’re establishing field-length strips of K to try to answer the question, ‘Can we predict when and where soybeans will respond to K fertilizer?’ Before I can figure out how much soybeans need, I need to know how much they are going to respond.”

The study will be looking at soybean rooting zones and the plant’s nutrient uptake. Soybean is similar to cotton where the uptake of K is high during a small window of time. Arnall believes the response is soil-related as well as limited soybean root zones. The study will be looking at conditions in the root zones that trigger potassium deficiency. 

Arnall says that he and his team will be going out to the fields and using every sensor, deep probe, and soil and chemical measurement they can acquire to answer these questions.

Not just one additional study emerged from Arnall’s previous nutrient work. He’s working on another research project that stemmed from these previous outcomes. Another study, funded through a USDA National Institute of Food and Agriculture (NIFA) federal grant, focuses on fertilizer application type and timing in double-cropped soybeans.

“One-third to half of Oklahoma’s soybeans are planted in June after a wheat harvest. We are looking at managing nutrients in this situation,” he says. “There are several options: adding all the phosphorus (P) and K in the fall for the wheat and the soybean, top-dress in January or February with the P and K for the soybean needs, or add fertilizer after the wheat is harvested prior to soybean planting.”

Even though Arnall’s research on potassium management has led to bigger questions, he still has some recommendations for farmers. His body of work is in precision nutrient management and his hope is to increase farmer usage of on-farm test strips. 

“If you don’t know whether you should be applying potassium or how much to apply, add a test strip. If you are already applying K, turn it off for 30 or 40 feet,” Arnall recommends. “Potassium is very environment-based and site-significant. Let those strips tell you these answers for your particular fields.”

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.