Research Highlights

Soybean Research Works Toward Improved Potassium Soil Test and Recommendations

By Laura Temple

Soybeans use potassium (K) to produce biomass and yield, and it impacts stress tolerance. Researchers continue to improve understanding of the uptake and use of this macronutrient in soybeans. With high fertilizer prices and tight margins, data on nutrient use can help farmers make fertilizer decisions to optimize yield and profitability.

“Soil testing routinely measures plant-available K,” says Dr. Bhupinder Farmaha, assistant professor and nutrient management specialist with Clemson University. “However, researchers focusing on nutrients agree that we need a better soil test to measure K availability. We have much to learn about how it interacts with different soils and crops like soybeans.”

To validate or recalibrate K soil tests and recommendations, the South Carolina Soybean Board is investing checkoff funds in research led by Farmaha that includes on-farm trials. The study started in 2021, and Farmaha hopes to continue it until clear cumulative and residual effects of K fertilization emerge.

“During wet summers, we see a dip in K-availability levels according to soil tests,” he says. “And in dry summers, we are likely to see higher K-availability levels. The change in K availability with varied soil moisture content makes it difficult to accurately predict K need and use. This warrants an improvement in soil K testing and providing better K fertilizer recommendations.”

Involving Farmers

The research started with trials at five on-farm locations and two research stations, all in fields that will use a soybean-corn rotation throughout the study. The locations provide a variety of soil types and production practices, including dryland and irrigation systems.

“On-farm trials provide more standard conditions than those at research stations,” Farmaha notes. “My team expects on-farm data to be more relevant to farmers as they look for ways to maximize the value of inputs, limit the cost of lost nutrients and achieve profitable yields.”

Jonathan Croft, county agriculture agent for Clemson Cooperative Extension, agrees. He has been instrumental in coordinating some of these trials. He notes that farmer participation helps effectively share research results.

“Farmers tend to have more confidence in larger-scale trials, compared to the small-plot work we do at research farms,” Croft says. “They want to see and try new things that can help improve efficiency, like potential changes in potassium fertilizer use that can either save costs or improve yields.”

And farmers become invested in the research, says Racheal Sharp, who farms with her father near Allendale, South Carolina, and serves as a South Carolina Soybean Board director. They hosted both dryland and irrigated K trials on their farm, and they are willing to continue the research.

“The data from on-farm trials is worth the effort, especially with high fertilizer prices,” Sharp says. “When we compared our soil sample results to those taken for the trials, they were very similar. That’s why I feel the trial results are accurate, and will continue to participate. We need more than one year of data, but we are learning new and valuable information.”

Surprising Soybean Yield Responses

“In research I’ve conducted in the past and in results from many other studies, an increase in the addition of K has caused a linear decrease in soybean yields,” Farmaha explains. “That is not expected.”

He says researchers in the past have attributed some of that to a salt effect because K fertilizer is often delivered as potassium chloride (KCl), a salt. However, soybeans in the study rarely show signs of salt damage. To test these concepts, his 2017 and 2018 station trials used KCl and non-chloride forms of K fertilizer. Different fertilizer sources had the same effect — higher K applications correlated with yield decreases. Farmaha currently hypothesizes that a balance between K and other soil cations might contribute to the unexpected results.

Results from 2021 on-farm trials show that K was not a limiting factor to optimize soybean yield at any site. In other words, there was no increase in soybean yield with K applications. This is an important finding from farmers’ perspectives, especially those who add K fertilizer as insurance, expecting an increase in yield.

Digging Deeper into K Use

Farmaha’s study is designed to improve understanding of K. Before and after each growing season, his team is taking soil samples at three different depths, up to 2 feet. The team will analyze these samples for K content both in plant-available and unavailable forms.

“These tests will help us understand K distribution in the soil,” he says. “We will learn about stratification, or if it is found in specific layers of the soil. And we will look at its fractionation, or the different forms of K in a variety of soil types and production systems.”

The team also monitored plant stand and height. They took plant tissue samples during soybean vegetative growth. This data will be correlated with K application rates and yield to better understand how soybeans absorb and use the nutrient as they grow.

“In addition to gathering data, the students on the team are getting valuable experience working with farmers,” Farmaha adds. “They are developing Extension skills that will prepare them to continue serving farmers throughout their careers.”

His plans for 2022 trials include repeating the trials in corn at the same sites and adding new trial locations to capture more soil types and diversity in testing environments. His study design includes some plots at each location will receive K annually, while others will receive K applications every other year.

“This approach will help us learn about the cumulative and residual effects of K fertilizer in a soybean–corn rotation,” he says. “As we gather data over time, we should learn how much K crops need, without over-fertilizing to decrease yields and allow more nutrients to leach.”

Published: Apr 4, 2022