Research Highlights

Finding and Feeding Hidden Hunger for Potassium

In this article, you’ll find details on:

• Soybeans can suffer from mild to moderate potassium deficiencies that cost yield without showing any visual symptoms.
• Developing and verifying dynamic critical concentration curves for potassium in soybeans during reproductive stages could improve the effectiveness of tissue tests at identifying potassium deficiencies.
• In-season potash applications have potential to correct potassium deficiencies and prevent yield loss.

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By Laura Temple

Potassium (K) helps soybeans fill pods, fight diseases, regulate water and more. Thus, K deficiencies make soybeans more susceptible to diseases, drought and other stress. Availability of K during reproductive growth stages allows plants to maximize yield potential.

“During my doctoral research, I focused on in-season potassium management in Arkansas soybeans, because trials showed that a deficiency can cause as much as a 40% yield loss,” says Carrie Ortel, assistant professor and extension soybean agronomist for Virginia Tech. 

She aims to learn if farmers can use tissue tests and in-season K applications to prevent soybean yield loss from K deficiencies in Virginia. Then, if cost, availability or timing forces farmers to back off on spring or fall fertilizer applications, they could determine if soybeans have the needed K and adjust as needed in-season without sacrificing yield.

The Virginia Soybean Board invested checkoff support into Ortel’s research, which builds on her doctoral work led by the University of Arkansas’s Trent Roberts, professor of soil fertility and soil testing at the University of Arkansas, as well as Nathan Slaton, associate vice president for agriculture and assistant director of the Arkansas Agricultural Experiment Station.

“We found that soybeans can display no visual symptoms for mild to moderate potassium deficiencies that impact plant growth and yield,” Ortel says. “With this continuing research, I hope to more efficiently identify and address this hidden hunger in expanded management systems.”

Analyzing Tissue Tests

Ortel explains that the amount of K needed in soybeans varies based on the growth stage. The critical concentration of a nutrient is the amount the plant needs to fulfill all functions at that stage. 

Slaton’s research in Arkansas identified the changing critical concentration of K that irrigated soybeans require as they mature. The development of these dynamic critical-K concentration curves for soybeans help predict when lack of K affects yield at any point in their reproductive growth stages.

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Best Practices for Tissue Tests

• Take leaf tissue samples from the uppermost fully expanded trifoliate on the soybean plant.
• Follow guidelines of the lab that will process the samples when collecting samples, to know if the leaf petiole should be included or not.
• Don’t pull samples in extremely dry weather, as K may be available, but soybeans can’t take it up without water.

Source: Carrie Ortel, Virginia Tech Extension

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“Most universities have data confirming potassium needs in soybeans at the R2 growth stage,” Ortel says. “The dynamic critical concentration curve identifies potassium needs in days after R1, so it provides a threshold for tissue tests taken at any point.”

Her Virginia trials aim to see how well the Arkansas concentration curves translate to dryland and irrigated soybeans. In 2024, six trial locations were split between irrigated and dryland soybean production, at both research stations and in farmers’ fields. The tissue tests aligned well with the dynamic critical concentration curves developed in Arkansas, and they showed very little difference between irrigated and dryland soybeans.

“I am optimistic that with additional data over more growing seasons, the work done in Arkansas will translate well to Virginia,” she says.

Adding Potassium In-Season

The 2024 trials revealed mild to moderate K deficiencies, or hidden hunger, in some soybeans. The second goal is to determine if such deficiencies can be corrected in-season to prevent yield loss. Ortel compared the use of both potash and foliar K sources.

“Foliar fertilizer applications don’t appear to provide enough potassium to make a difference,” she reports. “For macronutrient deficiencies like potassium, soybeans need a granular fertilizer source.”

Soybeans also need water, either rain or irrigation, to make the K from potash available to the plants.

“Slaton identified a window of 20 to 45 days after reaching R1 to correct potassium deficiencies with minimum yield loss,” Ortel explains. “The more severe the deficiency, the shorter the window to act.”

She adds that any K needs to be available to the crop before the R5 stage, when seed development begins.

Though more data from various environments is needed, she believes this research could support more cost-effective K decisions. She is conducting trials at five Virginia locations in 2025 and plans to continue the trials for at least another year.

In addition, the Science for Success partnership of soybean extension specialists selected this research as the shared project for 2025 and 2026. Researchers in 13 other states have trials using  Ortel’s in-season K management protocol.

“I am excited to see this work expanded to a larger scale,” she says. “In just a couple years, we will learn how a dynamic concentration curve translates across very different environments and maturity groups. And, we will have much more data about when and how in-season potassium applications can address hidden hunger.”

Additional Resources:

Potassium (K) Deficiency of Soybean – Crop Protection Network article

In-Season Potassium Management in Arkansas Soybean – University of Arkansas fact sheet

Soybean Research Works Toward Improved Potassium Soil Test and Recommendations – SRIN article

Soil Fungi Support Potassium Uptake – SRIN article

Science for Success: In-Season Potassium Management – Science for Success project description

Meet the researcher: Carrie Ortel

Published: May 12, 2025