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Manage soybean’s ‘hidden hunger’ for potassiumManage soybean’s ‘hidden hunger’ for potassium

Carrie Ortel, Virginia Tech Extension’s new soybean specialist, said farmers can’t rely on visual symptoms to manage potassium. They need to apply the right amount of potassium at the right rate and right time before the soybean plant’s “stomach begins to growl.”

John Hart, Associate Editor

January 27, 2025

4 Min Read
Hidden Hunger Potassium soybeans
Do these soybean plants show signs of hidden hunger? Carrie Ortell

Farmers concerned about potassium deficiency in their soybeans need to look beyond the visual sign of yellowing leaves to determine their “hidden hunger.”

Carrie Ortel, Virginia Tech Extension’s  new soybean specialist, said farmers can’t rely on visual symptoms to manage potassium. They need to apply the right amount of potassium at the right rate and right time before the soybean plant’s “stomach begins to growl.”

“By the time we see these visual symptoms, this is going to be a severe deficiency,” Ortel said at the South  Carolina Corn and Soybean Growers meeting Dec. 17. “We want to be addressing these issues before our soybean crop starts feeling wimpy, angry, irritable. We want to give it what it needs before it hits that severe deficiency of those visual symptoms.”

Managing potassium in soybeans begins with taking a soil test and then following the test’s recommendations for fertilizing. If concerned about a potassium deficiency, do a tissue test to determine where the deficiency lies. Tissue can be tested in two ways:

Reactive tissue sampling. Samples are collected from the uppermost, fully expanded trifoliate leaf of the plant. Gather a composite sample from bad and good areas.

Proactive tissue sampling. The field is divided into management zones. Collect 18 composite samples from the uppermost,  fully expanded trifoliate leaf within each zone and compare samples.

Related:Group 5 beans by May best bet in South Carolina

“If we are concerned about hidden hunger, we need to be out there before we have those bad areas in the field,” Ortel said. “Within each zone, it becomes very important to collect at least 18 from random points across that zone. The reason is because we know we have infield variability, so we need to be collecting at least 18 leaves to make sure that we are capturing that infield variability and representing that area.”

Send samples to the lab and make sure to interpret the results correctly to apply the right amount of potassium, Ortel said.

Doctoral work

Before returning to her native Virginia in April to accept her new job, Ortel completed her doctorate at the University of Arkansas, where she conducted “hidden hunger” potassium research with farmers.

Ortel was part of the U of A soil fertility research team to identify proactive sampling methods for tissue testing.

Two of the five research fields were dryland and three were furrow-irrigated. In the Southeast, there tends to be more dryland soybean production than in the Midsouth.

In short, the research revealed that the window of opportunity to make a corrective application and achieve a yield response in soybeans is 20 to 45 days after the plant reaches the R1 growth stage. 

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“Whether we are in the 20- or 45-day window is going to depend on the severity of the deficiency,” Ortel said. “If we have a really severe deficiency with those visual symptoms, we have 20 days after R1 to apply a corrective application and incorporate it with either irrigation or rainfall, and get about a 90% yield response. However, if we have a ‘hidden hunger’ situation, [with]mild to moderate deficiency, now that window of opportunity expands all the way out to 45 days after R1 to get an almost full yield response.

“That is going to allow us to have the time that we need to go out there and be proactively tissue testing if we are concerned about a deficiency, sending those samples to the lab, interpreting those results and responding accordingly. We have the time that we need to actually incorporate these management practices.”

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It all boils down to the 4 Rs of nutrient management: right source, right rate, right place, right time. Ortel said 20 to 45 days after R1 is the right time to correct a potassium deficiency. As for the right source and right place, Ortel said potassium is a micronutrient, which means the soybean plant needs potassium in large quantities.

Related:Tool helps Carolina farmers pick best soybeans for profit

“Because of that, we cannot rely on a foliar fertilizer. To apply the rate needed, we need granular applications,” she said.

An economic analysis of the Arkansas study revealed that if you do have a potassium deficiency in soybeans, the in-season applications are profitable. She said her task now is to do more research in Virginia.

“A lot of this relies on soil moisture so we need to make sure we are thinking ahead about all of these different considerations that we have in dryland production systems to make sure that this is a good use of inputs,” she said.

Ortel encourages farmers to use soil testing to avoid potassium deficiency and following the recommendations for applying the potassium at the right rate at the right time.

“However, if we are nervous about having hidden hunger because of different soil textures that we have, field history, whatever your reason is, then you need to go out and collect a proactive tissue test and then make sure you are interpreting those results correctly.

“If you do find yourself in a deficient situation, we now have the tools to, hopefully, be able to correct that deficiency and do so profitably.”

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About the Author

John Hart

Associate Editor, Southeast Farm Press

John Hart is associate editor of Southeast Farm Press, responsible for coverage in the Carolinas and Virginia. He is based in Raleigh, N.C.

Prior to joining Southeast Farm Press, John was director of news services for the American Farm Bureau Federation in Washington, D.C. He also has experience as an energy journalist. For nine years, John was the owner, editor and publisher of The Rice World, a monthly publication serving the U.S. rice industry.  John also worked in public relations for the USA Rice Council in Houston, Texas and the Cotton Board in Memphis, Tenn. He also has experience as a farm and general assignments reporter for the Monroe, La. News-Star.

John is a native of Lake Charles, La. and is a  graduate of the LSU School of Journalism in Baton Rouge.  At LSU, he served on the staff of The Daily Reveille.

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