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trent-roberts-soybeans-potassium-deficiency-nick-kordsmeier-uark.jpg Nick Kordsmeier
Soil scientist Trent Roberts demonstrates how to collect soybean tissue samples for potassium concentration analysis.

Potassium deficiency in soybeans explained in video

Potassium is the most yield-limiting nutrient for soybeans.

Potassium is the most important yield-limiting nutrient in soybeans, and the plants can suffer a deficiency of it at multiple stages of growth.

A new video produced by the University of Arkansas System Division of Agriculture explains the importance of potassium, also called potash, and how growers can make certain they are providing the fertilizer their crops need.

Potassium aids in water regulation in plants, said Trent Roberts, associate professor of crop, soil and environmental sciences for the Division of Agriculture.

“It’s essential for a lot of the pathways tied to water and water regulation,” Roberts said, “including transpiration, canopy temperature, carbon dioxide capture, and others.”

Nitrogen fertilizer is often the focal point of most row crop systems, he said. But soybeans are different in that they are legumes, which absorb nitrogen from the air and convert it into a nitrogen metabolite through a symbiotic relationship with soil microbes. The process is called biological nitrogen fixation.

Since soybeans essentially make their own nitrogen fertilizer, Roberts said, potassium becomes the most limiting fertilizer nutrient for yields.

Potassium-deficient light-textured soils

Light textured soils — sandy, sandy loams and silt loams — tend to be potassium deficient, he said. Clay soils normally have adequate soil test potassium levels. This makes soil sampling key to successful potassium management, he said.

“The first step to detecting and correcting potassium deficiency is soil testing,” Roberts said.

Nick Kordsmeiersoybeans-potassium-deficiency-nick-kordsmeier-uark-2.jpg

These soybeans show symptoms of potassium deficiency in the upper canopy.

Decades of research by the Arkansas Agricultural Experiment Station, the Division of Agriculture’s research arm, have resulted in calibrated soil test-based data that allow the Soil Testing and Research Laboratory to provide accurate fertilizer recommendations, Roberts said. Soil testing, available from the Division of Agriculture’s Soil Testing Program can help growers know how much potash they need to apply before planting their crops.

“This is the first step to ensuring that potassium will not be limiting and helps get the crop off on the right foot,” Roberts said. Farmers should contact their county Extension offices to obtain soil testing information, supplies and services.

Growers should follow up during the growing season by scouting their fields to look for symptoms of potassium deficiency, Roberts said. In the video, he shows where to look for common symptoms and what they look like.

Hidden hunger for potassium

But beware of “hidden hunger,” Roberts said.

“I think there is the potential that hidden potassium hunger may be robbing soybean yield on our lighter textured soils,” Roberts said. “Hidden hunger” refers to nutrient deficiencies that can reduce yield before visible symptoms appear in the plants, he said.

In the video, Roberts shows where and how soybean tissue samples should be taken to make sure the plants have sufficient potassium throughout growth and reproductive stages.

Current interpretations of tissue-sample data are based on the R2 growth stage, Roberts said.

But research data from the Arkansas Agricultural Experiment Station permits nutrient interpretations across a broader range of plant growth, from the vegetative stages through the end of the growing season, he said.

Roberts said that data was used to develop the Dynamic Critical Concentration Threshold, a graphed curve that indicates the ideal potassium concentrations as they change throughout a soybean plant’s vegetative and reproductive stages. Tissue samples can show whether potassium concentrations fall above or below the curve at every stage.

Greater flexibility, accuracy

“Our system allows greater flexibility and accuracy to identify possible potassium deficiencies and corrective measures that can be taken, to assure maximum yield potentials,” Roberts said.

This means that farmers can adjust potassium concentrations with applications of up to 60 pounds per acre of potash through the R5 stage, or to about nine weeks past the R1 stage, Roberts said.

The video can be viewed online: https://youtu.be/s9aKOfcc4UE.

More information on potassium management for soybeans and other crops can be found in Division of Agriculture fact sheets and crop production handbooks. These are available for download from the Cooperative Extension Service:https://www.uaex.edu/farm-ranch/crops-commercial-horticulture/default.aspx.

Funding support for the video and potassium research was provided by the Arkansas Soybean Checkoff and the Arkansas Fertilizer Tonnage Fees.

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