Soybean South

 - Soybean Fertility -

Don’t Skimp On The K

Potassium is essential for fertility in silt loam soils.

By Fred Miller

Growers looking for ways to trim down expenses for soybean production sometimes include potassium fertilizer among the cuts.

That may be a mistake, especially in eastern Arkansas fields with silt loam soils, said Dr. Nathan Slaton, director of the Arkansas soil testing and research program for the University of Arkansas Division of Agriculture.

“Soybeans are heavy users of potassium, an essential macronutrient that enhances drought tolerance, stem strength, disease resistance, nitrogen fixation and other factors that affect yield,” Slaton says.

Silt loam soils have less clay and organic matter than other soils, and that profile reduces their resistance to change.

“Potassium fertility doesn’t change much over time in clay soils,” Slaton says. “But in silt loam, if you leave out the potassium fertilizer a few years, the fertility level drops.”

Slaton has run test plots for five years in soybean-rice rotations with varied potassium rates. After three years, in silt loam soil at Pine Tree Branch Station near Colt, soybean test plots that have been treated with medium to high rates of potassium have yielded 10 to 20 bushels per acre more than untreated control plots, he said.

Farmers’ help appreciated
Division of Agriculture recommendations are updated as soil test methods change and different, improved soybean varieties become available.

Slaton conducts fertilizer rate studies to determine the proper fertilizer rates to apply for phosphorus and potassium. He also analyzes soil samples to see if there’s a correlation between soil test potassium and crop yields. This information is critical to growers, who use soil test results to determine how much potassium fertilizer to apply.

“We must use growers’ fields in different production areas to broaden the scope and range of data to make it more applicable to the whole state,” Slaton notes.

“Growers have been very supportive. They tend our plots on their land, basically giving up about an acre of their crop to help us out,” he explains. “They know it will help them make informed decisions about managing their crops.”

Fred Miller is with the University of Arkansas Division of Agriculture. Contact Miller at (479) 575-5647 or Contact Slaton at (479) 575-3910 or