When some people saw soybean leaves turning brown on the upper part of plants recently, they weren’t sure about the cause. Those with scouting experience likely thought about a potassium deficiency, but most scouts are used to seeing that show up on lower soybean leaves first. Often, outer edges of older leaves turn brown during vegetative growth stages if the plant runs short on potassium.
“During pod fill, upper leaves can show the same symptoms,” says Jim Camberato, Purdue University Extension soil fertility specialist. “From the samples we’ve seen and fields we’ve looked at, we’ve diagnosed various instances where soybean plants were short of potassium.”
If the problem is severe enough, it can lead to lower yield, a higher incidence of purple seed stain, and misshapen and wrinkled seeds, according to the Purdue Corn and Soybean Field Guide.
Other notes on potassium
At least one scout reports finding a fair amount of potassium deficiency in the blackest soil in a cornfield. Many people assume black soils would be the last place to find a nutrient deficiency.
That’s not always true, says Jason Webster with Precision Planting. He’s helping set up a Precision Planting Institute demonstration farm on roughly 180 acres of dark soil near Pontiac, Ill. He was surprised when they pulled soil tests. Many areas were low in both nutrients and pH. Potassium levels were low in places.
If black soils aren’t tested regularly and fertilized properly, they can become deficient too, he notes.