July 15, 2005 | By: Laura Skillman

Potassium deficiencies in corn and soybeans are common this year, creating the potential for stunted plants and reduced yields.

“I’ve been here 34 years, and this is the worst I’ve ever seen,” said Lloyd Murdock, Extension plant and soil scientist.

Likewise, the UK Plant Disease Diagnostic Lab at Princeton has not seen samples of potassium deficiencies in corn prior to this year, although they do see it yearly in soybeans, which require more potassium, said Paul Bachi, plant diagnostician. 

The primary cause of the deficiency is low levels of potassium in the soil, which can be determined by soil testing. While UK recommends that soil testing samples be limited to 20-acre plots, some single samples are collected from much larger areas. This allows areas with deficient levels to be masked by those with sufficient levels.

Other factors adding to the deficiency include anything that restricts or slows root development such as compaction or dry soils, both of which are factors this year. Plants take in potassium with water from the soil. The problem can be more pronounced in no-till and minimal tilled fields because much of the potassium is in the top few inches of soil and does not move down into the soil structure.

“Even with these other things, if a farmer had a good soil test, the crop would be alright,” Murdock said. “I haven’t seen a field yet that had a proper soil test, that’s had a potassium deficiency. What I have seen is fields that when you test the overall field it looks good, but there are parts of the field that may be parts of an old field where potassium was depleted and they’ve never built it back.”

This year has revealed some practices that farmers need to reconsider. No. 1, Murdock said, is that some people are not soil testing as often as they should. In addition, they are using too large an area for a single sample for testing, and compaction is not being checked but is building up in some fields.

Farmers should pay more attention to zone or grid soil testing, he said.

“You need to do it by zones, especially this year when you see where the problems occurred,” Murdock said. 

Potassium is important for top growth and root growth, and without sufficient levels plants can become stunted with minimal root growth. Potassium deficiency started with corn and has moved into soybeans. The deficiency in corn shows in the bottom leaves as a yellowing around the edge that will become necrotic and can kill the leaf and move to the upper leaves of the plant. In corn, it can be mistaken for chemical damage.

“Basically, if a person has potassium deficiency, they are going to lose yields,” he said.

Yield loss varies depending on the deficiency. Small plants can face a tremendous yield loss because they will not recover. Most of the severe deficiencies will reduce the yields by a third to one-half, but if the corn has grown well with only some mild deficiency symptoms showing at the bottom of the plant, losses are more likely to be about 5 percent, Murdock said.

Farmers generally should soil test every two to three years. Any areas of fields showing potassium deficiencies should be sampled and tested this fall so corrective measures can be taken.



Writer: Laura Skillman 270-365-7541 ext. 278

Contact: Lloyd Murdock, 270-365-7541 ext. 207