By Susan Jongeneel, University of Illinois
Potassium deficiency symptoms are developing in corn and soybean crops in many parts of the state according to University of Illinois assistant professor of crop sciences Fabian Fernandez.
Many farmers are observing this phenomenon for the first time. Conducting nutrient-deficiency diagnostics for crops based solely on visual symptoms is not always easy, but potassium deficiency symptoms are very distinct for corn and soybean.
"For both crops, symptoms start to develop in the older leaves with yellowing of the leaf margins," Fernandez explained. "The yellowing normally starts in the leaf tip and extends to the base of the leaf along the edges of the leaf."
In more severe cases the leaf edges look dead (brown) while the new leaves remain green because the potassium in the older tissues is remobilized to supply potassium to the newer tissues.
"Sometimes people confuse potassium deficiency symptoms in corn with nitrogen deficiency," Fernandez said. "While nitrogen deficiency also occurs in the older leaves starting at the tip of the leaf, yellowing develops along the midrib toward the leaf base forming a 'V' shape."
What is causing the deficiency?
The symptoms that are showing up in so many fields reflect either 1) insufficient supply due to low soil-test potassium levels or 2) reduced potassium availability due to less-than-ideal soil conditions and slow root activity.
A survey conducted in 2007-2008 found that potassium levels in approximately 45 percent of nearly 550 randomly selected Illinois fields were below the critical level needed to maximize yield. "When soils are below the critical level there is a strong probability of yield reduction even if the crop does not develop deficiency symptoms in the leaves," said Fernandez.
He said that this year, the appearance of deficiency symptoms is a signal that potassium should be applied this fall. "The crops in most of these fields will likely not recover even after growing-season conditions improve," he cautioned.
Some fields are showing potassium deficiency despite having adequate soil-test potassium levels. This problem is due to the dry soil. Potassium ions need water to diffuse in the soil solution from areas of high concentrations near the soil particles to areas of low concentrations near the crop roots. When soil pores have more air than water, the potassium ions cannot reach the root because they cannot diffuse through air.