I came across this article yesterday, “Corn, Soybeans Showing Signs of Potassium Deficiency“ and I think the information presented in the article is right on. Some good points are made in the article.
“For both crops, symptoms start to develop in the older leaves with yellowing of the leaf margins,” Fernandez explains. “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 K in the older tissues is remobilized to supply K to the newer tissues.
“Sometimes people confuse K deficiency symptoms in corn with nitrogen (N) deficiency,” Fernandez says. “While N 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.”
The cause for this deficiency as stated in the article, is really due to less than ideal soil conditions and slow root activity. In most areas, lack of moisture and drought-like conditions are apparent. Dry conditions, typically limit root growth and activity. This can explain why the soil may have good levels of potassium, but the nutrients are not getting into the plants.
Right now, the best thing to do is check out the areas in your fields where the “yellowing of leaves” is present and mark those areas on your map for fall application of potassium for next year. There really isn’t much you can do at this stage with respect to correcting the problem. The article looks at the area of foliar potassium, but does not give evidence that it can correct this issue.
The other course of action might be a plant tissue test. This test may provide helpful information of how the good spots in your fields compare to the less-performing areas and if there are other nutrient deficiencies.
A good article with some practical advice. I would highly suggest you read it.