Someone shows you stalks from a field of corn that's just tasseling and shooting ears. The stalks are big-around- good diameter- and the plants are dark green and healthy. The roots are healthy with lots of root hairs. All systems appear to be go.
There's just one blip on the radar screen. On a leaf or two on some of the plants, usually older leaves near the bottom, there's a distinct, narrow yellow to brownish fringe all the way around the outside of the leaf. What could be causing it?
Grab a copy of the Corn & Soybean Field Guide put out by the Purdue University Crops Diagnostic Training Center, directed by Corey Gerber. Find general crop management in the index, then nutrient deficiencies. Look up potassium deficiency. The picture greatly resembles the plants the person just showed to you.
When you mention you think there might be signs of potassium deficiency on the plant, the person's eyes light up. The stalks came from a 'red clay bank' in rolling ground, probably an eroded Miami soil. It's possible potassium levels weren't very high on that soil.
The secret is that you won't know for sure unless you test the soil. Ask the person who showed it to you to go back to the field, then pull a soil sample form areas where he pulled the stalks. Move into an area where he doesn't see any leaves with the discoloration around the outside edge. Pull a soil sample there. Send the two samples off to a lab for a soil sample analysis. If your hunch is right, the potassium reading should be higher in the soil where there weren't any symptoms, and could be medium to low where the plants were taken that show symptoms.
Whether one or two leaves showing signs of deficiency, especially older, lower leaves, will amount to much yield loss is hard to say. One view is that the plant has good stalks because it has made good use of what potassium was present. It's given you all it can for what it has to work with. The real test will come when ears develop. Will the plants be able to keep stalks together and still put on good ears?
Most soil consultants say they've seen more cases of potassium deficiency over the past few years in Indiana than in decades past. For years farmers fertilized with 6-24-24, yet corn needed more pounds of potassium than phosphorus. The fertilization pattern is likely why many Indiana soils for years were typically high to very high in phosphorus, but only medium to high in potassium. While fertilizer choices have changed, there are still soils where phosphorus levels are relatively higher than potassium levels. Potassium levels may even be low enough to produce deficiency symptoms in level, black soils thought to be very productive if these nutrient levels haven't been monitored properly and maintained through the years.
With fertilizer prices skyrocketing, keeping an eye on nutrient levels through soil testing will be more important than ever. The temptation may be to skip broadcast phosphorus and potash applications, hoping prices will go down at some future point, before your soil test levels drop low enough to significantly impact yields.
That strategy may work as long as you know what levels are in each field. If you don't have current soil test results for each field, it's kind of like trying to find your way to bed in the dark. This could be a year when soil testing is extremely crucial.