Believe it or not, the age-old practice of applying 6-24-24 fertilizer that began with modern tractor farming and continued into the 1990s has left many Indiana fields with adequate phosphorus levels. Those fields that have been manured heavily through the years also tend to be very high in phosphorus. The nutrient that is likely to be more limiting is potassium, especially in fields where manure hasn't been applied on a regular basis.
Potassium deficiencies began to show up more frequently a few years ago on both corn and soybeans in Indiana. Agronomists believe it's because potash levels did not build nearly as high as phosphate levels. As farmers have taken off good crops and not returned as much fertilizer to the field, some fields have slipped into the low to moderate levels for potassium. The fact that potash fertilizer once very cheap, is now on the high side probably hasn't helped either in terms of whether K has been applied to fields of even spots of fields where it is needed most.
Judging stalks at the Morgan County fair last week, I found as many leaves that were suspect for early signs of potassium deficiency as I did leaves with lesions- that's about two or three total, because most of the corn was healthy.
Potassium deficiency begins with browning and yellowing on the outer margins. Sometimes the browning runs around the outside margin of the entire leaf is distinct. The potential deficiency symptoms I saw were thin bands of yellowing and browning on one side of the leaf. Without tissue testing, it's impossible to know if it was truly potassium deficiency, or if something else was at work.
If you find these symptoms while scouting fields, pull a soil sample from the affected area, and pull a tissue sample as well. Private labs can handle both leaf samples and soil samples. You may also want to pull corresponding samples from a good area in the field which isn't showing these symptoms.If potassium is low, consider a full soil testing program before applying fertilizer this fall, unless the field is on a routine soil sampling program, and you can refer back to those samples. If the field was sampled by soil types of by grids, you should be able to tell whether you would expect the entire field to respond to potash fertilizer or only portions of the field. If it's only portions, then variable-rate application would likely be your best option for correcting the problem.