Potassium is a mobile nutrient inside the plant. If new growth or if the seed being produced demands more potassium, the plant will rob it from the lower leaves.
Leaves with brown starting on the outside edges are showing a visual deficiency in potassium.
If your soil is short in potassium, this kind of problem wouldn’t surprise you. What if your soil tested high in potassium? We’ve pulled soil tests at each inch going down looking at all the nutrients and studying nutrient stratification.
One thing we’ve noticed in many fields is a high percentage of the potassium is located in the top three inches of the soil profile.
Obviously, this is also where soil first dries out, making it difficult to impossible for plants to extract soil potassium.