Are we really seeing K-deficient corn, soybean?
In other fields showing K deficiency, the soil-test K levels are adequate for corn and soybean production, but environmental conditions are causing a temporary deficiency that should disappear soon after growing-season conditions improve. Potassium ions move in the soil solution by diffusion (from areas of high concentration near the soil particles to areas of low concentrations close to crop roots). The diffusion distance is very short, and where water is limited (soil pores have more air than water) the distance that K ions have to travel to reach the root becomes too far, as ions cannot diffuse through air. So under droughty conditions, as far as the crop is concerned it is as if the ions were not present.
Dry conditions also limit root growth and activity, which further reduces the capacity of the crop to take up K. Other factors that can reduce K availability by limiting roots include soils that are either too loose or compacted, root damage by disease or insect pruning, shallow seed-planting depth, and seed-furrow sidewall compaction that occurs when planting in wet soils (not a common problem this year).
What can be done about K deficiency? The best way to supply K is in the soil before planting is done. Similarly taking place before planting are management practices to avoid soil conditions that intensify the negative effect of dry weather on K availability, even in well-fertilized fields. There is thus little that can be done in an economically feasible way to correct a K deficiency for this year's crops.
Remember that if the deficiency is caused by factors other than low soil-test K, applying K fertilizer to the soil would not be profitable for this year's crop. Even in low-K soils, a liquid or dry application between crop rows is not a good option unless there is sufficient rain to move K into the root zone. A positive outcome, however, is that even if there is not sufficient rain to move K to the root zone, the application can begin to correct soil K levels for the next crop.
Another potential advantage to soil applications is that higher rates of fertilizer can be applied than with foliar applications. Both old and new research in midwestern states has shown that foliar K applications produce results too limited and inconsistent to be profitable in most years, especially given that deficiency tends to occur in isolated and small areas of the field. To my knowledge, none of the trials showing response to foliar applications occurred under droughty conditions. Still, if a foliar K application is deemed necessary, I recommend using fluid products that do not contain potassium chloride, potassium sulfate, or other ingredients with a high salt index so that higher K rates can be applied without concern for canopy injury. In general, for trials where there was a yield response to low-salt foliar K applications, the response was obtained without canopy damage at rates of 10 to 12 lb of K2O per acre or less.