Apparently, plants will take up excess potassium with no ill effect. Here's a quote from the soil class I'm taking:
"The amount of potassium needed by the plant also varies with the stage of growth. For example, lower levels of this nutrient are needed during early growth stages, and higher levels of potassium during fruiting and maturation. If potassium levels are sufficient, stems are stronger, there is less succulent vegetative growth, and fruit quality is improved. However, if there is an abundance of soluble potassium available, plants will take up more than they actually need. This is called luxury consumption and represents a waste of the nutrient because it is beyond the amount needed for optimum growth. "
I need to do some more research.