Potassium

Potassium is the third most commonly supplemented macronutrient.  It has important functions in plant metabolism, is part of the regulation of water loss, and is necessary for adaptations to stress (such as drought and cold).  Plants that are deficient in potassium may exhibit reductions in yield before any visible symptoms are noticed.  These symptoms include yellowing of the margins and veins and crinkling or rolling of the leaves.  An excess, meanwhile, will result in reduced plant uptake of magnesium, due to chemical interactions.

Potassium deficiency (photo courtesy of Department of Soil Science, University of Wisconsin (image source))