Potassium is one of the big three nutrients plants receive from soil and fertilizer; it is the third number in the NPK (nitrogen - phosphorus - potassium) ratio listed on commercial fertilizers. Once absorbed, potassium improves the overall hardiness of the plant by improving the rigidity of the stalks and increasing disease resistance. Thus, potassium is important in helping the plant overcome drought stress and survive winter. There is no level at which potassium becomes toxic to plants. But when plants get too much potassium, the absorption of other nutrients is inhibited, which leads to the symptoms caused by the deficiency of these nutrients
The primary risk of too much potassium is a nitrogen deficiency. This will stunt the growth of the plant and lead to chlorosis, a yellowing of the foliage that first appears on older growth lower on the stem. The veins on the leaves will have a red tint. Newer leaves will be smaller in size. These effects can be countered by adding compost or applying a primarily nitrogen-based fertilizer while discontinuing application of potassium-rich fertilizers.
Magnesium and Manganese Deficiency
The other major risks from too much potassium are magnesium and manganese deficiencies. The symptoms of magnesium deficiency are a yellowing that begins from the veins of the leaves. Manganese produces a similar yellowing that begins on the youngest leaves and develops into gray or black spots of dead tissue (necrosis). Eventually the leaves wither and die entirely. Watering with diluted Epsom salts can help remedy a magnesium deficiency. Manganese sulfate or manganous oxide can be mixed with fertilizer, but the best remedy when the cause is too much potassium is to discontinue use of potassium-rich fertilizers and add perlite or vermiculite to increase aeration of the soil and aid absorption of the already present manganese.