Potassium nitrate is a form of the mineral potassium added to or naturally occurring in some soils. Potassium is necessary for plant development and overall health, as well as proper photosynthesis. However, too much potassium can have detrimental effects on plants in both simple potassium form and potassium nitrate form.
A toxicity in plants is like an "overdose," when too much of one thing is introduced to a plant's system and causes serious side effects. While excess nitrogen and phosphorous can destroy or poison a plant, excess potassium does not harm the plant in itself. In fact, potassium is hard to get for plants and they will normally take as much as they can get.
If too much potassium is applied to a plant, it may result in a calcium or magnesium deficiency. This is because all of these elements are in competition, and too much potassium prevents enough calcium or magnesium from being stored. Similarly, too much calcium or magnesium can cause potassium deficiencies in plants if there is too much of either mineral in the soil.
Too much potassium in potassium nitrate form can harm plants because of the added nitrate, which is a form of nitrogen. If the soil is already high in nitrogen, the added nitrogen can burn the plants.