Potassium (K) is an essential plant nutrient. Because it is required in large amounts by plants, potassium is referred to as a macronutrient (Black, 1957). The terms primary and secondary elements also refer to macronutrients.
The major portion of potassium is contained in minerals such as feldspar and mica, and clays such as montmorillonite, vermiculite and illite. The total amount of potassium is important. However, of immediate concern to crop production is the portion of this nutrient that is in an exchangeable (available) form for plant use. Potassium is an exchangeable cation. The potassium ion has a positive charge and binds with the negatively charged soil particles.
Potassium is absorbed by plants in larger amounts than either magnesium or calcium; in fact, nitrogen is the only nutrient absorbed in larger amounts than potassium. Potassium is a vital component of numerous plant functions, including nutrient absorption, respiration, transpiration, and enzyme activity. Potassium is unique because it does not become part of plant compounds, but remains in ionic form in the plant. Potassium remaining in plant residues after harvest and in manure are quickly returned to the soil by water leaching through the plant materials and manure.