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Potassium, symbol K, atomic number 19, is a soft, silvery-white alkali metal. It is highly reactive, especially with water, and forms a caustic solution of potassium hydroxide. In nature it is not found free but occurs in minerals such as sylvite (KCl) and orthoclase (KAlSi3O8). The element has three naturally occurring isotopes, with K-39 and K-41 stable and K-40 radioactive.

Industrial production is mostly by electrolysis of molten salts; major uses are in fertilizers (potassium chloride, potassium sulfate), and in chemistry and industry for glass, soaps, and specialty compounds. Potassium compounds such as potassium hydroxide and potassium carbonate serve as reagents and processing aids. Potassium metal finds limited use in research and alloying.

Potassium is an essential nutrient for most living organisms. It acts as the major intracellular cation, helping regulate nerve impulses, muscle contraction, and fluid balance, and contributes to acid-base homeostasis. The kidneys control its level. In humans, dietary sources include fruits, vegetables, dairy, and meats; imbalance can cause hypokalemia or hyperkalemia with cardiovascular risk.

The term potash refers to early production from wood ashes. Humphry Davy first isolated potassium in 1807 by electrolysis of molten K2O. In the periodic table it sits in Group 1, the alkali metals, characterized by high reactivity and low density.