Potassium is the chemical element with the symbol K (Latin: kalium), atomic number 19, and atomic mass 39.0983. Elemental potassium is a soft silvery-white metallic alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.
Potassium and sodium are alkali metals and are chemically very similar. For this reason, historically their salts were not differentiated. They were finally realized to be different elements when the metals were isolated by electrolysis in the early 19th century. Potassium in nature occurs only as ionic salt. As such, it is found dissolved in seawater, and as part of many minerals. Industrical chemical applications of potassium tend to employ potassium ion's extreme water-solubility as part of chemicals which depend for activity on their non-potassium components. Potassium metal has only a few specialty applications, being replaced in most chemical reactions with sodium metal.
Potassium ion is necessary for the function of all living cells, and is thus present in all plant and animal tissues. It is found in especially high concentrations within plant cells, and in a mixed diet, it is most highly concentrated in fruits. The high concentration of potassium in plants, associated with comparatively very low amounts of sodium there, historically resulted in potassium first being isolated from the ashes of plants (potash), which in turn gave the element its modern name. Heavy crop production rapidly depletes soils of potassium, and agricultural fertilizers consume 93% of the potassium chemical production of the modern world economy.
The functions of potassium and sodium in living organisms are quite different. Animals, in particular, employ sodium and potassium differentially to generate electrical potentials in animal cells, especially in nervous tissue. Potassium depletion in animals, including humans, results in various neurological dysfunctions.
