Potassium is a highly reactive alkali metal with a faint bluish-purple color when exposed to air. It is the second least dense metal after lithium, is soft enough to be cut easily with a knife, and has a low melting point. It is relatively common in the universe and the seventh most abundant element in the earth's crust, comprising about 2.5 percent by mass.
Potassium is essential for all known forms of life. It is well suited for biological functions because it is highly soluble and easily transported through cell membranes and because its weak bonds allow rapid chemical reactions. Among its important roles are supporting enzyme functions and protein synthesis, maintaining fluid balance and cell volume, facilitating the transmission of nerve impulses and facilitating muscle contractions.
By far the largest industrial application for potassium is as fertilizer. It is one of the three main macronutrients, along with nitrogen and phosphorus, that are necessary for plant growth. Commonly applied in the form of potassium chloride or potassium sulfate, it aids in water retention and drought resistance and improves disease resistance, thereby improving overall crop quality and yield.
Among the many and diverse additional applications for potassium and its compounds are chemical production, soaps and detergents, glass manufacturing, batteries, pH control, explosives and pyrotechnics, metal treatment, water treatment, pharmaceuticals, textile dyes, fracking and electronics.
The largest negative environmental effect of potassium is likely its disruption of the pH balance of soil from the heavy use of manufactured fertilizers. This can reduce crop quality by inhibiting plant growth and flowering and by harming essential bacteria in the soil. Likewise, when released into enclosed water bodies such as ponds and lakes, potassium can cause a sudden rise in pH levels, thereby harming aquatic life.