"Potash" is a catch-all term for types of fertilizer based on potassium. Potash has been used as fertilizer for centuries and got its name from the nutrient-rich ash derived from plants, which was soaked in pots and allowed to evaporate. The name "potassium" was derived from this practice.
Potassium is an abundant element necessary for the health of plants and animals. It is a component of several types of potash, including potassium hydroxide, which is also called potash lye; potassium carbonate, which is also called pearlash or salts of tartar; potassium chlorate; and potassium nitrate, or saltpeter. Other kinds of potash are sulfate of potash and permanganate of potash. Potash used as fertilizer can be potassium carbonate, potassium chloride or potassium sulfate. Potassium oxide is not used for fertilizer, because plants can't use it.
Potash is also used to recycle aluminum, electroplate metal, and melt snow and ice. It is used to treat people with low potassium levels and is added to water filtration systems to remove minerals that cause hard water. In addition, it is used to make glass and soap and to bleach fabric; it has been used for these purposes since at least 500 A.D. Currently, potash is mined on an industrial scale largely from deposits that have evaporated from inland oceans.