Potash is the common name of potassium hydroxide (KOH), a substance that has been used since antiquity in the manufacture of glass and soap, and as a fertilizer. The name comes from the English words pot and ash, referring to its discovery in the water-soluble fraction of wood ash.

The term has become somewhat ambiguous due to the substitution in fertilizers of cheaper potassium salts such as potassium chloride (KCl) or potassium oxide (K₂O), to which the same common name is now sometimes also applied.

The element potassium derives its English name from potash.

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