Since ancient times, potash was obtained by putting leaf or wood ashes
in a pot, leaching the ashes with water, and evaporating the solution.
Saltpeter was obtained from decaying vegetation, sewage, or farmyard manure,
also leached and evaporated.
Potassium salts and sodium salts are chemically similar,
so people did not realize that they were compounds of different elements.
See also in The book of science:
Readings on wikipedia: