Potash

From DwarfFortressWiki

Potash is created from ashes or lye at the ashery by a dwarf with the potash maker skill. Potash has a value of 15 and does not have quality modifiers.

Potash is used for fertilizer on farm plots. Fertilized farm plots yield larger stacks of produce.

Potash can also be baked in a kiln or magma kiln to make pearlash, which in turn is used to make clear and crystal glass.