Fertiliser deriving from potassium (6)

I believe the answer is:

potash

(Other definitions for potash that I've seen before include "Powerful alkali used in gardening" , "chemical compound" , "Potassium compound used in agriculture and industry" , "Strong alkali derived from burnt wood" , "Potassium carbonate or hydroxide" .)

I've seen this clue in The Guardian.

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