Chemical compound used as fertiliser (6)

I believe the answer is:

potash

(Other definitions for potash that I've seen before include "Strong alkali derived from burnt wood" , "Compound used in agriculture and industry - shop at (anag)" , "Potassium compound used as fertiliser" , "The strange pathos of a potassium compound" , "chemical compound" .)

I've seen this clue in The Guardian.

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