Strong-smelling gas used to make fertilisers (7)

I believe the answer is:

ammonia

(Other definitions for ammonia that I've seen before include "Powerful cleaner" , "Gas compounded of nitrogen and hydrogen, NH3" , "Pungent gas used in the manufacture of fertiliser" , "Nitrogen-hydrogen gas" , "Colourless pungent gas" .)

I've seen this clue in The Guardian.

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