Why is the Haber process not directly used to produce NPK fertilisers, despite ammonia being a key component of many fertiliser salts?

Because the Haber process only yields ammonia; to produce NPK fertilisers, ammonia must be further reacted with acids or other chemicals to form ammonium salts, and phosphate rock and mined potassium salts must be processed separately, so the Haber process is just one step in a larger integrated fertiliser production chain.

A strong answer should directly address the approved learning objective to distinguish NPK fertiliser manufacture from the Haber process that produces ammonia. This question belongs to Production and uses of NPK fertilisers within The Haber process and the use of NPK fertilisers, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to NPK fertiliser, and keeps the wording specific to AQA GCSE revision.