Explain why phosphoric acid is preferred over nitric or sulfuric acid for producing soluble phosphate salts for fertilisers.

Phosphoric acid reacts with phosphate rock to give soluble phosphate salts without introducing harmful nitrate or sulfate ions into the soil. It also produces a product that is less corrosive and more stable for agricultural use. In contrast, nitric or sulfuric acid would leave nitrate or sulfate residues that could cause soil salinisation or environmental problems.

A strong answer should directly address the approved learning objective to describe treatment of phosphate rock with phosphoric acid to produce soluble salts for fertilisers. (Chemistry only). 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 chemistry only, and keeps the wording specific to AQA GCSE revision.