What is phosphate rock and why can't it be used directly as a fertiliser?

Phosphate rock is a naturally occurring mineral that contains high levels of phosphorus. However, it cannot be used directly as a fertiliser because it is insoluble in water, making it unavailable for plants to absorb.

A strong answer should directly address the approved learning objective to recall that phosphate rock is obtained by mining but cannot be used directly as a fertiliser. (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.