Here's why:

* Fine-grained: Phosphate rock is a sedimentary rock made up of very small particles.

* Fertilizer: It's a primary source of phosphorus, a crucial nutrient for plant growth.

* Not directly usable: Phosphate rock itself isn't directly usable by plants. It needs to be processed into more readily available forms like superphosphate or triple superphosphate.

Let me know if you'd like to learn more about phosphate rock or its processing!