Here's why:

* Phosphorus Cycle: Phosphorus is a crucial nutrient for life and cycles through the Earth's systems.

* Rock Formation: The original source of phosphorus is within the Earth's crust. Volcanic activity and the formation of igneous rocks bring phosphorus to the surface.

* Weathering and Erosion: Over time, weathering and erosion break down these rocks, releasing phosphorus into the soil and water.

* Biological Processes: Plants absorb phosphorus from the soil, and animals get it by consuming plants. Phosphorus is essential for building DNA, RNA, and bones.

* Sedimentary Rocks: Some phosphorus eventually ends up in the ocean, where it can become incorporated into sedimentary rocks. These rocks can eventually be uplifted back to the surface.

While we can find phosphorus in other places, like guano (bird droppings) and marine sediments, igneous rocks are the primary source.