Where is the main store of phosphorus ?

Phosphorus is most commonly found in rock formations and ocean sediments as phosphate salts.

The largest reservoir of phosphorus is in sedimentary rock. It is in these rocks where the phosphorus cycle begins. When it rains, phosphates are removed from the rocks (via weathering) and are distributed throughout both soils and water. Plants take up the phosphate ions from the soil.

answer and explanation;

volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet.  

plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma. plutonic rocks make up the base of continental crust as well as oceanic crust. their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock.

the textural difference is that plutonic rocks are more coarse grained than volcanic rocks. plutonic rocks are coarse grained while volcanic are fine grained ; glassey or fragmental. since fine and coarse are adjectives the decision as to whether a rock is fine or coarse grained is generally made on the bases of whether or not the individual crystals can be seen in most if the rock.