The largest reservoir of phosphorus in the biosphere is the

The largest reservoir of phosphorus in the biosphere is the Rocks.
Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals.
The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxide (P₂O₅). Areas of oceanic upwelling cause the formation of phosphates.