Unlike other elements, phosphorus is found primarily in sedimentary rock as opposed to the atmosphere, though it too is critical for life and necessary for DNA.
Rainfall and runoff cause erosion which in turn removes phosphorus from phosphate rock and supplies it to soil. The phosphate can then be absorbed and incorporated by plants which are consumed by animals on up the food web. Decomposing organisms return organic phosphorus to the water and soil.
The phosphorus in soil is dissolved in water that generally flows into larger bodies of water until it reaches the ocean. Some of this phosphorus is used by plankton but the majority of it sinks to the ocean floor creating a large resevoir.