What is phosphate released from?
Phosphate is released from the weathering and erosion of rocks.
Rocks containing phosphates uplift to the surface, where weathering and other erosion takes these phosphates and deposits them in waterways. The phosphates move along these waterways, however, phosphates are not soluble in water, so they stay as they are until they reach the ocean and sink to the bottom.
Over time the ocean floor is covered by dirt, sand, and dead organic matter, and the phosphates become part of the limestone rock that is formed when these components are compressed. The limestone, and the phosphates in it, will remain underground for thousands of years until natural uplifting gradually brings it back up to the surface and the cycle repeats.