Phosphorus is in pretty much everything: bombs, toothpaste, cheese. It's irreplaceable. Nothing can live without it and it's only economically recoverable in a few places. Like Morocco, and China.
Most of our phosphorus—or phosphate, which is its usable form—goes into fertilizer. The farmers pile it on, and then the bulk of it just washes right off into the rivers and then ocean. It's really hard to get phosphorus out of the ocean, which means, as far as we're concerned, that phosphorus is pretty much gone once it's in the water.