. . .and modern agricultural practices rely on using large amounts of the stuff, the excess of which runs off into rivers and oceans and causes nasty algae blooms.
"At current rates, reserves will be depleted in the next 50 to 100 years. Phosphorus is as critical for all modern economies as water."
Click here for more info and for a lovely photo of a phosphorous strip-mine.
Personally, I prefer to collect phosphorous by having Chris pee on the compost pile.