For those who have a very alkaline or calcareous soil, phosphorus is not very available, and soft or hard rock phosphate will not easily become soluble. It can be added to a
compost pile instead to become incorporated into microorganisms, which thus makes it available to plants once the compost is spread.
I would assume that a worm bin would do a really good job of this, and the phosphate might be good for the worms too.