Oz would be a good one to elaborate further on qst2know's answer as he has attempted this from what I gathered in another thread.
To elk,
Blood meal and bone meal both work, as both contain significant quantities of phosphate. Bone meal is obviously preferred as it contains calcium phosphate, but blood meal will have phosphate present as well (think BPG).
Traditionally, putrefied urine was used for the synthesis of white phosphorus; today however, most of it is made using a mixture of sand, aluminum powder, charcoal, and a phosphate of some sort--all fused at very high temperature. The phosphorus distills over.