I found this:
"Because of its high reactivity, phosphorus exists in combined form with other elements. Microorganisms produce acids that form soluble phosphate from insoluble phosphorus compounds. The phosphates are utilized by algae and terrestrial green plants, which in turn pass into the bodies of animal consumers. Upon death and decay of organisms, phosphates are released for recycling."
It talks about the 'weathering of rocks also and how that moves P eventually into the oceans,,, but then we already knew that in 14 million years the entire land mass of the planet will be eroded below sea level...
Here is the final 'summary statement' of the short article:
"Because of the steady diversion of phosphorus into the oceans, the element must be added (in fertilizers) to soils to maintain fertility and agricultural productivity."
My idea is that this article is saying that microbes DO break down insoluable P to become soluable P and that this would be even more likely when the P was already in a once living organism... unfortunately I haven't found much of that scenario on the internet...