What kind of decaying organism turn into phosphorus?

Phosphorous is a highly reactive element and is not found in nature. Indeed some varieties of phosphorous will spontaneously ignite when exposed to air.

As an element, is cannot be transformed, or changed into anything.

This element is commonly found in bones, and thus bird guano contains a concentration of phosphorous compounds, (from the fish the birds ate) and which may be easily released into agriculture.

The 'rag and bone' man of the 18th and 19th centuries collected the scrap household bones for fertilizer manufacture.