Actually a large portion of the of Phosphorus from the chemical fertilizers become unavailable to the plants due to their high tendency to form compounds with Ca, Mg (in alkaline soils) and Fe, Al ions (in acidic soils) making the P immobile. Further if their is sufficient Phosphate solubilizing organisms, it will made the P soluble and available for the plant uptake. The Mycorrhizal symbiosis also play important role in the P uptake due to their increased root surface area to volume ration as well as their well documented ability to transport P to the host plant.

the following paper may be useful in this regard.

http://www.idosi.org/aejaes/jaes3(6)/5.pdf

May be the following attachment will help you.