The present article reports the correlation of the physical and chemical properties of natural rock to the availability of phosphorus to plant in powdered phosphate rock.Resuits revealed that the availability of phosphorus in rock powder is much more depended on the compactness and regularity of the crytal lattice,while the graininess and the status of fissure of the rock are of secondary importance.The writers found that there are a serves of changes in the rock bed induced by the substitute of PO₄^- by CO₃^- in the phosphate deposit.The increasement of CO₃^- in fluo-appatite increases the availability of phosphorus in natural rock.A system for the classification of rock phosphate has been suggested according its phosphorus availability.