POROUS VOLCANIC TUFF - A SUPPORT FOR PHOS PHATIC AGGREGATES OF AGRONOMICAL INTEREST

Different phosphatic materials were synthesized by a reaction between a phosphoric acid solution and the volcanic tuff or a mixture of volca nic tuff + Ca(OH)2. After synthesis, the elements present in solution were analysed, and the solids were studied by XRD, SEM and EDS. The fo rmed products were : amorphous phosphates, aluminium phosphates (berli nite type), calcium phosphates (apatite), calcium-aluminium phosphates and silicophosphates. The crystallization of these mineral phases and the amount of amorphous phosphates in the materials depend on the tem perature and on the nature of the treated material. For the different products, the P2O5 ratio varied from 6 to 18%. The extraction of solub le phosphorus in simulated pedological conditions shows a low phosphat ic solubility in water, and a good solubility in 2% citric acid and in ammonium citrate. This provides an agronomical application of these m aterials, for highly leached acidic or basic soils.