Process for the calibration of the supply to the agronomic cultures of fertilizer, phosphoric acids comprising a response curve mitscherlichienne diazotrophe describing the activity of the flora azotobactérienne in a ground grass in the vicinity of the cultivation residues recently buried, and a method of fertilization phosphoric comprising the application of a dp of phosphorus and calibrated directly to the cultivation residues, advantageously in combination with an inoculum azotobactérien.