Abstract: A process for oxidizing aqueous elemental phosphorus containing residues (sludges) to produce orthophosphate containing slurries suitable for subsequent reaction with ammonia to produce nitrogen- and phosphorus-containing fertilizer products. It comprises reacting aqueous elemental phosphorus containing residues with certain special mixtures of concentrated nitric acid and sulfuric acid to effect the conversion of the elemental phosphorus content of the residues into mostly orthophosphoric acid and very little orthophosphorous acid with the relative ratios of orthophosphoric acid to orthophosphorous acid produced being dependent upon the weight ratio H.sub.2 SO.sub.4 :HNO.sub.3 employed in the processing. The reaction is conducted at the boiling point of the resulting aqueous reaction medium.