Method for production of granular ammonium polyphosphate fertilizer in which ammonium polyphosphate melt is prepared in a simple inline reactor and distributed onto a bed of solids in a drum granulator to bind smaller fertilizer particles into granules. The desired polyphosphate content can be obtained in the granular product by adjusting the total heat input of the feed reactants to the reactor by means of energy conservation modifications to the system. Energy losses from the reaction system are controlled by use of a predetermined quantity of insulation so that varying amounts of the maximum theoretical heat input can be utilized to produce products of polyphosphate contents as desired. The quantities of insulation and the resulting heat inputs to the reactor have been identified which will produce the given critical polyphosphate content material, determined to be 12 percent of total phosphate as polyphosphate in a granular material, which will give 10 percent polyphosphate in a suspension fertilizer made from the granular material. No external heat is required and energy savings can be readily realized as well as production cost savings.