Abstract: An injection control system selectively to activate circuitry for controlling injection of additive chemicals into an existing automatically timed irrigation system having an irrigation timer. This control system includes a plurality of storage reservoirs for storing one or more additives. Conduits deliver these additives to the existing irrigation system. The control system includes a central processor programmed for intended operation, and sensors sensing the flow condition valves in the existing irrigation system. The control system thereby controls supply of additives during the operation of the existing system.