Abstract: An automatic fertilizing device is installed inline with a below ground water line for an underground sprinkler system, thus placing the entire device below the surface. The device comprises a fertilizer tablet chamber with an inlet and outlet at opposite ends, which are connected to the inlet and outlet side of the water line. A fertilizer replenishment passage extends upwardly from the chamber, with an upper end positioned at or below ground level. One or more inlet and outlet screens may be provided at each end of the chamber, thus preventing the entrance of small particles of the solid fertilizer tablet into either the inlet or outlet line as the tablet is slowly dissolved by water flow therearound. The closure for the passage may include a fertilizer tablet retainer extending downwardly therefrom, to retain the tablet within the full flow of water through the chamber and to prevent the tablet from rising into the replenishment passage.