An apparatus for injecting liquid fertilizer into a water flow line in a sprinkler system, includes an injector mechanism body having a water inlet and a water outlet adapted to be connected in the water flow line of the sprinkler system so that water flows through at least a portion of the injector mechanism body, a plunger chamber, a plunger movably positioned in the plunger chamber, a paddle wheel positioned to be rotated by water flowing through the injector mechanism body, a plunger gear coupled to the paddle wheel and the plunger so as to rotate when the paddle wheel rotates so as to move the plunger back and forth within the plunger chamber and a valve assembly secured at a fixed location in the plunger chamber.
