Apparatus for adding liquid fertilizer to a water line of a sprinkler system includes a mechanical injector device powered by a paddle wheel turned by water flowing through the water line. The mechanical injector device includes a plunger that moves back and forth in the plunger chamber. Movement in one direction allows liquid fertilizer from a liquid fertilizer reservoir to flow into the plunger chamber and movement of the plunger in the opposite direction injects the liquid fertilizer from the plunger chamber into the water. The mechanical injector device is coupled to the paddle wheel which causes movement of the plunger in the plunger chamber in at least one direction. Spring bias can cause movement of the plunger in the other direction. The mechanical injector device can be disabled to stop fertilizer injection while still allowing rotation of the paddle wheel. The paddle wheel is coupled to the mechanical injector device through a plunger gear which rotates, and interacting camming surfaces which project from the plunger gear and from a ratchet. A preferred fertilizer for use in the apparatus includes a bio-stimulant.