A fertilizer injector is a tank hooked up to your watering system that puts fertilizer into the water. This allows you to both water your plants and fertilize them at the same time. Fertilizer injectors only work with chemical fertilizers that are water soluble, not organic fertilizers. Building your own fertilizer injector is possible, but it won’t have all the fancy features of an expensive fertilizer injector that you could buy.
Place a backflow preventer on the main water source faucet for your plants. The backflow preventer will screw onto the faucet. This stops any fertilizer from getting into the city water supply.
Tighten a 1/2-inch siphon valve onto the threaded end of the backflow preventer.
Slip a 1/2-inch siphon tube into the siphon valve’s hole and tighten the nut over it on the outside of the valve. This keeps the tube from slipping out of the valve.
Connect the watering hose to the end of the siphon valve by screwing it in place.
Cut a hole in the lid of the plastic container. The hole should be slightly larger than the circumference of the siphon tube. Slip the tube all the way into the container until it touches the bottom of the container.
Remove the lid and fill the container with the amount of liquid fertilizer you want to use on your plants. Turn on the water to start the injector. As the long as the siphon tube is connected and there is fertilizer in the container, the injector will pull fertilizer from the container and feed it to your plants at a rate of one part fertilizer to 15 parts water.