Once you've decided to fertigate your garden, you should select a fertilizer injector. An injector is required to add the fertilizer to the system. Whether you have one bed or a hundred, there is a suitable model. The price of injectors ranges from a few dollars to several thousand. Fortunately, the less expensive models are compatible with most small-scale drip irrigation systems. The least expensive injector is the brass siphon mixer, which is sold under various brand names and is available at most garden centers and nurseries for around $15 to $20.
The siphon mixer is nothing more than a suction device. As water flows through the tapered orifice within the siphon mixer, the velocity rapidly increases, creating a vacuum that siphons the fertilizer concentrate from a tank or bucket into the system.
Although reasonably priced, brass siphon mixers have limitations. They are designed to be coupled with watering wands and sprinklers having flow rates of 3 gallons per minute (gpm) or higher, far in excess of many backyard drip systems' flow rates. Before purchasing a siphon mixer, determine the flow rate of your system.
If your drip system delivers between 1/2 and 3 gpm, consider using a Grow Pro brand injector which costs around $30. Unlike the siphon mixer, the Grow Pro has a built-in 12-ounce reservoir that you fill with a concentrated fertilizer solution before use. The existing water pressure forces the concentrate into the system and exhausts it in 50 gallons of flow. Models capable of handling higher flow rates are also available. (See the appendix for a listing of companies carrying Grow Pro and Add-It injectors.)
The EZ FLO injector operates on a similar principle as the Grow Pro injector. A small amount of irrigation water is diverted through the fertilizer tank where it mixes with soluble fertilizer before being injected into the irrigation system. The EZ FLO is an upgrade over the Grow Pro as it is equipped with an adjustable injection rate control. EZ FLO injectors are available in 3/4-, 1 1/3- and 3-gallon models. The 3/4-gal model is priced around $70 which includes a faucet switching kit. The switching kit is recommended for folks who need a quick way of switching out the injector from one drip system to another. (See the appendix for a listing of companies carrying EZ FLO injectors.)
A fourth type of injector compatible with both small and large drip systems can be assembled from a new or used pump-up hand sprayer and a few pipe fittings for under $50. This contraption, affectionately referred to as the "pump-it" injector, relies on air pressure generated by the hand-operated pump to force fertilizer concentrate into the system.
Because fertilizer is corrosive, choose a sprayer that has a polyethylene tank and a pump housing made entirely of plastic. For the typical backyard garden, a 1-gallon sprayer will suffice. A 3-gallon sprayer is recommended for gardens larger than 1,000 square feet.
Before operating the pump-it injector, fill the tank with the desired amount of fertilizer concentrate. Insert the pump into the tank and tighten the handle. Insert the injector manifold into the head assembly of the drip system, making sure the manifold ball valve is closed prior to turning on the system. Once the system is fully charged, pressurize the tank by using the hand pump. When you are ready to inject the concentrate, simply open the ball valve and adjust it to control the injection rate. The vacuum breaker adjacent to the ball valve prevents irrigation water from entering the tank if the tank pressure falls below the system's operating pressure.
The disadvantage of the pump-it injector is that emptying the tank can require several pressurizations, the number of pumping strokes depending on the pressure differential between the tank and the system, the volume of concentrate in the tank and the desired rate of injection.
Don't try to force all of the concentrate into the system rapidly; rather, attempt to inject fertilizer at a uniform rate over a 30-minute period. While operating the injector, take time to inspect your garden, catch up on a few chores, visit with your next door neighbor, or perhaps meditate on the meaning of life. After each use, be sure to rinse the tank thoroughly, fill it with clean water and flush the hose and manifold.
At the Noble Foundation, we have used the brass siphon, Grow Pro, EZ FLO and our homemade pump-it injector successfully. Regardless of the model you select for your system, the injector must be installed below (downstream of) the head assembly vacuum breaker or check valve and ahead (or upstream) of the filter.
When using an injector, always wait until the system is fully charged before injecting fertilizer. When finished fertigating, run the system for a few more minutes to flush any remaining fertilizer out of the system.