We've devised a simple plant food injector that adds a bit of dissolved nutrient into the watering system with each ebb and flow of the automatic watering system. We've done this by clipping the hose from a standard pressure sprayer and added a simple 1/4" barbed splice connector. This is then connected to the water source hose. When the water system comes on its pressure adds water into the canister where it combines with the plant food concentrate. When the system turns off the pressure in the tank forces water into the hose where it waits for the next irrigation cycle. In time the water will become clear and you will know to add more plant food to the container. Be sure to use water solluable fertilizers so you don't clog your emitters.
Here's the pressure tank we like to use: Roundup 1gal Garden Sprayer (Amazon) ($20)