The better soil injection systems have a removable (screw in) carbide point with multiple outlet holes on the side of the stainless steel pipe with extra thick walls for rigidity and strength.
The trigger assembly is "invariably squeeze to inject", and provides good control of the injection volume.
Those little features add up to quite the problem to make for yourself. But yes, some pipe and a ball valve would get liquid squirted into the ground.
Myself, I never soil inject tree fertilizer unless it is specified by a government contract, and I have all the tools to make it easy. You will get just as good results by throwing fertilizer on the ground and watering it in.
(Your results with broadcast fertilization will probably be better than soil injections, but you won't get those lovely green spots all over the yard showing where you made the injections. )