I thought of that too, a venturi creates a vacuum pressure (Under pressure) and therefore a suction instead. You might not need a valve, but the opening in front of the venturi is rather tiny. I guess it will clog even sooner. What I do not know is, if the vacuum pressure from the venturi has a strong enough lift to create a constant flow of fert solution even without the pump running.
The same could be happening with a 45°T placed in the correct flow direction that is reduced with a, for example, 12x4x12mm connection since it is an open system and if the outlet/spray bar isn't providing counter pressure the 4mm nozzle might have suction instead. Since the T in itself is a reducer in the hose and it could function as a venturi.
But it will all stand and fall with intensive maintenance to make it stable... Might be more work maintaining it than dosing by hand. Neglection will only result in unstable fert administering.
It's a trial and error. But about every automated fert system whatever design I've seen is prone to salt deposits and clogging.
