You could also use the gear they make for drip irigation systems but hook the source hose to your tank instead of a faucet. Most such kits come with little spray heads with controlable valves that you stick in soil as appropriate.
It would take some time to get things adjusted, and no compensation for changes in weather, but it's be cheap. the water would go on top of the soil not down on the bottom of the pot where it could collect and make the roots rot. Only hard part I can see is a tight connection between the plastic hose and the tank, below the water level. Of course, as Max says, the tank must be above the outlet levels, but that too could be adjusted to get the pressure that works. Tom
