Should be simple enough. I want a pump trigger either manually by a push button or by a relay when soil moisture is below a certain point. I don't really want the extra wiring in my pump enclosre...so would the following cause me problems?
Ignore the relay in the picture...it was all that was in the bin!
Obviously power out to the right is going to the pump.
Yes, that will work as drawn, provided the switch is rated at sufficient volts/amps for the pump. Also, provided you don't connect anything to the other terminal on the relay, because the button connects all three terminals together when the relay is in the released position.
Would @Dougie's suggestion work? I'm concerned about forcing an active Arduino output to the opposite level. I don't see how a 10k resistor could pull down an active high output from the Arduino. If you make the resistor low enough to pull the logic level down, won't you risk damaging the I/O port?
The relay module you show is for a WeMOS D1 Mini. It includes a transistor to switch the relay (as do all relay modules of course). If you connect the switch to emitter and collector of the transistor it will switch the relay (as long as the power to the circuit is on) with no danger to the Arduino or ESP.