The DS2438 actually has two inputs where voltage can be read but generally only one is used as the other is designed for measuring current (it was originally created as a smart battery sensor, hence it also includes a temperature sensor) and I can’t recall if there are any limitations on it’s use.
There is also another 1-wire chip available with 4 voltage inputs. I believe places like Hobby Boards may stock a board with it on it, but I don’t recall it’s chip ID because it had a reputation for not being as reliable as the DS2438. It might be good enough for this usage though as the DS2438 is robust beyond belief. We’ve had enclosures fail and the chip kept working under water with salt build up across connections and everything was fine until eventually a pin corroded away or the buildup shorted the data line or ground to 12V. Truly unbelievably solid units.
I guess what I am getting at is that 1-wire has already solved most or all of what you are looking for and a probe that returns a 0-3V range is nothing more than soldering three wires to a board from the electronics side. A simple program (or maybe ospi.py plugin) that periodically monitors the soil moisture and then tells ospi.py to do a run-once program when needed should take care of the rest. And you can talk to the 1-wire bus through a GPIO pin, a USB or serial dongle or a network hub like the LinkHubE, though personally I’d love to see Ray add a 1-wire header to the OSPi with 5V out (and even better with a 12V pin as well!)…