Nice... but by my reckoning you'll definitely be replacing the battery before you have to fill up the water tank.
I really love it when these
instructables tell you to do silly things like use a crappy PP3 battery.
Typical mAh of a PP3 = 565mAh (http://en.wikipedia.org/wiki/Nine-volt_battery
Idle current draw of an Uno off a 9V battery: 50mA (http://www.gammon.com.au/forum/?id=11497
Not counting the sketch imposed current draw, or the relay's current draw:
565mAh / 50mA = 11.3 hours.
I get a feeling that you may want to "refine" the project a little. Yes, run it from an external power supply if you can. Alternatively (and even as well as) look at low power operation. You don't need to be sampling the soil 10 times a second - why not sleep for an hour, then sample it once, and only if it is below your threshold do you then need to worry about more frequent sampling. Also that would help reduce electrolysis of your soil probes (and needless aggravation to your worms).
Even better, replace that probe with a capacitative one that wouldn't suffer from those kind of problems...