Hi,
Had been considering buying one of these probes for my greenhouse, though no idea how effective they are, no micro really needed, but suppose you could connect a switch up to them ..?
**broken link removed**
Re your active probes, have you tested the worms on them - if they are as you describe sensitive beasties they might react against the electrical current, small to us but possibly dangerous to them ..??
From a backup point of view, if you are using a sealed enviroment, then perhaps a humidity sensor like the DHT11 or DHT22 might act as a fail safe incase the soil probes / wires corrode.