@ray,
I like your idea of capacitance measurement using an ATTiny44. Neat.
I have no objections to using any components either off the shelf or home built. What I would aim for first is a solution that works. After that I’d look for cheap or home brew methods. However my main concern is making a device that can be used outside in all weathers. One variant would need to be lawn mower proof too and that would seem to mean burying it. Ain’t sure right now how to get 433Mhz through 6 inches of dirt. Cables are the obvious alternative. Even if I ignore the problems of in ground reliability of thin cables, there are two major worries with cabling. The first is making reliable connections under the ground. Obviously this can be solved with a decent case but that brings the cost up. Second is the chance of a nearby lightning strike inducing a power spike into the cabling spread out over a wide area – makes a great antenna. MOVs and optocoupler/isolators will minimize this but it will never eliminate it especially if the MOV gets damaged and not replaced.
Having done all the doom and gloom stuff above I will now go an buy a couple of sensors and have a look at how I think this may be done and then I’ll chuck it back to you in here and have you rip it apart. That way we might get a workable solution.