I also thought about doing a project like that, thanks for the write-up. I somehow can't really believe that it's that hard to measure soil moistures.

Are there any industrial plant moisturing robots? What approaches do these use?

Edit: I met this guy a few months ago at a faire: https://lambdanodes.wordpress.com/ The project doesn't seem to have advanced since then, but maybe he'll get better results with his epsilon node.