I have a pretty similar project. But the problem with the mainstream moisture sensors is that they break after some time (or I did not find a good one). For myself I found the expensive solution a flower power from parrot http://global.parrot.com/au/products/flower-power/ a handy solution. It workes with bluetooth and they have some documentation. I did some working for python: https://github.com/Dlotan/flower-master-fab/blob/master/app/... and get good results over a long perioud without too many outliers