I have a Rapsberry Pi 3 A+ with an Adafruit Capacitive Touch HAT for Raspberry Pi (MPR121) and the idea is to plug it to the wet soil of a plant and it detects when I touch the plant (or a leaf) and plays an MP3 but it's not detecting the touch at all to have a result I have to touch the alligator clip connected to the plant directly.
The other thing that I tried is use a MakeyMakey and It works pretty well but you have to have a ground attach to the person.
I also tried a cheap version of the MPR121 Breakout (AliExpress) that have the same problems as the Adafruit HAT.
My code is in this repo if you want to check it out. I'm using Python3.6 with the Adafruit library for MPR121.