We know both the 555-based capacitive soil moisture sensing and the approach to "just stick an ESP8266 on it" to make a wireless sensor. However the question needs to be asked why the antenna is always OUTSIDE the soil when the 2.4 GHz band lends itself...
There's a video of this revolutionary device, able to bring led light in remote areas by using the "energy of the soil, stored in its nutrients and microorganisms released by the plants".No more details, I am a bit skeptical, but it looks like...