The most critical component of this project is the capacitive sensor that detects the permittivity of the surrounding soil to calculate the moisture content.
After doing some research online I found this study that showed promising results by using a capacitive touch sensor to detect soil moisture content. After already deciding that I would be using an ESP32 as the MCU for the project, I knew that the chip had several I/O pins that could be routed to capacitive touch pads.
To see if the ESP32's touch pins could be used, I created a crude capacitive probe by mounting two copper plates to each side of a wooden stake. Each plate was isolated from the steak and soil by wrapping the stake and the plates in electrical tape. Wires were also placed in the tape to connect the two plates to the ESP32-DevKitC. One plate was connected to ground while the other was connected to the pin that is Touch0. I then compiled and ran the Touch Pad Read Example from Espressif on the dev kit.
The results from running the program look promising. Touch0's value definitely had a significant change when placing the probe into a container of damp soil. The next step will be making a container of dry soil to compare the results to. Once I have data from both containers I will make a follow up post and upload the values.