These documents will help although the applications are different.
Direct vs Remote Liquid Level Sensing (somewhat similar application): www.ti.com/.../snoa935a.pdf
Ice Buildup Detection with FDC2214 (walks through a full system design with the FDC2214…
Hugo,
There is no TI design for soil sensing. Here are a couple of possibly relevant ones:
Liquid level sensing:
www.ti.com/.../TIDA-00317
Capacitive ice/frost detection:
www.ti.com/.../TIDA-01465