Ok, right up front I'll admit that combining an $1 ADS1115 board with these cheap soil moisture probes is already a decent soil moisture solution for most projects. So I was just messing around here to see what I'd get:  

https://thecavepearlproject.org/2020/10/27/hacking-a-capacitive-soil-moisture-sensor-for-frequency-output/

Pin powering the low-power TLC555 worked well, but at the low-ish output frequencies I was using it's really only good as a water level sensor because the probe polarizes over time in soil. But I'm sure I'll find a problem for this solution sooner or later  :-)