I think the comments you make are good. I do have a question about the removing the regulator. I assume you mean the 5V to 3.3V regulator.
I connect to the 5V and I see lot of variation in moisture readings due to the variation in the output of the power supply (solar panel/stepdown). I have a calibration routine for that. The voltage can go as high as 4.2V, which I believe may be bad if I connect the power supply to 3.3V.
Of course, this is not a concern if using two 1.5V batteries. With batteries, the moisture reading would be more consistent over a day. Lot of advantages there. I just don't like changing batteries and I really hate it when they leak. I have every reason to believe that my power supply will last 10 years and I am fortunate to live in a place with consistent sunshine :) . Someone replicating my project will have to take this into consideration. My design easily accommodates a different power supply.
And my power supply could be used in other projects.
Resistive vs Capacitive sensor. I'm glad you report good results. This gives people options.
Water moisture: I've had good experience with "spraying" the Arduino with clear electronic coating and sealing the enclosure with liquid tape.
-OSD
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