Overview
Most low cost soil sensors are resistive style, where there's two prongs and the
sensor measures the conductivity between the two. These work OK at first, but
eventually start to oxidize because of the exposed metal. Even if they're gold plated!
The resistivity measurement goes up and up, so you constantly have to re-calibrate
your code. Also, resistive measurements don't always work in loose soil.
This design is superior with a capacitive measurement. Capacitive measurements use
only one probe, don't have any exposed metal, and don't introduce any DC currents
©Adafruit Industries Page 3 of 19
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