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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@allObjects
The challenge with a DC powered simple sensor such as 2 wires stuck into the ground is that various effects take place that have over time an impact on the measured values... Electrolysis/Electrophoresis/...deposition on/deterioration of electrodes. To have a lesser effect, apply power only when measuring and apply alternating polarity (AC). Another variable is calibration, which could be done by just having properly watered and waited half period and then measured... (Im sure there are publications out there about this subject in industrial/agricultural production settings).