My plant watering kit moisture readings jump up 10 to 15% every night, and then back up in the morning. This seems to correspond to when my thermostat turns my heat down to 63 degrees F and then back up to 68.
Does anyone know if this temperature dependency is an issue with the Nano RP2040 Connect or the capacitive moisture sensor?
water condensation?
I agree with @GrandPete
I guess it's possible moisture is condensing on the sensor in the soil. Kind of surprising it's that sensitive. I would think the thermal mass of the dirt and moisture in the soil would keep the temp fairly constant.
if it is actually water condensation, your proposal is correct but you must think this is atmospheric water condensating (and therefore capturing heat from the surroundings).
You may see it as a foreign activity (the atmosphere) applied on a static system (your soil).
At least, yes, the signal shift is surprisingly strong. you actually made a dew point measurement system!
The relative humidity in my house is 25%, seem like a long way to go to get condensation especially since I'm only dropping the temp at night by less than 10 degrees F.
to change the temp in a system is a good way to produce condensation or evaporation, and you don't need to have a boiling bath in the middle of your home for the mechanism to appear (just think about the condensation on the window in the early morning).
If I zoom on the sensor itself, the conductive part is a thin metal plate, the perfect place to drive shift in temperature, and directly link to the sensor.
You may try to put just a drop of water there (without changing temperature) and see the result. It could be consistancy with what you see during the night.
How was the probe calibrated?
I stuck my dried probe into the air got an average of 100 readings and use that value for dry. I stuck the probe into a glass of water, took an average of 100 readings and use that value for wet. My capacitive soil probes, have the electronics covered in silicon and are buried under the soil by an inch. The plants are in an enclosure to keep the humidity high and pot heaters are ran 24/7 and get 13 hours of plant light per day. I experience minor fluctuations of soil moisture content but no more than 2% over 24 hours.
I'm on vacation for a bit and have set my thermostat back permanently to 60 degrees F. I'll confirm the fluctuation goes away. If so I can always modify the software to not read the moisture sensor at night. 
yeah good idea to test!
at least, your hardware is not an issue, so everything is fine!
Not clear to me why you don't want to see the moisture variation but actually: your module, your needs!
did you see the idahowalker answer: his module's sensitive parts are covered, so what could be the temp variation, no water would have the possibility to fall on it.
As far as the variation, I just figured I should read a fairly straight line slowly falling as my plant dries out. Since that isn't the case the engineering in me wants to know what's going on.
My ultimate goal is just to monitor when my plant needs water...... and to mess around with micro controllers.
Contact Arduino Customer Support via Contact Arduino, they will assist you resolve the issue.
Starting at 23 Feb this shows the increase in moisture level at night when my home thermostat sets back from 68 to about 63. Around the 27 Feb is where I dumped a bunch of water on the plant since I was leaving for vacation. The very last dip and then straight line when I put my thermostat on permanent hold since I'm out of town.
If I want to smooth out the curve looks like I should only take readings in the day when the ambient temperature is constant.
Which soil moisture sensor are you using?
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