My mother has several outdoor potted plants and she worries about over / under watering her plants.
Would sild moisture sensor or something like this http://www.keyestudio.com/blog/cat/arduino/post/soil-moisture-measurement/ be helpful ?
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The linked sensor type corrodes quite fast if used continuously. If you want to measure outside soil moisture choose a sensor that measures capacitively (example).
pylon:
The linked sensor type corrodes quite fast if used continuously. If you want to measure outside soil moisture choose a sensor that measures capacitively (example).
This device can be left in the pot?
If I use the other device, I would insert into the soil and measure then remove?
Is one more accurate than the other?
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Capacitive sensors are much more accurate, don't degrade, and (at least the proper ones) are designed to be kept in the soil permanently.
I am going to buy two to try out.
I wonder how well they differentiate between wet and moist soil.
Or make one yourself 
GitHub - codewrite/arduino-capacitor: Capacitance measurement library for Arduino (or search for "capacitor" in the Arduino IDE library manager) to measure the capacitance.
Interesting to see if that works (stable) fine with soil as well
But as far as I can see from multiple YouTube video's those capacitive sensors works quite fine. But you might want to add some sealing around the edges (and protection for the electronics).