You can find these types of soil moisture senors all over the internet. They measure the capacitance in the soil around them and returns a value. Coupled with a wifi enabled arduino you could handle the data in a lot of different ways.
I chose to send it as a HTTP packet to a tiny flask server running on a computer. The WeMos D1 board might be overkill for this project but with its low price and available battery shield I found it suitable.

With a battery connected and deepsleep implemented you can with a few lines of code monitor your plants remotely, or setup python-telegram-bot so that you get a reminder to water your plants. The battery life on my device lasts slightly more than a month which is good enough for a old 500mAh battery I had lying around.

Images:

The Soil sensing egg

From above

Parts needed

A look inside

STL modells

Back of shell

Lid

Front of shell

You can find the repo here.

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