I can never remember to water my household plants. Why not let them do some of the legwork?
That’s where the Chirp! Plant watering alarm comes in. It’s a really simple sensor for detecting soil moisture. Installation amounts to sticking the sensor into the soil you want to water and clicking the button to calibrate it to a dry value. When it’s time to water, the device emits a “beep” using the onboard buzzer. The folks at Catnip Electronics also designed a light sensor into the board to disable the buzzer at night. How thoughtful! There’s a whole bunch of documentation available for the Chirp! at Catnip’s homepage, including Github links to the PCB files, schematics, and source code.
I already want one of these for my houseplants. I’d love to replace the buzzer with a simple speaker so it can deliver one of my favorite Bill Murray lines: “You can’t go! All the plants are gonna die!”. Lucky for me, Catnip also makes a sensor module version for use in your own projects. Just communicate with it using I2C and be as creative as you want with how you formulate your plant-in-distress alerts.
Of course there are many other options if you do want to prototype soil moisture sensing. Seeed Studios has a soil hygrometer built for their Grove module system; this one uses CANbus for communications. Icstation has a couple of different versions depending on your needs. And Mmm999 builds one for Arduino that has hardware adjustable sensitivity.