A trio of Grow soil moisture sensors, for use with Grow HAT Mini or other devices.
These capacitive moisture sensors work using Pulse-Frequency Modulation (there's more info on that in the Grow tutorial). They're a lot less vulnerable to corrosion over time than old school resistive sensors, as there are no exposed electrodes.
There's a handy indicator line to show you how far they can be pushed into the soil, and they even have a little area so you can label them with plant names (we think 'Pete' is a great plant name).
It's possible to use Grow moisture sensors on their own with non-Raspberry Pi devices (like Arduino), as long as they're capable of monitoring pulses on an IO pin in the range of ~2Hz up to ~30Hz accurately.