This project doesn't require much to get started, but you will need some hardware. It uses one Raspberry Pi, a Pimoroni Explorer Hat Pro, a submersible water pump, and some jumper wires.
The Pimoroni Explorer Hat Pro acts as the backbone of the project. It's a hub, connecting everything that interfaces with the Raspberry Pi. The moisture sensor (which is just two jumper wires pressed into the soil) is connected to analog inputs on the hat, while the water pump motor is activated using the output.
The Explorer Hat isn't necessary to complete the project, but it does make things easier. It's a compact solution that eradicates the need for a relay. It also comes with a python library, making it much easier to program on the Raspberry Pi.
You can read all about the project in detail on Reddit here. You definitely want to keep an eye on Lord-of-the-Pis. This isn't their first Pi project and it won't be their last. Lord-of-the-Pi's anticipates a part two to this project as they complete a dashboard for monitoring light, soil moisture, humidity, and temperature over time.
A developer known online as Remoheadder has created a Raspberry Pi-powered What-If Machine from Futurama.