This sensor that SparkFun sells is an analog sensor — I think there are digital versions, which are basically the same probe but combined with an op-amp and potentiometer to trigger a digital output when the soil moisture level reaches a certain threshold (set by the potentiometer). Like this one:
On Amazon you can get a pack of 5 for less than 10 bucks:
If you search on tindie.com, you can find more choices, including this I2C soil moisture sensor:
or even a wireless one:
The I2C would allow you to connect multiple sensors on the same I2C bus (as long as you can give each sensor a different I2C address, which I assume it can be done since sensor is based on a microcontroller that can be re-programmed). The WiFi version is probably even more convenient as there is no wiring involved (although battery usage can be a big issue).