Yes, we’ve considered supporting soil sensor but the difficulty is that the sensor ports on OS are all digital pins, so they cannot read analog signals. Technically capacitive soil sensors are digital sensors and they can output a digital signal where the frequency is correlated with the soil moisture. However, I am concerned that as the frequency can be very high, it’s not feasible to have OS read that signal as it will be wasting a lot of CPU cycles just to detect the signal frequency. So at the moment the best way is to use an analog soil sensor connected to a analog to digital threshold converter which converts the analog signal to a binary digital signal with a user defined threshold. This way the soil sensor becomes similar to rain sensor. This is also what firmware 2.1.9 supports — it allows a soil sensor with binary output.
Hunter’s Soil Clik sensor basically works this way, albeit quite pricy:
it’s equivalent to a soil moisture version of rain sensor.