First, I'm no electronics expert, but I suspect it would damage your humidity sensor. It's designed for the air, not soil.
Second, by doing so you'd be mixing apples and oranges. I believe humidity sensors are still designed to work in concert with the thermometer to give the relative humidity reading. Both need to be exposed to air in the same vicinity. Imagine placing the thermometer outside and placing the humidity sensor in the bathroom with a hot shower running. Suppose the station says the relative humidity is 80%. Does that mean the humidity outside or the humidity in the bathroom is 80%? Neither. The reading is pretty meaningless because the two sensors that are meant to work together are exposed to different conditions.
Third, you can get soil moisture sensors for some stations (Davis, for example).
Regards,
Kevin...