Thanks for the tips. My main goal is to get an accurate sense of when the air close to the surface freezes. The second purpose would be to have a pseudo-solar sensor that shows the solar heating during bright sunny days, although I would sacrifice that if the freeze sensing was more accurate.
I’m not sure if I need to protect it from moisture or if I need to sense the evaporative cooling when the ground gets wet.
Maybe I attach it to a heat sink, painted to match the reflectivity of the soil, then invert the heat sink with the fingers into the surface of the soil.