They don't live in the soil, only on plants. They do, however, fly and move very fast so they can seem to come from out of nowhere. Also, being small and dirt colored they can hide almost anywhere. If you have a bad infestation and want to get rid of them for months, use a systemic insecticide with imidacloprid. The plants absorb it through their roots and translocate it throughout so it gets all of them even in those hidden places you can't reach.
