Black fly in soil?

[LemonVerbenaGeranium]

Does anyone know whether blackfly live in soil as well as on plants? I have a plant that is riddled with blackfly year after year, and I’m finally getting rid of it, but it’s in a big pot and I wonder if I need to get rid of the soil as well?

If the soil has been in the pot a few years and you want to plant something else in it, you would need to replace the soil with fresh compost anyway.

As far as I know, blackfly doesn't live in soil, but you can get aphids on roots.