if its just the soil would not think spider mites but the do live in soil also you just usually see themm on the
plant.i get them on my hibiscus but not this year*knock on wood*I use garden safe insecticidial sopa& this is the time of year to buy it as its on clearance most places.I buy the concentrate and mix it up.it kills aphids,mealybugs,mites,leafhoppers,scale,thrips& whiteflys and a few others
I had a goast like looking bug in soil last year in my cyclamens& used chamomile tea on them and it worked.was actually for a
seed damping off solution and figured what the heck.least my soil smelled good& the bug whatever it was did go away.