going out on a limb here but id say he needs to let his plants dry out far more, when i didnt let mine dry out i got soil mites, fungus gnats and springtails like hella crazy, i did some research (which i highly recommend you do as well, look for pictures that match what you have) and found what looked like the mites i had in the soil and they were predatory, infact after reading that i looked longer and deeper in the pots and actually found a little force of mites swarming a gnat larve, and just a minute later the larve wasnt moving anymore, so i checked the mites off as non threats to my plants, i also read up on springtails, again non threats, they eat bad molds and fungi in the soil, both of these the mites and springtails are actually signs of healthy soil, however i did still have fungus gnats so i went with the azamax treatment, and i actually went with the strongest strength soil drench mixture in the azamax book, just 2 days later there was 0 movement in the soil.
also as a side note Rosebud you are half right, the mature thripes that do the visable damage to the plant are white or slightly tanish and do not live in the soil, however when ready to reproduce the mature thripes go into the soil and lay their eggs, the larva and immature form of thripes live near the roots and eat them, thripes are hella annoying and can be super destructive just like spider mite, but they effect the entire plant from roots to leaf tips.
all in all unless you have fungus gnats, these white-ish soil mites arent a threat to the plant, but still i prefer to control whats in my house and i dont like little critters, good or bad, i still highly recommend an azamax soil drench just to be 100% sure there arnt harmful critters like the bad soil mites or root aphids, or fungus gnats in your soil. better to be safe then sorry i say.
i would also say to modify your watering schedule, lengthen the time between waterings by a few days, MJ for one needs a wet/dry cycle, and second making sure your soil drys out enough is key to controlling the insects, if you keep the soil wet enough your just keeping the insects habitat in the perfect conditions for them to thrive and populations explode uncontrollably, let the soil dry out well enough and the little insects will die off naturaly, they depend on the proper conditions, and a proper wet/dry cycle destroys the perfect constant conditions for the insects. it took me a while to get used to waiting longer between waterings but in the long run your plants will thrive and the insects will never be able to get a foot hold to thrive.