Hey mg,
Not a pro here by any stretch of the imagination but soil dwelling mites are not always spider mites, I learned this the hard way. Soil mites can be quite beneficial and actually help prevent fungus gnats infestations. Spider mites are usually 2 spotted spider mites and (I think) they prefer foliage. Try to identify them first.
Are they in the foliage?? Or just the soil??
I had a plant with tiny whiteish or off white soil mites they were line dancing on bucket rims, I nuked the crap out them. Sulfur dust, oil dormant spray, soil drenches with Spinosad, neem oil, SNS and for weeks no matter what I tried - I could not get rid of them. I actually burnt the plant will all the sprays and developed root rot from nearly daily soil drenches. In the end the mites lived and the plant died. I tossed that soil outside
fast forward I had been slow cooking a large batch of LOS and planned to put that into use, ordered nematodes and sprayed all my soil bins, grow bags and containers of the new soil with the nematodes. Also ordered hypoaspis miles which is predator mites and released them into my two 30 gallon grow bags, there is a 2 inch layer of rice hills as ground cover and I’m using birdseed to grow grass as a ground cover and nitrogen fixer. I still see occasional fungus gnats which I think are visitors that come into basement via doggie door to escape heat but don’t think they are able to get a foothold in the soil due to the predators. Check out arbico organics or build a soil both are pretty well known I’m sure there are others too. Nematodes, earthworms, hypoaspis miles predator mites and rove beetles all to work the soil; they tunnel, nest, mate, poop, eat pests and die in the soil which also puts nutes back in.
brb with link....saw this video about this time last year so I’ve been working in this direction for a while. I still don’t have the rove beetles established in my soil but that’s next on the list, plus I want a colony of lady bugs to work the foliage. Nunyabiz is a very skilled grower - this is his soil, see post #4
Nunyabiz soil critters video < < <
also see post # 424 in my journal for close up pic of predator mites
013 predator mites