Others have already chimed in about thrips occupying the foliage but for your question specifically -
I’d probably look into an entomopathogenic nematode like Steinernema feltiae from a reputable distrubuter if I was going to treat the soil. Rove beetles and soil mites ( Stratiolaelaps scimitus Womersley ) if you want to get crazy with more preventative layers.
Gotta second @misterbee rec of
Hey lusid, no bud it wasn’t any particular reason tbh.
I was thinking I like your approach to them, they’re to be hit hard from the start.
Nope, was a nuken joke. If you look at the instructions only says for suppression on thrips so who knows. Definitely wouldn’t be best approach. I can’t stand them $!#@ers. 
Blue sticky traps and nematodes. Thrips
For now I’d try this and you can get a sample size for cheap.
I used this outdoor last summer and indoor in veg w/ little issue. Neat product smells like surf wax.
In the future I would recommend a better IPM regiment. I won’t say we all get these issues from time to time, but cleaning and a proper regiment should help. If you’re bringing in plants ( clones ) rethink that.
Sierra Natural Sciences has some decent products I have used as well. Beneficial oils and stuff you could do a drench with.
Hot pepper capsaicin may actually eat through the exoskeleton of those bugs too if they’re soft. Your plants will like that too if you spray it in the foliar just be careful because if you fuck with super hot it’ll take your breath away.
In the end best defense is a good consistent offense.
Btw I use beneficial insects as well as a regular IPM regiment.
Good luck.
When im saying soak the soil im personally using BTIs i use a product called Microbe lift.It uses a spore that attacks soil bourne larvae as well as mosquitoes.Does a pretty good job with dirt gnats and squiggly shit in the soil.I also have Dr Zymes which i have heard works well as a drench also.I have a bunch of soap nut too with all that saponin
Relentlessly shock and awe them with these. Bomb them a few more times after you think they are finally gone.
I watched a very long Youtube interview with an expert on thrips. Key takeaway I got was that in addition to the every three days treatments 4x, they are spectacularly good at developing resistance. So it can’t be the same treatmentc twice in a row.
I planted green beans, which they liked better than my cannabis, then removed all the bean leaves to depopulate the thrips.
Sprayed every leaf and stem of every plant top and bottom with spinosad, then 3 days later with isopropyl alcohol which burnt the leaves badly - I didn’t really think about how the lemon tree is fine with it because the cuticle is thick. Online it says to dilute it, but I don’t know it that would have worked. Also of note, the damage in the leaves matched the location thrips like to v insert their eggs. I have no proof, but I think the alcohol got into the invisibly damaged parts of the leaves and killed the thrip eggs along with or by drying that part of the leaf into a crisp.
Then I did Spinosad at the same drenching level again and removed the bean leaves again.
Meanwhile, 1,000 ladybugs arrived. They don’t mind spinosad, and must have eaten any last stray thrips.
I had tried beneficial mites and sprays before to no avail.
Also I noted that my attempt to close down my grow to starve them would possibly have worked if I had kept the temp in there up around 80f so they couldn’t just hide out in a sort of hibernation state.
Good luck everyone!
Mine are gone 100% now.