Advice needed - (spider)mites in soil
Last year was the first year my partner and I attempted some balcony gardening. It went great until we went on holidays for a few weeks and came back to a completely out of control infestation of spider mites, which killed almost everything we had. We had someone come in and water for us, but they hadn't noticed the spider mites until we came home and then it was too late. We obviously threw all the dead ones out, disposed of the soil and washed all of the pots. The exception was the big planter box shown in the picture because of the sheer volume of soil in it, it felt like a waste to throw all that out, to to mention a lot of work.
Jump forward to this year, we did a test plant of a raspberry cane in the same planter box, again went away for a couple of weeks and came back to it covered in mites (presumably spider mites again, but I'm not sure). I unfortunately don't have a picture of it when it was covered in them, but I tried to take a good picture of some of the dead ones after they had been sprayed a couple of days ago with the spray in the image (apologies for the German on the bottle).
The raspberry cane I'm pretty sure is beyond saving, but I want your guys advice on what we should do next. Do we really need to dispose of all of this soil and thoroughly clean the inside of the planter, or is there a way we can somehow treat the soil to stop them swarming all over the tomato plants we will soon put in there?
Thank you all very much!