The used soil should be ok, but I would thoroughly wet it and then let it sit for a few days before tilling it in. Sometimes old soil dries out to the point where it is very difficult to get it to take water. I would add a bottle of peroxide to the water I wet it with to kill off any mold. I would not use miracle gro to fertilize with. I would add in lime, bone meal, and epsom salts at least and worm castings if you have them available. See how your plants do with that. If they look like they could still use some nitrogen, add blood meal or manure tea.