Yes you can reuse soil spider mite infested plants grew in after you treat it the right way. Spider mites need live plants to eat so they die within days in plain soil. A few weeks of drying or some heat treatment kills any mites or eggs that might remain in your mix.
I reuse my potting soil all the time and have tested different ways to make it safe for my plants. Drying it out in the sun for three weeks works but takes a lot of time to finish. Baking it in your oven for half an hour kills everything fast when you need soil right away. Both methods have saved me money on fresh soil over the years.
Spider mites in potting soil cannot live there for long on their own without food. They only eat plant sap and cannot feed on soil or dead plant bits like some other pests can. Without a live host they starve within 2-5 days under normal conditions in your home. The soil itself does not harbor mites like it does some other garden pests.
The one risk you face comes from sleeping female mites that hide in cracks and old root bits. These can stay alive for weeks in cool spots waiting for your new plants to arrive. You should remove all old roots and stems from your soil since this cuts the risk way down before you treat it further.
To treat soil after spider mites through drying you should spread it in a thin layer on a tarp in full sun. Let it sit for 2-3 weeks without any water at all. Turn it every few days so all parts dry out well in the heat. The combo of heat, dryness, and time kills mites and their eggs hiding in your mix.
For faster results you should try the bag method in hot weather during summer months. Put your soil in clear plastic bags and seal them tight with no air gaps. Leave them in direct sun for 4-6 weeks to cook. Temps inside your bag can reach 140°F (60°C) or more which cooks any pests hiding in your mix.
My go to method is oven baking when I need soil ready fast for a new plant. I spread the soil in a pan no more than four inches deep and bake at 180°F (82°C) for 30 minutes to kill all bugs and eggs. Then I let it cool and mix in some fresh compost to bring back the good microbes I cooked off.
You should always pick out all roots, stems, and leaf bits before you treat your soil. Mites can hide in these scraps and survive treatments that would kill them in bare soil alone. Sift your soil through a coarse screen to catch pieces you might miss by hand.
Add 25% fresh compost to your treated soil before you use it again for new plants. The heat treatment kills helpful soil life along with the pests you wanted gone. Fresh compost brings back the good bugs and microbes your plants need to thrive in their new home. Your reused soil will work just as well as new with this simple step.
Read the full article: Ultimate Spider Mite Control Guide
