If they're just on the soil, don't assume they're bad for the plants. Where they've infested your regular plants, do they just stay in the soil or can you see them eating the plants? Some mites are beneficial because they eat and break down organic matter in the soil. They usually leave plants alone because they'd prefer the mostly rotted stuff in the soil.
If they are eating up your regular plants try diatemaceous earth. You can find it at most large greenhouse supply places. It kills pests two ways,.. by cutting up their exoskeleton and by drying them out and preventing them from hydrating. They're little tiny organisms based on silicates that have razor sharp edges. They also provide some trace elements for your plants. Use a dust mask when you apply it, or mix it into water and pour it onto the soil surface and let it dry.
You can also find it in some health food stores,... the food grade stuff helps people get rid of parasites like worms. I'm not sure the food grade is as good for pests, because it may not have as much as the razor variety as the pest killer. It's cheap too,... bout $10 for a few pounds which is more than you'll ever need, unless you're growing pounds of bud.
It can also be used on pets to kill ticks,..etc. It's completely organic and beneficial to your plants and pretty much ALL pests hate the stuff. Don't get the type used for filtration. That's chemically and heat treated harmful to your health and mostly useless for what you need.
