The easiest is to simply remove the old plants from their containers, fluff up the soil and replant. You can also mix it with compost and use it in containers, or bake it in the oven for 30 minutes before cooling completely and using it again. Microwaving is another option – put moistened potting soil in quart-size microwavable containers and cover them with microwavable lids. Finally, you can sterilize and amend the soil to guarantee that it’s pest- and disease-free before reusing.