Instead of discarding old potting soil when you're transplanting your plants or repotting your container garden, reuse it to save yourself money. Although old potting soil is often devoid of minerals and nutrients and sometimes infested with diseases and insect pests, you can sterilize your potting soil and restore its nutrients to make it ready and safe for re-filling your containers.
Preheat your oven to 180 degrees Fahrenheit. At that temperature, according to the Washington State University, most plant disease-causing bacteria and viruses will be killed, as well as weed seeds and insect pests. You can "bake" old potting soil in the oven to sterilize it.
Pour the potting soil you wish to reuse into a metal or glass baking pan. Layer the soil no deeper than 4 inches. If you have more potting soil than that, you'll need to do this step in multiple batches.
Place the pan in your oven and let the potting soil bake for 30 minutes. Remove the pan after the time is up and allow the soil to cool to room temperature.
Pour the sterilized potting soil into a bucket and add perlite and compost. Measure the compost and perlite so they each equate to approximately 30 percent of your sterilized potting soil's volume.
Use the sterilized, amended potting soil just like you would any other potting mix.