If you raise houseplants or tend a potted garden, you'll often find yourself throwing away buckets of potting soil whenever you repot your plants. Reusing old potting soil is generally not a good idea because such soil is spent of nutrients and may harbor pests and plant diseases, but budget-conscious gardeners can rejuvenate old potting soil to improve its composition and make it safe for use again.
Sterilize the potting soil. This kills any disease organisms or pests that may have grown in it over time. Spread the potting soil on a baking pan. Cover the pan with aluminum foil and bake it at 180 degrees Fahrenheit for 30 minutes, according to Colorado State University.
Remove the potting soil from the oven and let it cool to room temperature. Scoop the cooled soil into a bucket using a measuring cup so you can estimate the general amount of potting soil that you have.
Add compost to the sterile potting soil in an amount that's equal to 30 percent of the potting soil's volume. This adds micronutrients and beneficial bacteria that were lost during the sterilization process.
Mix in coarse sand, perlite or vermiculite at a rate of 30 percent of the potting soil's volume. This improves the soil's drainage.
Use the resulting mixture as you would any standard potting soil. After planting plants in the rejuvenated soil, remember to fertilize the plants according to their species-specific needs.