Yes, You Can Reuse Your Potting Soil Instead of Tossing It at the End of the Season
When your flowers fade, and the temperatures drop, it's time to empty your containers and put them away for the winter. It's tempting to keep and reuse the old potting soil, which can be pricey, especially when you have a lot of potted plants like I do. But this lightweight mix of compost, peat, perlite, and other materials doesn’t last forever. Plants use up the nutrients in it as they grow, and the mix can become compacted and filled with roots. Sometimes pests, diseases, and weeds can take up residence, ready to pop back up when you replant in the mix. However, you can remedy each of these issues and get another use out of your potting soil with a little extra work.
How to Reuse Potting Soil
It’s generally fine to reuse potting soil if whatever you were growing in it was healthy. If you did notice pests or diseases on your plants, it's best to sterilize the mix to avoid infecting next year’s plants. First, remove any roots, grubs, leaves, and other debris from the old potting soil. Then, decide on the best method for banishing microbes and insects.
One technique for sterilizing soil is called solarizing. It involves putting old potting soil in lidded, five-gallon buckets ($8, The Home Depot) or black plastic bags that are tightly tied shut and leaving them in the sun for 4-6 weeks. The heat builds up inside the buckets or bags just enough to kill bugs and pathogens.
You also can sterilize old potting soil in your oven. Place it in an oven-safe pan, cover with foil, and bake it at 180 to 200 degrees F for 30 minutes. (I tried this once, but didn't like the earthy smell it created.) It’s also important to check the soil temperature with a candy or meat thermometer ($21, Williams Sonoma) to make sure it stays below 200 degrees. Higher temperatures can release toxins. When it’s done, take the soil out of the oven and keep it covered until it cools.
Microwaving is another option. Put old, moistened potting soil in quart-size, microwavable containers. Cover them with microwavable lids (never use foil) that you can poke ventilation holes in or can leave cracked to allow steam to escape. Heat at full power for about 90 seconds per two pounds of soil. Remove the containers, cover the vent holes with tape, and let the soil cool completely before using it.
Once your old potting soil has been sterilized, you'll need to replenish its nutrients. You can do this by combining equal parts of new potting soil with the old and adding a dose of slow-release fertilizer pellets ($18, The Home Depot) according to package directions. Or, you can mix in one part compost to three or four parts of your old potting soil. Besides adding nutrients that plants need, both the fresh potting soil and compost will help keep the mix from compacting.
What to Do with Used Potting Soil
Reuse your clean potting soil in containers for vegetables, flowers, houseplants, or whatever you'd like to grow. If you're not up for sterilizing and refreshing old potting soil, you still can put it to use instead of throwing it out. It can be dumped directly out of your containers and into established beds and borders. I like to use mine in my raised beds or wherever I need to fill in holes or eroded areas in my yard. It can also be mixed into compost piles. The old potting soil you reuse can help you save money for what all gardeners want: more plants.