Many of us plant gardens in the spring and might be wondering if we can use old potting soil in our gardens. If you left potted plants outdoors with the soil still inside, you can still mix it into the garden or actually reuse it for potted plants. Since potting soil is technically soil-free -- it's a mix of peat moss, perlite and compost -- all you have to do is mix it with fresh organic compost. You'll end up with a nutrient-rich mix perfect for either your garden or new houseplants. Just shake out the roots from the old plant before mixing in the new compost, and you'll be good to go.