Don’t throw away the soil from your containers. It can be reused! Photo: Gary Pilarchik
Question: I’ve been emptying my outdoor planters for the winter and am wondering what to do with the soil. Should I keep it or send it to the recycling center? Can I reuse it to redo my planters next spring? If not, could I use it on my lawn in the spring?
Response: I once heard a garden center employee explain to a customer that she had to throw out used soil from her planters at the end of the season and buy fresh soil each spring. I was horrified: what a waste of a good product! Because, in fact, this soil can be used again and again for years, even for decades. Clearly, the garden center was putting financial gains from selling extra potting soil ahead of the interests of its client, but that might explain why you commonly hear this bit of unfortunate advice being bandied about.
However, it is true that soil in a container garden will become more compact over time. That’s because much of its content (peat, coir, wood residues, etc.) is organic and will therefore decompose. Therefore, it will occasionally be necessary to add more organic matter—compost, manure, fresh potting soil, etc.—when the level of the soil in the container drops.
If you don’t want to reuse the soil in a planter, it is perfectly fine to use it for almost any gardening purpose: to repair a lawn, to add to a vegetable or flower bed, etc.
But only use recuperated planter soil outdoors: never bring outdoor soil into the house (for starting seeds, taking cuttings, repotting houseplants, etc.), as it may contain microbes, diseases or insects that have no place in our homes.
What About Used Houseplant Soil
Houseplant soil is a bit different, especially if the soil is 4 or 5 years old.
Indoors, where the water you apply is essentially added to a closed-circuit system (the surplus drains into a saucer to be reabsorbed by the soil later), mineral salts—coming from tap water and fertilizer—accumulate bit by bit, to the point where the potting mix eventually becomes toxic and begins to damage plant roots. This doesn’t happen with outdoor planters, as salts are regularly leached out of the soil by rain and irrigation water and the excess drains away into the ground below. Therefore, minerals don’t tend to build up in outdoor containers.
The logical place for used soil from houseplants would be in the compost bin. As it’s very rich in minerals, adding it will “feed” the microbes, pushing them to work even more diligently and resulting in faster composting.
You could also mix used houseplant soil with regular soil in the vegetable or flower bed in a proportion of no more than one part of “contaminated” potting soil to four parts of garden soil. Again, once diluted, the “contaminated” soil is actually good for plants.
And, of course, if you can’t find any other use for it, you can also take used houseplant potting mix to your local recycling center.