Don’t throw out your used potting soil: reuse it! Photo: pistilsnursery.com
By Larry Hodgson
As you repot your houseplants into fresh soil, remember the old soil you’re removing, now too dense and excessively rich in mineral soils for reuse directly as a potting mix, can still have a second life.
The most obvious thing to do is simply to add it to your compost bin. Since it is extra-rich in minerals (mineral salts tend to build up in houseplant soil over time), it will stimulate the microbes present in the compost to work even more diligently, resulting in faster composting. In addition, any roots it contains will decompose quickly, adding yet more humus to the compost you’re creating.
Don’t worry about the presence of perlite and vermiculite in the mix. They are simply expanded rocks and no more harmful to the environment than particles of sand would be. They’ll help structure the compost when you use it in the garden later.
Another possibility is to mix old houseplant soil into the ground as you plant or transplant perennials, shrubs, vegetables, etc. Consider it to be similar in its results to adding compost to the soil.
Or simply spread the old mix over the soil among established plants (perennials, shrubs, etc.), again just as you would do with compost. They’ll then be able to profit from the minerals the used potting mix will release.
In fact, about the only thing you shouldn’t do with old potting soil is to throw it in the trash: that would be a waste!
Adapted from one originally published on this blog on April 23, 2016.