Gardeners start eyeing their containers at about this time of year, wondering if they can reuse the potting soil that is in those pots and windowboxes. I wrestle with this each year too. The conventional wisdom I’ve heard (or thought I’d heard) was that the potting soil should be replaced each year. This is fine when all one has is a couple of small pots or maybe a windowbox or two. But what about when you start to get pretty serious in terms of container gardening? People are growing impressive vegetable gardens entirely in containers. The popular self-watering large (think Earthbox) containers hold about 40 quarts of container mix. Replacing that each year can really start to add up both in terms of money and in space needed for dumping the old stuff.
Perusing many of the online gardening discussion groups, I discovered LOTS of different opinions on this issue. Some people swear by just planting right into the old stuff without even mixing it up – just punching through that dried crust and planting (and then, I assume, watering the heck out of it until the material is damp again). More nervous gardeners get rid of every speck of old potting soil and then sanitize their containers with boiling water. Others sanitize the actual potting soil either with boiling water or by dragging it into the house to bake in their oven in batches (a rather smelly option). Yet another group dumps the old potting soil into the compost bin, figuring that way it actually will all be reused eventually. And finally, another suggestion is to mix the old stuff with some compost and slow release fertilizer and reuse that way.
Next, I investigated what Cooperative Extension programs had to say about it. I was somewhat surprised to discover widely varying recommendations once again. Iowa State University Extension recommended what I thought was the conventional wisdom, to NEVER reuse to the potting soil. But the generic Extension web site (www.extension.org) offered a recommendation I’d not heard before, to dump the potting soil out of containers in the fall, stir it occasionally throughout the winter, and then reuse it the next season. (A caveat is that if any of the plants in the containers had disease or insect problems, then the soil should be sterilized before reusing it. The soil can be sterilized by solarization – spreading and covering the soil with clear plastic to bake in the sun for about two weeks.)
So…the good news is that (maybe) you can reuse your potting soil. Obviously there is no one clear answer on this issue. I’d say if you did have terrible results in the containers in question last year, toss out the old stuff and start with some new. Otherwise, if you’re open to experimentation in reusing the potting soil, you’ll have to choose your method to try this year. If you aren’t pleased with the results, you’ll need to try something else next year. I’m going to risk it with some experimentation this year. It’s obviously too late for me to do the “dump it out in the fall” option, but I can dump all the pots out, stir the mix, add a bit of compost, and refill the pots.
Contributed by Carey Harrington, Certified Colorado Gardener
Extension information mentioned in this article was found at the following sites: