Fresh potting mix for container gardening protects against spreading disease from sick plants to healthy ones. But you don’t have to toss all your potting soil from last year.
I’ve reused old potting soil for a few years and there’s a variety of ways to go about it.
For starters, I’ve been very lucky with disease in my containers – I haven’t had any. If you suspect a failed plant had problem soil, don’t reuse it for containers.
Casual Notes for Reusing Potting Soil
1. Collect it in a bin. I have an old city recycling bin that I’ve always dumped spent soil into. We don’t have a compost bin so I throw away any substantial root balls.
2. Put it in the shade or place a lid on it. I actually store my rolled-up open bags of fresh potting soil on top on my open bin, the whole thing is under our stairs out of the sun. Some folks bake their soil (in the oven) to sterilize it but it’s difficult to rejuvenate once it’s totally dried out.
3. If you have earthworms elsewhere in your garden (or a friend with a worm bin), throw a few in there. This is especially good if your bin is outside and you’ll be in and out of it often, mixing it up, adding and removing soil. They’ll eat the small roots as they breakdown but this isn’t a proper worm bin – don’t go nuts with them. Worms don’t like to be sealed away without air so consider your bin and whether it’s a worm home or worm tomb.
4. Seed starter mix from failed seed starting is great to mix into your spent soil bin. If any seeds sprout when you use your old soil, just pull them like weeds.
5. When using your old soil for containers, mix with new potting soil. I do this right in the container and usually eyeball it at about 50/50. Some gardeners use very little new potting soil, some use very little used potting soil.
6. Used potting soil is great for working into your flower beds when planting new items. I don’t fuss over it much – I dig a hole, add some used potting soil, scratch it around with a hand tool, place the plant, fill, water with deodorized fish emulsion and let the wild worms do the rest.
7. I let it sit over the winter and mix in a little water come spring if the top dried out.
8. I mulch the top of my pots with pine bark chips and these end up in the bin. This helps keep the used soil light enough for container use.
9. Gardening on an apartment balcony? Did you just leave the old soil in your pots over winter? As you replant your pots, turn them out into a tub (or trash can or empty larger pot) and mix with fresh potting soil pot by pot. Stash extra used potting soil in a bag alongside any leftover fresh potting soil. You just saved yourself money (or bought more pots and plants).
Gayla Trail from You Grow Girl offers seasoned advice and, in short, you don’t need to throw it out and blow your budget on all new potting mix every season. Especially nice if you’re trying to keep it organic on a dime.