New Life For Old Potting Soil

Refreshing weary soil (or medium) benefits plants indoors and out

Waking Up Worn Out Soil

As autumn arrives, many of us clear out planters and containers, tossing dead or aging plants on the compost heap or into the green waste bin. Some people also toss out the spent soil every year or even every season, but unless the plants were actively diseased, that’s not necessary. Indeed, many green waste collection services specifically limit or ban used soil. In part, that’s because dirt is very heavy, and even just adding lighter-weight potting soil (especially in large quantity) can make the carts difficult or impossible to empty. Also, some shredding equipment can get clogged or damaged by an excess of soil; the amount clinging to a plant’s roots isn’t a problem but adding the whole planting container’s soil might be. If you have a compost heap or system already in place, tired out potting soil can be layered in a few inches at a time, along with the usual layers of brown/green plant materials. As the compost matures, the potting soil will add body to the compost and gain new life and nutrients from its biotic companions in the composting process.

Where there’s no room for a compost heap, old soil can still be reinoculated with healthy soil biota by mixing it half and half with compost and layering it on dormant vegetable beds. Sow a cover crop like fava beans, field peas, or annual clover and by late winter, you can chop up the cover crop and let it rot in place. Scatter on some granulated humic acid, then top off the bed with more compost before setting out spring starts. You can also layer old potting soil at the back of beds, or anywhere you plan to build up soil for new beds. It’s also a good base layer for areas where you’ve removed turf and want to start a pollinator patch come spring.

Refreshing Soil & Cleaning Pots

In really tiny yards like mine, you can use a wheelbarrow or even a huge tree pot as a mixing bowl for renovating old potting soil. If the pot that’s being emptied is very deep, the upper half of the used soil can be blended with a mixture using a third by volume of old soil, a third of compost and another third of fresh potting soil. Wet it thoroughly, which may take quite a while. Especially if most of your pots are on the smaller side, the necessarily frequent watering schedule has probably stripped out nearly all the nutrients in the soil. Once the re-mix is evenly moist, blend in some slow release dry fertilizer such as Dr. Earth All Purpose before using the mixture to refill cleaned or new pots.

When plants have died for mysterious reasons, and/or when pots have crusty mineral deposits, especially on the inside, cleaning is in order. For really suspect pots, recycling is the best option. To restore good ones that are just grungy, soak them overnight, then remove the crusts and stains with a stiff wire brush. Now soak and rinse them once more before refilling and planting. If you have a large number of nursery plastic plant containers, this is a good time to prep them for re-use by you or local growers. Knock out any dirt, spiders, old leaves, etc., the sort them by size and color/type. I can always give away quarts, gallons, and even 4-inch pots that are clean and sorted, and some local growers who sell starts at the farmers market will even accept clean 6-packs or pony packs.

Aerating The Airless

The soil at the very bottom of large/deep pots tends to be compressed and airless, and may even get smelly, in which case it should be spread on a tarp and wetted down before doing anything else with it. Once it’s aerated, add it to your compost system or put a few gallons at a time in the green waste bin. To keep the soil in very large containers sweeter, add a cup of activated charcoal (the kind used in fish tank filters) to every gallon of potting soil. You can also use a 1-2 inch layer of activated charcoal on the bottom of large containers before adding the base soil. Fill the upper portion of the container with fresh potting soil mixed half and half with compost, then mix in some fertilizer as described above.

Houseplants also need periodic repotting, which is best done before the plants start to struggle or fail. Repotting is harder on a plant that’s in bloom so it’s also best to do it when a plant is finished blooming. Potbound plants are often super dry as well, so put the potted plant into a bucket of water and let it soak until it doesn’t bubble anymore. While it soaks, prepare a pot at least a few inches wider and deeper than the one the plant is already in, keeping the drainage holes clean of soil with a few bits of broken pots or washed rocks. Put in a few inches of moistened soil mix, gently tamped down, set the plant in the new pot and gently tamp fresh potting soil along the sides. Put a little fresh potting soil on top (like about half an inch) and set the pot in a deep saucer. Pour on water until it seeps out into the saucer. Let it stand for 15-20 minutes, empty the saucer and add a little liquid fertilizer. Within a surprisingly short time, your plant will rebound with new vigor. What’s not to love about that?