The Components of Potting Soil
Technically, potting soil is not soil. It doesn’t contain a speck of the good old familiar dirt found in lawns and gardens. Instead, it’s a carefully formulated growing medium designed to nurture plants in containers—i.e., pots. More accurately referred to as “potting mix,” this stuff is comprised of sterile filler plus organic enhancements that help plants thrive in close quarters, including:
Peat moss and/or coconut coir help retain moisture that would otherwise be lost through a container’s drainage holes. Perlite (volcanic glass that appears as tiny white balls) and/or vermiculite (which resembles sawdust) also help retain moisture while assisting aeration for healthy roots. Pine bark helps prevent disease and maintains a stable environment for plants during temperature extremes. Fertilizer may also be added to promote new growth and nourish plants.
As these components are depleted, the mix may become compacted and be more prone to infection, weeds, and pests. This can make the growing medium less efficacious at best and harmful to plants at worst.
The Lifespan of Potting Soil
A 25-quart bag of potting soil costs about $10—hardly dirt cheap! Especially when you consider that a 10-inch standard clay pot typically needs about three gallons of potting soil. Fortunately, most potting soil can last up to two years because that is the lifespan of the peat moss it likely contains. (Tip: To keep it as fresh as possible, store it sealed in its original bag or another covered container in a climate-controlled environment.) As long as used potting soil still looks fairly fluffy and doesn’t emit a rotten odor, gardeners should be able to use it again with good results. However, if the plants formerly grown in the potting soil struggled with disease or insect problems, it’s probably best to discard the mix and start fresh next season. Beyond a hard consistency and unpleasant smell, there are other signs that potting soil is over the hill, including white patches that would indicate mold growth and the presence of visible pests, such as gnats. However, some used potting soil may be past its prime without visible evidence, which is why some gardeners recommend sterilizing it before reuse.
The Pros and Cons of Sterilization
The purpose of sterilizing potting soil before reuse is to get rid of disease-causing microbes. There are two main ways to sterilize potting mix:
Solarizing entails putting the used mix in buckets or black garbage bags, sealing it tightly, and leaving it out in the direct sun for four to six weeks. As heat builds up inside, it kills bacteria and other pathogens. Baking or microwaving is a much faster sterilization method. Put the potting mix in a baking pan, cover with foil, and place in the oven until it reaches between 180 to 200 degrees Fahrenheit. Bake for 30 minutes and then let it cool completely before use. Alternately, you can put potting mix in microwavable containers, cover with vented lids, and nuke at full power for about 90 seconds per two pounds of soil. Remove from the microwave, tape over the vent holes, and let cool fully prior to use.
Not all gardeners advocate sterilizing potting soil, however. Some say the practice will also kill desirable organic material that might have found its way into the mix, such as worm castings, which combat disease and assist in nutrient uptake. What’s more, leaving black bags out in the yard for weeks won’t exactly beautify it, and baking or nuking potting soil can give off an unpleasant smell.
Refresh Before Reuse
One key to success when reusing potting soil is to follow the farming practice of crop rotation and simply grow a different type of plant the following season. This is especially true with potting soil used to grow tomatoes since these high-energy plants tend to zap the growing medium. Once you decide what to plant, try these tips to boost used potting soil:
Start by examining the potting soil, stirring it with your fingers or a hand rake, a few quarts at a time in a large sieve. This will fluff it back up and allow you to remove and discard any large seeds, roots, or other clumps. Fertilizer included in fresh potting mix only lasts about three to six months, so you may wish to mix in fertilizer with a ratio that will suit the plants you intend to grow next. Depleted peat moss may have made the potting mix more acetic. Either grow acid-loving plants such as azaleas (Rhododendron x Gable) or bleeding heart (Dicentra Spectabilis) or stir in some garden lime to raise the pH. Hedge your bets by mixing some new potting soil with the old stuff.
Potting soil that looks and smells fine after two years may retain some benefits. But rather than risk it in containers, consider beefing up ordinary garden soil by mixing it into raised beds or in-ground plots. Used potting soil can also add desirable volume and organic matter to a compost pile.
