Can You Reuse Potting Soil?
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Replacing container garden soil every year is expensive and messy. Here's what you need to know about the viability of last year's potting soil.
About 21.2 million U.S. households tried container gardening in 2019. As the newbies among them found out, growing plants in containers requires a different type of soil than good old garden variety dirt. And while reusing it from year to year may be tempting, there are some considerations that may cause you to think twice.
What Is Potting Soil?
First, some basics. Potting soil is also referred to as potting mix because of the blend of components that go into a good quality potting medium. Typically, potting soil contains organic matter like peat moss, compost and worm castings; something to create air pockets and improve drainage, such as vermiculite, perlite or rice hulls; and some contain moisture-retaining crystals or coconut coir.
Those extra ingredients help plants thrive inside a pot or container, which doesn’t provide the same organic matter, nutrients or moisture that plants in the ground enjoy.
Can Potting Soil Be Reused?
Yes, with a caveat. While all the experts we talked to agreed that potting soil can be reused, they said that it may affect how well your plants grow.
Jessie Keith, a gardening communications expert with Sun Gro Horticulture, says, “I always find that fresh potting soil performs better, and there are several reasons why.” For starters, she says, the soil “loses its air pockets and its ability to drain.” She says that by the third year of reusing potting soil in her containers, she noticed a really big difference in performance.
Luckily, there are some easy steps you can take to get your potting soil back in good shape for planting, at least for another year or two.
Does Potting Soil Need to Be Refreshed Before Reuse?
Yes. If you want to reuse your potting soil, you first need to improve the porosity and fertility.
Porosity, or what Mark Highland of Organic Mechanics calls “the fluffiness factor,” means the presence of air pockets and drainage space that your plants’ roots need to stay healthy. Potting soils will eventually shrink down and become more compact as the organic materials in them decompose, Highland says.
Fertility refers to the level of nutrients in your potting soil. Even if you use a soil mix that contains fertilizer, Keith says, “It’s important to note that those added fertilizers generally last three to six months.” Plan on adding more, depending on the needs of the plant you are growing. Annuals and veggies need the most; shrubs and plants adapted to low-fertility can get by with less.
The pH of potting soil may also change from a neutral level over time. “As peat-based soils age, they can become more acid, and some plants simply don’t like that,” Keith says. You may want to apply some garden lime or grow acid-loving plants like azaleas and blueberries in reused potting soil.
Finally, Amy Enfield, Ph.D., associate product manager for The Scotts Miracle-Gro Company, cautions against reusing potting soil that is too dried out. “It can become very difficult to re-wet [because] sphagnum peat moss becomes hydrophobic if it gets too dry, meaning it repels water,” she says.
How to Refresh Potting Soil
To prepare your potting soil for replanting, Highland says to start with what’s already in the flower pot or container from the year before. “Come springtime, fluff it up,” he says. “Pull out any remaining roots or root balls. Chuck those in your compost pile.”
Highland says you will likely need to add fresh potting soil to refill the container. As a bonus, that layer of new soil will help block any of last year’s weed seeds from germinating.
Besides adding new potting soil, Enfield suggests also adding a soil-revitalizing product designed to improve old soil’s ability to hold moisture, provide needed air spaces and replenish nutrients.
Should You Sterilize Used Potting Soil?
No. If you’re thinking of sterilizing your soil to remove any soil-borne plant diseases, don’t. Highland warns that sterilizing soil kills beneficial microorganisms found in worm castings, compost and other organic soil amendments.
“If there is any chance that there are soil-borne pathogens or insects in the soil, the best thing to do is start with fresh soil,” Enfield says.
What Else Can You Do With Used Potting Soil?
There are other reuse beyond container use, Keith says, “I always add it to my garden beds to add organic matter and help fortify my garden.”
What shouldn’t you do with used potting soil? “Don’t put it in your compost bin,” Highland says. “It will cool it down too much.”