You notice an old bag of potting soil in the corner of your garage…next to a pitchfork, dried-up paint cans, and the transmission from your old ’57 Rambler, and wonder…is this stuff still good? Does potting soil even go bad? What’s its lifespan? Should I re-gift it?
I’m not sure why it’s even called “soil,” since it doesn’t contain “real” soil. It’s a blend of various organic and inorganic materials, which usually include: peat moss, coconut coir (husks, great for water retention); compost or well-rotted manure (for water retention); perlite or vermiculite (improve aeration and drainage); and bark or wood chips (helps with drainage), and even worm castings.
Potting soil, aka potting mix, is a growth medium meant for container-grown plants. It’s designed to be a balance between oxygen, water retention, nutrients, and drainage.
Potting soil is completely different from garden soil. If you are looking for the perfect soil for your houseplants, seed starting, potted plants, and smaller container garden, use potting soil, not garden soil.
If you purchase new potting soil, it’s considered “sterile” because it has been treated with heat, steam, or chemicals to kill weed seeds and pathogens.
Garden soil is topsoil that may be enriched with compost, fertilizers, and organic matter. Garden soil may carry pathogens, weed seeds, diseases, insects, and fungi. Look at the ingredients so you know what you are buying.
According to the International Society for Horticultural Science (actahort.org), “potting soil degrades over time and eventually goes bad. On average, the expected shelf life of an open bag of potting soil is about 6 to 12 months. Unopened bags can keep for one to two years.”
While potting soil doesn’t come with an expiration date, it doesn’t mean it lasts forever in its prime condition. Over time, the nutritional effectiveness of potting soil can degrade, but it does not “expire”. Old potting soil loses its quality over time since it contains fewer nutrients which can cause stunted plant growth.
Once the bag is opened, the main concern affecting its longevity is nutrient depletion. But it can be rejuvenated by adding new potting mix, fresh organic matter, 25% compost, worm castings, or a small amount of all-purpose slow-release organic fertilizer. Add perlite or vermiculite to improve drainage and aeration. Mix vigorously…and enjoy.
Good potting soil should be lightweight to facilitate soil aeration, provide good drainage, contain essential nutrients (nitrogen, phosphorus, etc.), and have beneficial microbes. Not all potting soils contain the same nutrients since they have different purposes. For instance, outdoor potting soils need to retain moisture longer, while indoor soils should drain water quickly.
Some main indicators as to the condition of the potting soil: healthy soil is loose, and has an earthy smell. Old, spent soil often becomes compacted, and has a moldy odor, or a white fungal layer.
If your potting soil bag has small insects, it’s likely infested with fungus gnats. These gnats feed on the soil’s decomposing organic matter. Treat the soil with an insecticide.
If the soil smells like a combination of mothballs, vinegar, and rotten eggs, you are likely down to one working nostril. Some gardeners will expose the soil to the sun by spreading out a thin layer of the soil (e.g., on a tarp) to let it air dry and kill the bacteria. Then they will combine half of the old with half of the new soil.
However, less than 1 percent of readers will actually do this. Ninety-seven percent won’t. Two percent are undecided; they used to be indecisive, but now they’re not so sure.
Freezing temperatures will not cause potting soil to go bad.
Store both opened and unopened bags of potting soil in a cool, dry place to make them last longer.
Transfer unused potting soil into a suitable air-tight container.
Whenever I have some old leftover soil, I will thinly spread it around in my orchard.
When some gardeners get a little older, we like to keep things simple. Reviving your potting soil is one thing; reviving yourself is another. The next time I’m at the nursery, I just pick up a bag of new potting soil, and will usually use it sometime that week.
Out with the old, in with the new. Some things just outlive their youthfulness. I know that feeling.
Schmidt is a Poway resident with over 40 years of gardening experience.
