Gardeners who grow in containers may find themselves asking “Can you reuse potting soil?” In this article, I’m going to answer that question, along with a few others you might have about recycling used potting soil. Commercially made potting soils are expensive, and even if you follow our DIY potting soil recipes and make your own mixes, the cost can still take a bite out of your gardening budget. Reusing potting soil can help. But it won’t work in every situation, and you have to follow a few guidelines to ensure the health of your container plants. Let’s dig in!
Potting soil 101
Traditional potting soils are made from a combination of ingredients such as peat moss, coir fiber, perlite, vermiculite, bark chips, compost, and other items. High-quality brands can be expensive, especially if you have a lot of containers to fill. The need to purchase new potting soil every season is one of the reasons why some gardeners use smaller pots; the big ones are simply too expensive to fill. The bigger the pot, the more growing media you’ll need and the more costly it becomes. Reusing your old soil whenever possible would seem like a smart, budget-friendly idea, but it’s important to understand that there are times when the answer to “Can you reuse potting soil?” is yes and other times when the answer is a clear no.
Can you reuse potting soil? Well… it depends.
Aside from cost, there are a few other factors to consider when determining whether reusing potting soil is a good idea. Potting soil can be quickly depleted of nutrients during the growing season, and if the old potting soil was used for a full season, those nutrients will need to be replenished before it is used again (more on this in a bit).
If your mix contained peat moss, it’s also important to remember that most peat moss is treated with a surfactant to help it absorb and hold water. As the potting soil ages, the surfactant becomes less and less viable, making it harder to re-wet the potting soil after it dries out. This is the reason why old potting mixes often shed water or hold it in a pool on top rather than absorb it.
Shrinkage is another factor that comes into play when considering the question “Can you reuse potting soil?” Old potting mixes lose their stability and, when dry, pull away from the sides of the containers, leaving a gap between the soil and the pot where irrigation water runs off rather than soaking in. Higher quality potting soils typically have less shrinkage than cheaper brands, but most brands will show signs of shrinkage as they age, regardless of their initial quality.
One final factor to consider is the health of the plants growing in the pot. If there were any signs of disease or pest problems on your container plants and you want to reuse the potting soil, you’ll need to sterilize the soil before reuse. I cover how to do that in a later section.
When can you reuse potting soil?
There are a few times when it is always okay to reuse potting soil, rather than investing in fresh potting mix.
- If the container is a long-term home for a plant, there is no need to replace the soil every year. For example, I grow a lot of tropical plants and cacti in containers on the patio, and I overwinter them in their pots in the garage or in the house. In this case, I only use new soil when I transplant them to larger pots every few years.
- If you are growing perennials, trees, or shrubs in containers, you don’t have to replace the soil until you are ready to divide or up-pot the plants every couple of years.
- If the container is very large there is no need to replace the soil completely each season. Case in point: my file cabinet planters. I have six old filing cabinets that I flipped onto their backs after removing the drawers, painted, and filled with soil. They are way too big and heavy to empty and refill each season, so instead, I refresh and top off the soil according to the instructions in the section titled ‘Refreshing old potting soil’ below.
You can also reuse potting soil at any time and for any plants if you are willing to properly sterilize it. Let’s talk about how to do that next.
How to sterilize used potting mixes
If you plan to start seeds, grow seedlings, or plant disease-prone specimens, like tomatoes or begonias, in old potting soil, sterilizing it is necessary to kill any potential pathogens it might harbor. It’s a messy, smelly job, but if you want to give it a go, here’s how.
To kill pathogens and weed seeds, bake the used potting soil on a tray in the oven at 180 degrees F for at least 30 minutes. Spread a 3-4 inch layer of soil on a pan, cover it with aluminum foil to keep it from drying out, and place it in a preheated oven.
You can also sterilize potting soil in the microwave on full power for 15 minutes. Word of warning, though, move your microwave outside or do it with the windows open. The smell is not pleasant.
Refreshing old potting soil
For gardeners who want to reuse their potting soil but don’t need or want to sterilize it, the focus should be on refreshing/refurbishing the mix. This is essential for replacing nutrients, reactivating the mix’s ability to retain moisture, and enhancing the structure and stability of the mix so it is less likely to pull away from the sides of the pot.
Whether you plan to grow vegetables, herbs, or annuals, you’ll need to refurbish your old potting soil before reuse. Here are the steps to take:
- Dump the used potting soil into a wheelbarrow or onto a tarp using a bucket as a casual way to measure the quantity you have.
- Sift through the pile with your gloved hands or a compost sifter to remove any old roots, dead stems, or other debris and break up any compacted chunks.
- For every two buckets of used potting soil you have, add one bucket of new organic matter. This could be homemade compost from your compost bin, worm castings, aged manure, leaf mold, or mushroom compost. The idea is to have two parts old soil to one part compost or other organic matter.
- Replenish minerals and nutrients with the addition of a complete organic granular fertilizer. I like to use Plant-Tone or Bulb-Tone when growing annuals, and Dr. Earth’s Home Grown when growing veggies. Add 1/2 cup of fertilizer for every 5 gallons of the soil mix.
- Blend everything together, using a shovel to mix it well. As you stir, spray the soil mix with water until it has the moisture level of a wrung-out sponge and keep stirring. This can take a while for mixes that were very dry or very old as they may repel moisture at first, rather than absorb it. Let the mix sit overnight. Add more water and stir again the next day if necessary.
- Refill your containers with the mix and plant.
When should you not reuse potting soil
Occasionally, you may see some problems with old potting mixes that should serve as a red flag. If you see any of these issues, do not reuse the soil.
- If water failed to move through the mix and the drainage was extremely poor, don’t reuse the potting soil. Too much soil moisture for long periods of time can result in pathogen build up and lead to root rot. Not to mention how smelly waterlogged soils can be.
- If your potting media sprouted weeds, it may contain more weed seeds and should not be reused unless you are willing to sterilize it as per the instructions above.
- Potting soils that showed signs of extreme compaction should not be reused. If your soil was difficult to break up after dumping it out of the pot, skip reusing it.
- Potting soils that grew mold, mushrooms, moss, or algae should not be reused. While the problem could be a simple pH issue, it could also be something worse. Better safe than sorry.
- If your plants’ leaves, fruits, or stems showed signs of disease, various pathogens such as bacteria, viruses, or fungi might be present in the soil. The risk of reintroduction is simply not worth it. Don’t reuse the soil.
- There are many pests that lay eggs in soil, including slugs, earwigs, flea beetles, cucumber beetles, and others. If insects plagued your container plants, replace the potting mix before the next growing season.
What else can you do with old mixes?
Used potting soil can be repurposed in other ways if you don’t want to reuse it in your containers. You can always add potting soil to your compost pile. I also like to use it to pot up divisions of perennials to share with friends. Since these new plants will only be in their pots for a short time, it’s okay to use the old soil.
Some gardeners opt to bury used potting soil deep beneath their garden soil, while others turn it into their flower beds. I’ve also used mine as a layer in my potato bins.
What’s old is new again
Whether you opt to replace or replenish, now you know the answer to the age-old question, “Can you reuse potting soil?” I hope you can now make an informed decision for yourself and your plants.
For more on cultivating a healthy container garden, please visit the following articles: