Studies have shown time and time again that having houseplants can help fight depression, and many also remove toxins from the air.
But having houseplants can get pretty expensive, and, let’s face it; the economy hasn’t been that great in the 21st century. This has led more and more indoor plant enthusiasts to consider reusing their old potting soil.
Even a quality economy brand such as Miracle-Gro can get quite expensive when you consider replacing every houseplant’s soil every 2 to 3 years.
But we’ve often warned against reusing soil, so what happens when you find you have no other choice?
Should You Reuse Potting Soil?
Reusing your old potting soil isn’t ideal, but it’s not impossible.
Let’s go over when soil may be salvageable and how to recycle it safely.
Why You Shouldn’t Reuse Potting Soil
Four very important reasons we try to discourage reusing old soil.
The primary reason is that the soil is drained of its nutrients over time.
This is often solved in your garden soil by using a compost pile, planting nutrient-enriching crops such as peanuts, or allowing the land to fallow. However, using some of these methods on potting soil can be more difficult.
The second reason is an obvious one: the risk of pests or diseases. There’s always a risk that fungal spores, bacteria, or pest eggs have invaded the soil, especially in your container garden.
Considering these contaminants can sometimes survive for months or even years in the soil, the risk is quite high.
The third reason is a lot harder to spot but no less important. As you fertilize your plants (and especially if you’re using unfiltered tap water), various mineral salts get trapped in the soil.
These mineral salts will eventually become toxic to your plants. While you can flush the soil to remove them artificially, this also leaches nutrients from the soil.
Reusing the potting mix can cause these salts to continue building until they affect your plant’s health.
Finally, there’s the matter of soil compaction. When you first open a bag of potting mix, it will feel light and fluffy because it’s full of tiny air pockets.
Unfortunately, these air pockets collapse over time, causing the soil to compact.
In nature, worms and other little critters help restore these air pockets, but you don’t have that luxury with potted plants.
As a result, your plants will slowly lose an important source of oxygen and have more trouble absorbing nutrients.
Simply breaking up the potting soil before reusing it won’t restore that initial fluffiness, making your plants work harder to sustain themselves. So it’s better to use fresh potting mix, as it performs better.
Reasons You Might Choose to Reuse Potting Soil
So now that we’ve looked at the potential problems let’s think about why you might want to reuse that old soil.
The most obvious reason is budget. As we mentioned earlier, good potting soil isn’t cheap, even for economy brands.
The average potted plant will need to have its soil replaced every 2 to 3 years, with a handful of plants needing it every 4 years or as often as annually, depending on how fast they grow and how heavily they feed.
Even worse, you can’t just buy any old cheap brand because many skip the sterilization process to reduce costs.
Thus, you can either throw away this soil after one use and waste all that money, or you can find a way to recycle it and recoup some of those losses.
Another factor is also related to cost but has to do with amendments. Chances are, you’ve amended your potting soil with an aggregate, such as coarse sand, perlite, or vermiculite.
This adds to the amount of money wasted when you discard the soil, especially since these aggregates don’t lose their functionality over time.
The third reason is simply a matter of convenience. Going to a garden center for a single bag of soil can be annoying, and there’s a good chance you’ll end up spending more money on additional items to justify the trip.
And when you order online, there’s no guarantee the soil will arrive in good condition, not to mention having to either pay extra for overnight delivery or wait several days for it to arrive.
Moreover, you can reuse potting soil to fill the holes when cleaning up container plants. It also helps pasteurize and get rid of potential threats, including pathogens, weed seeds, and bug eggs.
Tips On Reusing Old Soil
So let’s say you’ve decided you want to try reusing the old soil for plants.
You can’t just toss the old soil in a new container and call it a day.
Instead, there are several important steps you’ll need to follow to recycle the used potting soil before it is ready for more healthy plants.
Step 1: Inspecting Soil for Reusability
So let’s take a moment to look at ways to tell if your soil can be recycled.
Inspecting the soil and keeping at least a mental record of the soil’s history is important.
Ask yourself the following questions:
- Does the soil show signs of fungal infection (such as green or white discoloration of the soil surface)?
- Does the soil smell rotten?
- Has the plant suffered from insect infestation or disease issues?
- If the plant has battled the disease, was it bacterial, fungal, or viral in nature?
- How old is the potting soil?
If you answered yes to the first three, you would need to sterilize the potting mix if you want to reuse it, although this process can be less than pleasant.
For the fourth question, if the disease is viral, you will need to discard the soil.
If the disease is bacterial in nature, you may or may not be able to salvage the soil, so be sure to check up on whether the bacteria can be easily destroyed.
Moreover, if you got your potting soil from a dead plant, removing its roots from the potting soil and container is important.
Finally, the more you reuse potting soil, the less you’ll be able to restore it, so you should try to avoid recycling the soil for container use more than 2 to 3 times (although you can still recycle it to add to your garden bed after this).
Step 2: (Optional) Flushing the Soil
This simple step isn’t necessary but can make a difference over time.
Before you flush the soil. Make sure to let the potting soil dry out. Next, place the soil in a sterile container that has drainage holes.
Next, sit the container in your tub or shower and water it slowly so you see moisture seeping out of the holes.
Pause for a few hours until the water stops seeping, then pour more water on top of the soil (you don’t have to go as slowly this time).
Continue doing this a few times (a common practice is to use approximately 4 times the amount of water the container can hold in total) to ensure all mineral salts are flushed out.
Step 3: Sterilizing Old Potting Soil
This is an important part of recycling your potting soil, although many people choose to skip it and take their chances.
Their reasoning is that sterilization will also kill beneficial microbes or use organic materials such as worm castings.
However, we’ll be addressing this very issue later on.
Of course, sterilizing isn’t always fun, and it can get a bit smelly, but it’s a step you should try not to avoid.
There are two main methods of sterilizing your soil: baking and solarizing.
Baking
This method is popular because it’s fast, but it can make your house smell like baked dirt for a while.
Set your oven to between 180 and 200° degrees Fahrenheit.
Spread the potting soil out on a baking sheet and cover it with foil.
Bake it for 30 minutes (you may wish to use a meat thermometer to ensure the soil temperature has reached 180 and 200° degrees Fahrenheit before removing it).
Avoid going over 200° degrees Fahrenheit, as this can cause the soil to begin releasing toxic gasses.
Once the soil is up to temp, sit it somewhere so it can cool off completely,
Note that you can also use a microwave in a pinch.
Place the soil in a microwavable container with a vented lid and nuke it for 90 seconds for every 2 pounds of soil.
Remove the bowl and cover the vent holes with tape, then let it sit covered until it has completely cooled.
Solarization
This is much slower than baking, but the smell will at least be outside.
Place the soil in 5-gallon buckets, black plastic bags, or a lidded plastic container, and seal them tightly.
Sit the container in a warm, sunny spot where they’ll catch the midday heat.
Allow the container to bake in the sun for 4 to 6 weeks to cook out bacteria or microorganisms.
NOTE: This method is best done during the summer when the baking process will be most efficient.
Step 4: Restoring the Old Soil
You will need to do three key things to the soil before it can be reused: refresh, amend, and fertilize.
Refreshing
Take your sterile soil and break it up as much as possible, then run it through a fine sifter.
This will separate all of the old aggregate materials as well as any broken roots or other debris.
It will also help to restore some of that original fluffiness found in fresh soil.
If you so choose, you can pick through the larger debris to recycle some of the aggregate material.
Amending
There are two types of soil amendment you will need to add to the soil for reuse
The first is an organic material, which can be organic compost, peat moss, orchid bark, or similar amendments.
The organic material should account for ⅓ of the final soil composition for most plants.
Unless you’re using compost, a good potting soil mix would include mostly orchid bark or coconut coir with a little bit of charcoal and either peat (for more acidity) or sphagnum (for more neutrality) moss.
Meanwhile, you’ll also want 1/3 of your soil mix to be an aggregate.
Aggregates improve drainage, help reduce soil compaction, and can include coarse sand, fine gravel, perlite, or vermiculite.
Fertilizing
Finally, unless you amend using organic compost, you’ll need to give the soil an initial fertilization to help restore lost nutrients.
Ideally, you should perform a soil test to find the soil pH and nutrient levels, then fertilize accordingly to make the soil quality appropriate for the plant you intend to give it to.
However, in a pinch, you can use a balanced houseplant fertilizer, slow release fertilizer, or some worm castings to help bring back the nutrient content. Other popular alternatives include liquid fertilizer or organic fertilizer.
Step 5: Reusing Your Old Soil
Finally, your recycled potting soil is ready for use.
After adding the soil to a sterile container, keep an eye on the plant for any signs of nutrient deficiency and further amend the soil as needed.
You should achieve a healthy balance for the plant within a month or two after reusing the clean potting soil.
Final Notes
Reusing old potting soil isn’t ideal, but it can be done with a little time and dedication.
However, each time you reuse the soil, it will become increasingly less viable. Reused old potting soil is known to become more acidic over time, so it’s best to grow acid-loving plants to raise the pH.
Regardless, this means you will want to discard the recycled soil after 2 to 3 reuses or recycle it into the garden compost pile where nature will use it.
