Gardeners spend at least $9 to $10 per container annually to replenish their potting soil. This is because most potting mixes contain peat-based ingredients, which break down fast.
Luckily, you don’t have to incur this cost yearly like most people do. It makes more sense financially and environmentally to start rejuvenating old potting soil instead of replacing it all every season.
Still, there’s a knack to it to make sure the soil is revitalized enough so that the next crops you grow actually grow well.
In this article, I’ll provide you with the best way to rejuvenate old potting soil.
What Did You Grow in It Last Year?
As you probably know, different plants vary in their nutrient requirements.
Heavy-feeding plants include cabbage, potatoes, peppers, tomatoes, and lettuce. Light feeders, on the other hand, are your garden peas, carrots, and most herbs.
In the middle ground are beets, baby corn, eggplants, chili peppers, broccoli, and sweet potatoes. Actually, there are a lot of plants that fit the medium feeder bill.
ResearchGate has a handy list detailing which plants need the least to most nutrients from soils.
The reason it’s worth paying attention to this list is that, ideally, you should use crop rotation with your rejuvenated soils every year.
This rotation is crucial when rejuvenating old potting soil instead of starting afresh. This way, your soil won’t need as many nutrients, so a lower-quality potting mix will still produce good plant growth.
As an example of crop rotation, let’s suppose that last year, the soil you’re replenishing was used to grow tomatoes. Then, this year’s replenished soil would be used to grow a medium feeder such as sweet potatoes or carrots, and the next growing season switched to growing some light feeders such as herbs.
This way, you can make a bag of regular potting mix go so much farther by rejuvenating the soil instead of replacing it.
Did You Have Any Soil Problems?
Some types of plants are notorious for bacterial or fungal infections or insect infestations. This is particularly probable for tomatoes.
If your soil is affected by insects or infections, it’s probably a good idea not to reuse it, just in case.
If you are concerned about pests or bacterial infections in your soil and still want to try rejuvenating it, using worm composting will make the soil much safer.
What’s Your Soil Level?
As the plants absorb a lot of the nutrients from your potting soil every year, you’ll notice your potting mix is below the lip of the pot and needs to be topped up.
Depending on the soil condition, you’ll need to incorporate new additions to get the original soil level.
Of course, you can easily use a new potting mix to make a 50/50 batch of potting soil made from mixing the old with the new.
In addition, the majority of potting mixes have pine bark that breaks down fast. So, you should top up the potting mixes with perlite, lime, and/or gypsum that last longer.
What’s the pH of Your Soil?
The final piece of the puzzle is the soil’s pH level. It’s a good idea to test your soil pH levels because they drop regularly, which is the reason for adding fertilizer.
Use a reliable test kit to measure the pH of existing soil. The pH of soils for growing vegetables should ideally be around 6.4 (0.2 on either side of that is fine).
For most crops with the soil revitalized for the first time, just perlite should be sufficient, provided you use the soil for a lighter-feeding plant (as mentioned about crop rotation).
Items Needed to Rejuvenate Old Potting Soil
Provided your soil isn’t suffering from excruciatingly low pH levels and hasn’t been affected by bugs or soil viruses, rejuvenating old potting soils is straightforward.
However, it does call for getting your hands dirty. Given that, you need a few items to be prepared to revitalize your soil. The following is a list of these items:
- A large tarp that can contain the amount of soil you’re working with.
- A garden rake or a hand fork if you’re okay with getting down to ground level.
- New bag of potting mix.
- A soil sieve (available here on Amazon in regular sizes for home gardening. If you need something extra large for tonnes of soil, see this page to make your soil sieve).
- Perlite, gypsum, or lime to boost the soil pH.
- A slow-release fertilizer (you only need a teaspoonful of this stuff per gallon of soil).
- A large compost bin to store your prepared soil.
- A large enough bucket to put the soil in with holes in the bottom. If you have a big enough plant pot with holes, use that.
Note: instead of using a bucket or a large plant pot, you can use a cheap plastic bin and punch nails through the bottom of the bucket to make the holes. Alternatively, you can drill holes in the base of plastic bins.
How to Revitalize Your Old Potting Soil
Now, we come to the actual process. Before starting, make sure to adhere to all the instructions to get the output that you’re looking for.
Using the list of items provided above, follow these steps to rejuvenate your old potting soil:
1 – Lay the Soil Out on a Tarp
The first thing to do is take the batch of soil you want to revitalize and pour it onto a sheet of tarp. Then, loosen clumps of soil using a hand fork or garden rake.
2 – Screen the Soil
You’ll need to clean the soil and remove any debris like rotted plant roots, weeds, dead leaves, and large chunks of material.
You should also remove any pests, such as grubs, that might be hidden in the soil.
3 – Clean with Water
This step is the easy part as you’re only adding water to the soil to flush it of excess salts.
To do so, use a plastic bucket or bin with holes for drainage. Fill it with the soil and then saturate it with water.
Leave the soil to drain for some time. Once the water stops draining through the base, pour the soil onto a tarp. Let it dry in the sun, then loosen it up with your rake or hand fork.
After that, repeat the watering process, letting the soil drain, drying it, and then working it loose a second time.
4 – Make a 50/50 Mix
Once the soil is clean, the easiest way to revitalize it is by adding a new potting mix to the old one. Try to keep the ratio of new to old soil around 50/50.
Using this method, top up the old soil with the same amount of new soil. Then, use a soil sieve to sift in your fresh soil, getting rid of any clumps.
5 – Test the pH and Adjust as Required
Once your new soil mixture is ready, mix a portion of the new and old potting mix. Then, test the pH of this mixed portion to ensure it’s in the sweet spot of 6.4, with 0.2 on either side.
If it’s below 6.2, add in a sprinkling of perlite, gypsum, or lime to increase the pH of the soil.
6 – Add in a Slow-Release Fertilizer
For fertilizing, all you need is only a teaspoon of fertilizer per gallon of soil. Add this after you’ve added any perlite or other pH adjuster.
Besides the slow-release fertilizer, you’ll need to add a bit of worm casting with a few worms as organic fertilizer.
7 – Mix the Soil Well
After adding the fertilizer, moisten the mixture with a brief portion of water to make it easier to mix and activate the beneficial microbes in the soil.
Then, gently mix the old and new soil with the other ingredients with your hands. Continue mixing until you distribute all the soil ingredients equally.
8. Let It Cure
The final stage is storage. Fill suitable containers (plastic or a cardboard box) with the rejuvenated soil and store it somewhere dark and away from moisture.
Leave it for at least two weeks before using it.
After a couple of weeks, your newly recharged potting soil will be ready to release a new lease of life, providing you another season of good plant growth at around half the cost.
Using the method described above will let you grow more from existing soil. This means cutting your growing expenses without compromising on quality.
Rejuvenating your old potting soil correctly, you can reuse it for three years by following crop rotation.
That said, I recommend you use fresh soil every second or third growing season.
This is because, after three cycles of rejuvenating old potting soil, most of the soil nutrients will be too depleted to be able to recharge it effectively.
Growing up with a mom who filled her home (inside and out) with all sorts of plants, Lisa got her start in gardening at a young age. Living now on her own with a home and yard full of plants (including an indoor greenhouse), she shares all the gardening tips she’s gained over the years.