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Every year, there’s at least $9 to $10 spent per container by gardeners to replenish their potting soil because most potting mixes contain peat-based ingredients, which break down fast.
With fast depleting potting mixes, it makes sense financially and environmentally to start rejuvenating old potting soil instead of replacing it all every season.
Heavier feeders of plants like tomatoes, cucumber and cabbage will use a lot of the nutrients throughout the year, causing faster nutrient depletion in the soil. Eventually, the soil levels drop and you need to top it up or replace the soil completely.
Rejuvenating old potting soil is a cost-effective way to grow plants, but there’s a knack to it to make sure the soil is revitalized enough so that the next crops you grow, actually grow well.
How to Know if it Makes Sense to Rejuvenate Old Potting Soil
Consider these questions as a checklist…
What did you grow in it last year?
Different plants have different nutrient requirements. Heavy feeding plants include cabbage, potatoes, peppers, tomatoes, and lettuce. Light feeders are your garden peas, carrots, and most types of herbs.
In the middle ground are beets, baby corn, eggplants, chile peppers, broccoli and sweet potato. There’s a lot of plants fit the medium feeder bill.
ResearchGate has a handy list you can reference detailing which plants need the least to most nutrients from soils. The reason it’s worth paying attention to is because ideally, you should use crop rotation with your soils every year.
Especially when you are rejuvenating old potting soil instead of starting afresh. This way, the soil you use 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 potato or carrots, and next growing season, switched to grow some herbs with.
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 fungi infections or insect infestations. Particularly problematic for tomato growers.
If your soil was affected by insects or infections, it’s probably a good idea not to re-use it just in case. If you are concerned about pests or bacterial infections in your soil and still want to try rejuvenating it, worm compositing would be your safer option.
What’s your soil level?
As the plants are absorbing 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. By how much depends on how much the plants have gone through.
The majority of potting mixes have pine bark that breaks down fast. You can top up old potting mixes with perlite, lime and/or gypsum that last longer without using new potting mix.
Of course, it is easier to use new potting mix to make a 50/50 batch of potting soil using old mixed with new, but you may still need a little something extra.
What’s the pH of your soil?
It’s a good idea to test your soil pH levels. You don’t want to skip this part because soil drops pH regularly, which is the reason for adding fertilizer.
Use a soil pH test kit to measure the pH of existing soil. The pH of soils for growing vegetables should ideally be around 6.4 (0.2 either side of that is fine).
For most types of crops that are having the soil revitalized for the first time, just perlite should be sufficient providing you use the soil for a lighter feeding plant (as mentioned about crop rotation).
Provided your soil isn’t suffering from excruciatingly low pH levels, and hasn’t been affected by bugs or soil viruses, then rejuvenating old potting soils is straightforward, although it does call for getting your hands dirty.
When you’re ready to play in the dirt…
Items Needed to Rejuvenate Old Potting Soil
Starting with a list of things you’re going to need…
- A tarp big enough for 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 own 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 enough bucket to put the soil in with holes in the bottom. If you have a big enough plant pot with holes, use that. Otherwise, use a cheap plastic bin and punch nails through the bottom of the bucket to make the holes, or easier still is to drill holes in the base of plastic bins.
- A large enough compost bin to store your prepared soil.
How to Revitalize Your Old Potting Soil
1 – Lay the Soil Out on a Tarp
The soil will need to be cleaned and any debris like rotted plant roots, weeds and dead leaves will need to be removed from the soil.
The first thing to do is take the batch of soil you want to revitalize, pour it onto a sheet of tarp and start getting rid of debris and also loosen clumps of soil.
Use a hand fork or garden rake to loosen the soil.
2 – Clean with Water
The next step is the easy part as you’re only adding water to the soil to flush it of excess salts.
Using a plastic bucket or bin with holes for drainage, fill it with the soil and then saturate it with water.
Once the water stops draining through the base, pour the soil back onto the tarp, let it dry in the sun then loosen it up with your rake or hand fork.
Then, repeat the watering process, letting the soil drain then dry, then work it loose a second time.
3 – Make a 50/50 Mix
Once the soil is cleaned and ready to be mixed, the easiest way to revitalize it is by mixing in the same amount of fresh soil potting mix to what you have already.
Using this method, you’re only topping up old soil with new soil. To mix the soil, use a soil sieve to sift in your fresh soil, getting rid of any clumps.
4 – Test the pH and Adjust as Required
Once your soil is prepared, test its pH to make sure it’s in the sweet spot of 6.4 or 0.2 either side of that.
If it’s below 6.2, add in a sprinkling of perlite, gypsum or lime to increase the pH of the soil.
5 – Add in a Slow-Release Fertilizer
You only need a teaspoon of fertilizer per gallon of soil. Add this in after any perlite or other pH adjuster has been added.
6 – 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, you’re newly recharged potting soil will be ready to release a new lease of life, letting you let get another season of good plant growth at around half the cost.
Done right, you’ll be able to reuse your old potting soil for three years using crop rotation.
Ideally, use fresh soil every second or third growing season because after three cycles of rejuvenating old potting soil, most of the nutrients that plants need will be too depleted to be able to recharge it effectively.
Using the method described above will let you grow more from existing soil, cutting your growing expenses without compromising on quality.