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There’s a big divide within the plant community between people that reuse potting soil and those that don’t.
Now, it’s best practice to not reuse potting soil. It can harbour bugs and bacteria, and it does have a finite life span.
So why reuse it?
I reuse my house plant soil for two reasons:
- I think it’s wasteful to chuck out perfectly good (kinda) potting soil on the off chance that it’ll harm your future plants.
- House plant soil can be expensive. I’m gonna reuse it until it’s nutritionally depleted, and then I’m gonna charge it up and keep right on using it.
If you don’t agree with reusing potting soil, FINE. I’m not here to debate the pros and cons of using it.
But I do, and here’s an article on the various ways you can add nutrients back into the soil mix so that it still provides your plants with adequate nutrition.
When should you add more soil to a potted plant?
I like to add a bit more nutrition into my soil on an annual basis, for no other reason than…I have decided that that’s how long it takes plants to use up whatever’s already in there.
But you can also add more soil if you think that there’s a problem with the existing soil.
Often when you buy plants, especially if you’re not buying them from a garden centre or other specialist, the plants aren’t in great soil. Sometimes they’ve been put in a really dense mix, or something too airy, or WHATEVER.
A great way to reduce the risk of this causing a problem in the future is to take out some of the soil and add a bit of something else in, be it perlite, compost, or something similar.
If your plant isn’t growing or is growing weirdly (yellowing mottled leaves, rotting cataphylls etc) it can indicate that there’s a problem with the roots. A good way to stave of root rot, or encourage the soil to hold more water, it to add amendments that are more suited to the plant.
It can take time to work out what kind of plant parent you are, and this is more likely to dictate the amendments you add to your soil than the preferences of the plant.
Plants grow in various different substrates in the wild, so they don’t typically care what they’re grown in – they just care about the availability of water and nutrients.
If you tend to forget to water your plants, or you travel a lot, you may want to keep them in a pretty dense soil mix. If you’re an overwaterer, you’ll want a super airy mix.
You could, of course, remember to water more or stop overwatering, but it’s waaaay easier to change some soil than it is to change your habits.
Soil amendments you can add
I have a whole article dedicated to perlite here, but in short, perlite is volcanic glass that’s been popped like popcorn and can hold a tonne of both water and air.
Adding perlite to your soil is a great way to add more aeration and improve drainage. It’s pretty cheap and easy to get hold of.
Perlite is pretty much inert and doesn’t add any nutrition to the soil but it’s a great addition to store-bought house plant soil for those people that have a tendency to overwater their plants.
Orchid bark is another way to improve aeration and drainage to your soil.
It has the added benefit that it will release nutrients into the soil as it rots, though remember that rotting matter can encourage fungus gnats (read about how get rid of fungus gnats here), so make sure you keep the top of the soil dry.
Vermiculite is similar to perlite but it can retain more water. I don’t use it on house plants, because I like the soil to dry out quite quickly, but it’s great to use in outdoor soil which can dry out suuuuper quickly on hot or windy days.
You could add compost (either bought or homemade) into your house plant potting mix, but I don’t recommend it.
It’s great for your plants, and they’ll love the nutrients, but you’ll end up being driven mad by the fungus gnats that’ll show up for the feast.
Compost is also very dense, so if you add too much you can end up providing an environment where root rot-causing bacteria can thrive.
If you have a plant that doesn’t need repotting, but has been in its current soil for a long time (like, over a year), adding a bit of store-bought house plant soil is a really easy way to add a bit more nutrition back into the soil.
Regularly fertilising your plant should provide enough nutrition to mean that this isn’t a necessary step, but it’s a great way to add a bit of supplementary feed (and it’s lifesaver for people that aren’t great at remembering to feed their plants).
Make sure you get the right kind of sand – I just buy horticultural sand – but if you have too many choices ask an employee that knows about plants. Most garden centres will sell horticultural sand.
Horticultural sand has large particles and doesn’t clump easily, so adding it to your soil improves drainage.
I don’t add sand to my potting mix, because whilst it improves drainage, it doesn’t really add aeration since the particles are too small to create air pockets.
It’s a great option for plants that like dry soil because it doesn’t retain any water itself (unlike perlite, and, to a lesser extent, orchid bark), so add it to succulent’s soil.
Leca is an alternative to perlite (I have a whole article about LECA’s properties here) – it’s much bigger chunks though.
I don’t add leca to soil, because I prefer perlite. I can’t really give you a reason as to why, I just…do.
Worm castings look like dense black soil, but are basically worm poop. Worm castings are very nutritionus for your plants, but won’t cause root burn like water-in fertilisers can.
I like to top dress my plants with worm castings, because they’re quite good at retaining water, and keep the moisture in the soil below. Adding worm castings on a six-monthly basis means I don’t have to worry too much about fertilising my plants. I still use a seaweed fertiliser, but only every couple of months or so.
Making your own worm castings
You can buy your own wormery and make your own wormery fairly cheaply. I haven’t done it yet, but it’s something I keep meaning to get into.
Can you put new soil on top of old soil?
Ideally, you’d mix it in, but I quite often add a bit of soil on top of the old stuff and haven’t had any problems with it.
To be honest, it doesn’t really matter if you mix it in or not. If you top water, the nutrients will leach down towards the roots over time anyway.
If you do decide to carefully mix the old and the new soil together, try not to damage the roots, if you can.
Can old potting soil be reused?
You shouldn’t really reuse old soil as it is, beause it can be harbouring bugs, mould, and other nasties.
But as I said at the start, I do.
And I just reuse it as it is, which is a big no no.
Do as I say, not as I do, as they saying doesn’t go.
There is a happy middle ground though, between wantonly throwing new plants into old soil and putting them at risk of god knows what, and yeeting the old soil directly into the bin.
You can heat your soil and kill any nasties, then reuse.
Obvs this is an extra step I’m not willing to take, but it’s actually pretty simple.
The method I’d use if I could be bothered is to put the old soil into a black bin liner and leave it in the sun for a few hours.
Alternatively, for those of us that don’t live in reliably sunny places, you can whack the soil in the oven for half an hour at 180oC/82oF.
Oh, and if you’re going to the trouble of cooking your soil, remember to wash your pots out properly. You don’t want to sterilise your soil and then have bugs enter the pristine soil by way of old soil clinging to the pots.
Should you replace potting soil every year?
Again, I just think that replacing the whole shebang is a bit wasteful, ESPECIALLY since many store-bought potting soils contain peat, which is non-renewable. It also deteriorates and breaks down over time, unlike perlite.
If your plant is growing well, you’ll probably need to up-pot it every couple of years anyway, and you’ll be adding new soil and therefore more nutrients anyway.
The old soil doesn’t need to be removed. It may be fairly devoid of nutrients, but it won’t pose a danger to your plants. Just add more, rather than replacing completely.
Should you remove old soil when repotting?
This is really a matter of personal preference.
In my experience, leaving the roots alone as far as possible is the least damaging way to repot.
I take the plant out of the old pot, soil and all, and place it in the new pot, and then infill with fresh soil.
I find that moistening the new soil, and waiting until the old soil is dry before repotting, encourages the roots to grow out into the new soil.
If your plant is really rootbound, it’s a good idea to loosen a few of the roots, just to make sure that the roots grow into the new soil, rather than continuing to coil around themselves.
Please let me know if anyone’s had any disasters re. using old potting soil. I’m yet to, and I’ve used some REALLY dodgy soil (including some v v old stuff that was actually meant to be for outdoor plants) and haven’t had any issues.