While snagging the last 2 bags of potting soil in the garden center, you begin to frantically calculate in your head. And no matter how you work it out, you’ll barely have half the soil you need for your plant containers?
So what do you do now? What can you do? Sure, you could hit every garden center, hardware store and big-box shop in town, but odds are they’ve sold out as well. It seems like the entire world has taken up gardening this year.
And even if you could – somehow – get your hands on more potting mix, would it be worth the price? There is definitely a more cost-effective solution: but can you reuse old potting soil? Is it safe for you plants? And is reused soil as beneficial for you plants as new potting mix?
Why should I change the soil in my pots?
There are some really important reasons why you should change the potting soil in your containers between plantings.
The most obvious reason is that used soil will probably lack some of the nutrients that your plants require. They’ve been leached out of the soil by watering or used up by last season’s crop.
You might also have noticed how the potting mix in your plant containers changes over the course of the growing season: as the root system flourishes, the roots begin to fill the pot. And as the nutrients are taken up and the root ball grows larger, the potting soil becomes dense and compacted.
The soil can also collect organic matter as plant leaves fall and decompose into the soil.
What some less-experienced gardeners may not know is that plant diseases and other pathogens (such as viruses or bacteria) can live on in the soil and may infect any new plants you place in that soil. Insect infestations can be transferred via planting containers too – as can weeds.
Can potting soil expire?
Potting mix is typically made up of vermiculite, perlite, organic matter such as tree bark, and peat moss. Peat is a highly-absorbent plant that allows the soil to retain water and nutrients. But, like any organic matter, it begins decomposing soon after it’s been harvested. It will remain at its best for a year or two after buying your potting soil.
So, how can you tell if your soil is expired? The most obvious answer is to check the packaging for an expiration date. You could also open the bag and look for signs that they soil has developed mold: mycelium is a type of white mold that will leave light-colored speckles dotting your soil. If you catch the smell of rotting eggs, your soil was stored in a damp space and has developed bacteria. And if small insects have taken up residence in your potting mix, this can be a sign that something has gone wrong with the soil.
How can I reuse my old potting soil?
If you plant to reuse your old soil, there are a few things you should keep in mind:
You will need to ensure that your soil is clean before adding new seeds or plants. You’ll also need to replace the missing nutrients so that your new plants can thrive.
The safest soil to reuse is one that has never held a diseased plant. And yet, those healthy plants will have taken up many of the nutrients and left a tangled mess of roots behind. So you’ll need to remove the root mass before reusing the soil.
If you live in a cold-weather climate, it’s best to bring your containers indoors over the winter – the soil will expand and contract as the weather fluctuates and could potentially crack or damage the containers. Another option is to empty the pots in the fall – removing any roots, weeds, or plant debris and adding them to your compost pile.
Because plants will soak up anything in the soil – including chemicals and pesticides – experts recommend leaching the soil (washing the chemicals away) whenever you change the plants in your container. If you plan to keep the same plant in the container over several growing seasons, it’s not necessary to leach the soil annually.
If you can, you should pasteurize your soil to kill any pathogens. While microwaving the soil, or baking it in a 200-degree oven will safely sterilize it, it could over-dry it and reduce its ability to retain water. It may also cause the soil to smell bad. Instead, you can simply solarize the soil – placing it in a sunny location inside a black plastic bag or covered buckets for 4-6 weeks.
If you’re lucky enough to have some new potting soil, you can replace about one-third to half of the old soil with new in order to replenish the nutrients. Turning in the new soil will also help aerate the mix in your container. Again, if you keep the same plant in the same pot, you’ll only need to replace the potting soil every 2-3 years.
Why can’t you reuse the soil in tomato plant containers?
These high-energy plants just sap nutrients from the soil. And aside from leaving nutrient-poor soil behind, tomatoes that have been affected by blight can also leave behind spores that will live on in the soil. They can then spread to the leaves of new plants as water splashes onto the soil.
What materials do I need if I plan to reuse my potting soil?
Again, you’ll definitely need to pasteurize and nourish your soil if you plan to reuse it. This means that you’ll need containers for baking or solarizing your potting soil.
You’ll also want to add a slow-release fertilizer to replace those missing nutrients. Many growers also enrich their potting soil annually with compost. Adding vermiculite will improve the soil aeration and its moisture retention and help promote a consistent release of the fertilizer over time.
So, while in the grand scheme of things, while you can reuse your potting mix, it may cost just as much money to cleanse and amend the soil as it would to buy a new bag of potting soil. Perhaps in this case, newer is better. But wouldn’t you like to give it a try anyway?