Picture the scene – you’ve returned from your local nursery with a car full of glorious, beautiful plants that you simply can’t wait to start potting up in containers. But, as soon as you start, you realize that you’ve completely forgotten to buy potting soil!
You do know that there is an old bag of potting soil lurking in the back of your shed, though. But is that suitable for your new plants? Does potting soil go bad? How long does it last?
Don’t panic! We’re here to help. Below, we’ll take a closer look at these important questions and, in the end, you’ll have a much better idea of whether or not you can use old potting soil for your plants.
How Long Does Potting Soil Last?
The first thing you need to know is that potting soil doesn’t technically have an expiration date. It’s not a consumable product and, as such, it doesn’t go bad. However, it does contain a mixture of nutrients and minerals that feed your potted plants and ensure they grow strong and healthy.
While these nutrients last a fairly long time, their quality does deplete the longer they stand dormant. When this happens, there are certain noticeable changes in the quality of the potting soil including texture and moisture content.
Put simply, since potting soil is an organic material, it’s susceptible to breaking down. This is one of the reasons why old potting soil can have a finer, dustier texture than new potting soil.
Depending on where you’ve been storing your potting soil, it may also be exposed to rain, which can speed up the break-down process and, ultimately, affect the quality of the soil.
Does Potting Soil Go Bad?
Again, since potting soil doesn’t technically have an expiration date, it isn’t considered to ‘go bad’ in the same way as food. However, the nutrients and minerals it contains do break down over time and this can make it less suitable for certain plants.
The way that your potting soil has been stored will also have an impact on its quality.
An opened bag of potting soil will retain its original quality for around six months. After this time, exposure to moisture and oxygen will cause the nutrients to break down and, ultimately, it will have less of a beneficial effect on your plants.
This doesn’t mean that it’s entirely useless, though. Old potting soil can be used as a mulch in your garden, and there are even some potted plants that thrive in poor soil conditions. These include:
- Heleniums
- Eryngiums
- Geraniums
- Thyme
- Asters
- Geums
- Sedums
- Nepeta
If you’ve got a bag of opened potting soil that you have no idea what to do with, any of the above plants are an excellent way of using it efficiently.
But what about an unopened bag of potting soil? Even though it’s sealed and no oxygen or moisture can get in, the nutrients contained in an unopened bag of potting soil will still break down over time, albeit at a slower rate.
It takes between 1-2 years for an unopened bag of potting soil to start losing its benefits. After this time, it’s best used elsewhere in the garden or as a potting mix for low-nutrient-loving plants.
Can You Reuse Potting Soil?
If you’re in a pinch and you have no other options, you can reuse potting soil that once had plants in it. However, this is something that you should do with caution, and there are few reasons for this.
First of all, if the potting soil you’re using was once the growing medium for annuals or bedding plants, it will contain hardly any nutrients. This is because the plants that were already growing in it will have used the vast majority of them.
In this case, it will be much harder for your new plants to grow strong and healthy. You may even find that you need to feed them with a liquid plant fertilizer to give them the nutrients they require.
Another reason why it’s not usually a good idea to reuse old potting soil is because it can harbor diseases and pests. These can wreak havoc on your new plants and completely decimate them before they’ve even had a chance to start growing to their full potential.
How Can You Tell If Potting Soil Has Gone Bad?
If you’ve come across an old bag of potting soil in your shed or greenhouse and you’re not sure whether it’s still suitable for use, there are some things to look out for.
Smell
First of all, check what it smells like. If it has a strong, eggy smell, there’s a good chance that it has absorbed moisture and, in doing so, bacteria has been able to grow in it.
This doesn’t mean that it’s unusable, though. Empty the bag out and spread the potting soil in an even layer in a warm, sunny spot until it’s completely dry. This will kill the harmful bacteria and make the potting soil safe to use around the garden.
Insects
If you notice tiny insects moving around or flying out of your bag of old potting soil, there’s a good chance that it’s infested with fungus gnats.
These can get in through even the tiniest hole and lay their eggs in your potting soil. Luckily, they aren’t harmful to mature plants but it would be best to avoid using the soil if you’re sowing seeds or transplanting seedlings.
Mold
If you’ve opened a bag of potting soil that has been sitting dormant for a while and it’s covered in mold, there’s no need to worry – this is quite normal.
Mold is common in potting soil as fungal spores are found naturally in both soil and fungus. It is an indication of too much moisture though, so it’s best to empty the potting soil and spread it in an even layer across the ground on a hot day. This will kill the mold and make it usable once more.
Final Thoughts
In summary, using old potting soil is fine as long as you’re planning to add it to existing soil in your garden, use it as a mulch, or plant some containers with low-nutrient loving plants. Since it will have almost no nutritional value, it’s not a good idea to use old potting soil for newly bought plants or sowing seeds.
In this case, you’ll need to return to your nursery and buy a fresh bag of potting soil that will feed your plants with all the nutrients they need to grow strong and healthy.
