If you’re new to gardening, then you might find it hard to understand what soil sterilization is. You may have also come across potting soil at gardening stores that say sterile on the label. It’s not a complicated concept, but there are some things you need to know before you decide whether or not you need to sterilize your soil on your own.
Not all potting soil is sterilized in the traditional definition of the word. In gardening terms, potting soil is sterilized as long as it has no pathogens, diseases, or microbes. So, if you’re buying nutrient-dense soil with microorganisms in it, it technically wouldn’t be considered sterile.
However, not all potting soil needs to be sterilized. In the rest of this article, I’ll explain what sterilization is and what isn’t. Additionally, I’ll give you some guidance on sterilizing your own soil and explain when sterilization may be necessary.
What Sterilized Soil Is and What It Isn’t
Contrary to popular belief, sterilization isn’t synonymous with “clean” in gardening. It’s less relevant to gardeners than it is to farmers, but it’s still a buzzworthy term in the gardening community.
In fact, dirt can’t really ever be “clean” – that’s why things that aren’t clean are called dirty. Instead, in the gardening world, sterilization means being free of microorganisms, fungus, mold, and things of the like.
This is why the answer to the above question is a little complicated. In technical terms, your potting soil should be completely sterilized upon purchase. There should be no molds, fungus, microorganisms, or life within the soil.
However, this isn’t a black-and-white or foolproof system like most things. Sometimes, bags of soil may pick up bugs or molding if they haven’t been properly stored or taken care of. If you have an outdoor gardening center where you typically pick up your potting soil and the bag has a hole in it, there’s a chance that something could have crawled in.
So, to summarize, sterilized soil means nothing is living within your soil. It should be completely free of life until opened and added to your pot. However, this doesn’t mean the soil is “clean” or that you can even count on your bag of potting soil to be totally sterile. Be vigilant for signs of contamination and sterilize if need be.
Sterilized Soil Isn’t Necessarily a Good Thing
Unfortunately, sterilization isn’t a process that works in black and white. Sterilizing potting soil on your own can result in depleting the soil of any helpful microorganisms. The process will also kill small, beneficial animals, such as bugs or earthworms.
Essentially, sterilizing soil sets it back to square one. It won’t have any harmful fungus or pests. Still, it also won’t have any of the nutrients or microorganisms (or larger organisms) that help your plant grow. You’ll need to re-add compost, nutrients, or anything that you wish to help support your plant’s growth.
Gardening Stores Typically Sell Sterilized Soil
While some brands may sell their products as sterile, it’s important to know that there is no industry standard for what constitutes “sterile” potting soil. The only way for consumers to ensure that they get a truly sterile product is by using either one of our methods listed below:
- Microwaving
- Sunbathing
- Oven baking
- Freezing
This is the safest practice because it’s hard to tell when the soil has become contaminated. Finding a hole in the bag means that something would have crawled in, and often bags that are sealed up may have condensation (which can result in moisture and molding). Unless the bag of soil has been vacuum sealed, there’s no guarantee it’s sterile.
Why You Should Sterilize Your Potting Soil
As mentioned above, potting soil doesn’t necessarily need to be sterilized. Sterilizing your potting soil is only appropriate in certain situations.
You should sterilize your potting soil if you’re worried about where it has come from. If you’ve gotten your potting soil from a store, there doesn’t seem to be any obvious rips in the bag, and the soil comes out completely dry, you likely don’t need to sterilize it.
Of course, your potting soil needs to be sterilized from disease, pests, mold, and the like, too. If your current plant has potting soil that’s gone bad, you should remove the plant and wash away as much dirt as possible before transferring it to a new pot. Additionally, you need to clean out and sterilize the pot it was in and then isolate your plant until you know it’s illness free.
However, taking healthy soil and sterilizing it will result in the loss of nutrients. For that reason, you should keep your soil sterilization to the following situations:
- Right after purchase, before you put it into your pot and add your plant.
- If you’d like to reuse the soil after the spread of the disease.
- If you want to get rid of mold that formed in an improperly sealed bag of soil
At any other time, sterilization may risk killing your plant or the helpful nutrients within the soil.
How Do I Sterilize My Potting Soil?
If it is the right time to sterilize your potting soil, you may wonder how to do it. There are quite a few methods to sterilize potting soil, and all are equally effective.
You can sterilize your potting soil by soaking it in boiling water, exposing it under the sun, heating it in the microwave, or baking it in the oven. Essentially, each of these methods uses extreme temperature to kill all life within your potting soil.
Remember, there are only a few instances where it’s actually appropriate to sterilize your potting soil. It doesn’t need to be done often, and some soil may be so healthy (especially if you maintain its pH and nutrients) that it’ll never need to be sterilized.
Additionally, it’s worth mentioning here that sometimes it may be easier just to throw your soil away. Suppose you intend to sterilize your soil because of a pest infestation, hard-to-get-rid mold, or any other illness. In that case, getting a new bag may just be easier and can also save you money and time.
Oven Baking Your Soil To Sterilize
You can oven-bake your soil to eliminate unwanted microbes and pests. To do this, find an oven-safe pan or dish you don’t mind having dirt in. Then, you’ll need to spread the soil thinly over the pan.
Bake the soil for around thirty minutes at a low temperature, such as 180 °F (82 °C). If you bake it too long at a high temperature, you risk burning the soil, which will kill any nutrients and microorganisms living in it and ultimately prevent new life from forming.
Using Boiling Water To Sterilize Your Soil
Another simple method for sterilizing soil is to use boiling water. You can use a pot or a tea kettle to boil water and pour it over the soil you want to sterilize. Let it dry out completely, preventing exposure to pests or moisture, and then use it again.
If you were sterilizing soil because of pests or mold, and the pests or mold pop up again, I’d highly suggest tossing the soil and getting a new bag. You tried, and that’s what counts! Some soil isn’t saveable, and getting a new bag is worth it rather than risking your plant’s life.
Freezing Your Soil To Sterilize
You can use the freezer to sterilize potting soil by putting it in a plastic bag and then putting the bag in the freezer. Leave the potting soil in your refrigerator for 24 hours, then remove it from your refrigerator and let it thaw. Once you’ve let it thaw, you can use it as normal. Freezing helps prevent mold and kills microorganisms.
The Sun Method To Sterilize Your Soil
Using the sun is one of the easiest ways to sterilize your soil, and it can be done right after you purchase it. To use the sun:
- Spread your soil out on a flat surface.
- Let it bake in the sun, doing what you can to keep away pests and moisture.
Some will do this before even taking their soil out of the bag to ensure nothing is getting in.
Conclusion
Potting soil only needs to be completely “sterilized” in certain circumstances. Typically, this is a practice used by homesteaders and farmers who have undergone some kind of plight in their plants or crops. Otherwise, it may be used by home gardeners to ensure the soil they’re using isn’t ridden with diseases.
Sterilized soil isn’t synonymous with good soil, and vice versa! You can count on most potting soils being sterilized in the traditional sense. Still, it becomes unsterilized the moment you start adding things to it. Knowing when to sterilize will save you time, money, and grief.
