Taking care of your indoor herbs takes time and dedication. That is why we use fertilizer, pay attention to how we water them, and make sure they receive adequate light exposure. However, what you might forget is that the soil might need to be sterilized; otherwise, you might face potentially severe problems with your herbs in the future.
Hence, how potting soil can be sterilized? Potting soil can be sterilized through a variety of methods. However, the most suitable for indoor applications are:
- Through boiling water
- Adding hydrogen peroxide
You need to know that soil might be the host of harmful bacteria or even insects. Indeed, especially low-quality soil, unfortunately, comes with herb-dangerous insects on it that, at first, might not be visible at naked eyes.
That’s when the sterilization comes handful. Sterilize means simply killing fungal diseases, harmful bacteria, or insects that might seriously damage your herbs in the future through high temperature.
Not all sterilization methods are quick or easy. Indeed, you cannot merely place the soil in the oven for a random amount of time and temperature. Indeed, overheating will damage the chemical and physical structure of the soil, making it useless or even dangerous for your herbs.
As clearly stated in this authoritative gardening study, 180F is the temperature above which you can guarantee to kill all harmful organisms in the soil, as shown in the table below. However, remember, the sterilization process should never bring your soil to a temperature above 212F (100C) for reasons that we will discuss later.
Now let’s dive-in in the best sterilization techniques for potting soil.
|Harmful Organism||Max Tolerable Temperature|
|disease-causing fungi||30 min at 140F|
|bacteria, most plant viruses, and soil insects||160°F for 30 minutes|
|weed seeds||between 180°F for 30 minutes|
Baking your potting soil takes little set up and preparation, as it is very likely that you already have at home all items you need. This process requires half an hour. However, given that the soil will just site in the oven, in the meantime, you can walk away to complete other tasks around the house.
- Aluminum pan or any oven-safe container you have
- Oven-safe thermometer
- Spread out the soil on a disposable pan or oven-safe surface that will not be used for food preparation. The depth of the soil should not be more than 4inches (10cm). Why? To guarantee a uniform heating process that will avoid the center to be too cold while the outer layers too hot. You also need to moisten the soil so that it is damp but not heavily saturated.
- Tightly cover the top of the pan/container with aluminum foil. Insert an oven-safe thermometer into the pan where it can be easily read while baking.
- Place the soil in the oven and set your oven to preheat to 180F (83C). Tip: If you know your oven well and you are aware that it will not reach the temperature, you can set a higher one (like 220F) and, after reducing the temperature.
- Monitor the temperature of the soil every 10 minutes. When reaching the 180F, leave it inside the oven for another 1.5 hours to 2 hours (someone even leaves it even more, but it is usually not necessary). You can turn off the oven after an hour and half and leave it inside for another half hour.
You may need to adjust your oven temperature so that the soil remains around 180F but never above 212F (100C). If you have an extra oven-safe thermometer (or non-contact one as this good one in Amazon) even better. Indeed, you can place it in the other layer of the soil to check that the temperature is not overcoming the 200F.
- Remove the pan/container from the oven. Then, wait for the soil to cool down until it reaches room temperature before attempting to use it for plants.
Steam sterilization is one of the most efficient methods. This is because it uses both steam and heat to kill fungi and bacteria within the soil, making it safe for indoor plants.
In this section, you will find how to sterilize your soil using a pressure cooker or, in case you do not like it (or you feel unsafe), a steamer pot.
In this process, the soil is placed inside a pressure cooker where heat and steam will penetrate and circulate through it.
- Steamer pot or soup pot with a wire rack
- Pressure cooker
- Heat-safe, shallow container that fits inside the pressure cooker
Procedure to sterilize your soil with pressure:
- Add around 1 inch of water in the pressure cooker. Place inside the steamer pot or the rack.
- Fill the heat-safe container with soil and place it on top of the rack, or steamer pot, inside the pressure cooker. Then close the lid and open the steam valve. Again, the soil should create a layer no deeper than 4 inches to guarantee a uniform temperature. In case you have a rack with multiple levels, you can sterilize more soil samples at once (each one in a heat-safe container).
- Place your pressure cooker on the stovetop on high flame
- Close the steam valve once the steam begins to escape and leave it for about 15 minutes. This is a sign that the water starts boiling, and the steam produced. The heat and steam trapped inside the cooker will penetrate and sterilize the soil.
- Turn off your pressure cooker and let it cool down before opening the valve, removing the cover and using the soil.
Sterilizing your soil with a steamer pot
In this case, the soil is sterilized with steam, but without pressure. This approach will take around double the time compared to the pressure-based counterpart.
Procedure to sterilize your soil without pressure:
- Put 3 to 4 cups of water into a steamer pot (a cooking pot that has small holes on the bottom that sits within a larger pot). If you do not own a steamer pot, just place a wire cooling rack on the bottom of a standard pot so that the rack is above the water level.
- Place the soil you want to be sterilized into a heat-safe container. Make sure that the soil is not thicker than 4 inches. Moreover, do not pack or press it down. This is important as otherwise, the steam and heat will not be able to circulate freely through the soil, potentially leaving part of it not-sterilized.
- Put the heat-safe container into the steamer pot. Turn on the stove and leave it for 30 minutes.
- Turn off your stove and remove the soil from heat. It should be ready to use after 30 minutes it cools down. However, if it still feels warm, just wait a bit longer. Herbs do not like warm/hot soil, as it can damage their roots.
Pressure cooker and steamer pot might sounds perhaps not very handy. However, good news for you, a microwave can be as good as the above. After all, it is one of the most common devices nowadays to warm up food.
You need to remember that in case the soil does not reach the 180F, you might need to split it into smaller batches. The size of such baches will depend on the maximum power and size of your microwave.
- Microwave-safe plastic bag with zip-lock
- Spread out the soil on a flat surface approximately ½ inch thick so you can moisten it uniformly. Why? You need to add water as microwaves work by moving around, at very high speed, water particles. This very juggling creates the friction that generates the heat you need to sterilize the soil. However, do not make the soil soggy.
- Fill the microwave-safe bag with soil. I would go for a maximum of 2 pounds (around 1kg) per bag. Why no more? Because this will cause the soil to be too hot on the surface (remember that the soil should never go above 210°F) while too cold inside.
- Microwave the soil for around 1 minute for one bag at full power. The time heavily depends on the power of your microwave and type of soil (the 1min is referred to as 1000W microwave and high-quality potting soil).
- Repeat step 3 with a shorter period of time. Indeed, the temperature is below 180°F (using a thermometer that you plant in the soil after microwaving) try a 2nd heating cycle of 10-20 seconds (depending on how cold is the soil) and recheck the temperature. Continue in this way until the temperature reaches the 180F. Do not close the bag during this stage. I prefer this approach rather than providing a specific time with just one heating cycle. Indeed, such timing strongly depends on the type of soil, microwave features that I cannot know a priori.
- Close the bag and let the soil sit until it has cooled down to room temperature.
As you might have noticed up to know sterilization is all about bringing your soil to a temperature high enough to get rid of unwanted guests in your soil. However, you might not want to use a microwave or oven to create the right conditions.
Hence, what about boiling water? Boiling water as the advantage to never get over 212F (the limit for soil, on this more later) and can be produced in many different ways (with a kettle, on a stovetop, etc…). This method, not very often known by gardeners, is also mentioned by a study conducted by the Kanagawa Institute of Agriculture Science, Japan.
- A heat-safe container, possible insulating to host all your land
- Sealing for the container (no need to be hermetic)
- Boiling Water
- Place the soil inside the container. Here you have to make sure to have enough space for the water. So, as a rule of thumb, the soil should not use more than ⅔ of the volume;
- Pour enough hot water to make the soil damp;
- Close the container and agitate it for a minute. This assures that the heat is trapped inside and uniformly distributed, so as to maximize the effectiveness of the sterilization. If the container is too large/heavy you can use a wooden spatula;
- Wait for 10-15 minutes and open the container;
- Let your soil dry by spreading evenly on a surface either under the sun or inside (if outside, make sure it does not get in contact with insects that might infest the soil).
The only trick to this approach is to keep the temperature high enough for a long period. Indeed, the capacity of the container to retain the heat is essential. However, even if the water cools down in the process (it started at 212F), the soil will still be sterilized if the temperature does not drop below 180F.
Ideally, you would like having a food thermometer planted in the soil (possibly with the sensor close to the central part, always the colder). A transparent container lid will allow you to see through.
This is the most straightforward technique among all. Ideally, you should use an insulated container that can keep the soil hot for a while. On Amazon, for instance, you can find many relatively cheap food containers (here, for instance) that are and can be used for a small amount of soil.
If you do not have such a container, a simple plastic box (heat resistant though) would be ok although some microorganisms might survive the sterilization process.
Tip: compress the soil once wet (with your hands with gloves as it will be hot or a spatula). Indeed, this will allow trapping the heat/steam for longer as there will be less air (inside the soil) through which it can escape.
Sterilizing your soil using hydrogen peroxide is the only method on the list that does not require heat as it based on a chemical process.
Indeed, hydrogen peroxide (sold in liquid form) has a higher amount of oxygen (one atom more per molecule if you have some basic chemistry, if not, just ignore this statement). This extra oxygen as the tendency to react strongly with carbon (every life form has carbon in it) damaging it quite significantly. That is how this substance can be used to damage spores, bacteria, and any other microbial life lurking in your potting soil. Moreover, as stated by an expert in the field, here the academic resource, “the free O atom can destroy dead organic material (i.e., leaves roots) that are rotting and spreading diseases.”
Hydrogen peroxide needs to be diluted in water (you will not even easily find 100% hydrogen peroxide). The majority of typical retailers, just have a quick look here to check Amazon, drug stores, and even pharmacies, sell this substance in 3 to 35% concentration.
- Hydrogen peroxide (3% or 35%)
- Large pan or flat surface
- Water (tap is fine)
- Measuring cups and spoons
- Bucket or glass bowl
- Create the solution of water and hydrogen peroxide. You should use 1 part of 3% hydrogen peroxide for each part of the water. In case you are using 35% hydrogen peroxide, then 10 parts of water for each of hydrogen. However, you can stick with the 3% version as a way safer to handle. A note of extra care by wearing gloves and goggles if available. Indeed, hydrogen peroxide, at high concentration, is corrosive and might also cause a strong reaction (production of a high amount of gas) if in contact with organic matter. It can also be an issue if it gets into your eyes or skin.
- Spread the potting soil out on a flat surface creating 0.4 inches (1 cm) thick layer. You may do more soil by stacking more 0.4 inches layer and watering each time. You can use a baking paper sheet. As extra precautions avoid metal as hydrogen peroxide reacts with it (although very limited given the low concentration we are using).
- Spray the soil until damp, not soaked. Make sure to spray all your soil uniformly without forgetting corners.
- Let sit until dry or almost dry, then mix so that new soil is on top. Repeat steps 2 & 3 until you are sure that all soil has been sprayed. This usually might take 3-4 iterations.
- Allow the soil to dry completely (even outside under the sun is fine) before use. Wait for at least 3-4 hours, even if dry before using it for planting purposes. Indeed, this solution takes a long time (2-3 hours) to kill spores. However, all other lifeforms around 50 minutes are in general enough, as detailed by The Department of Health of Louisiana.
You need to know that temperatures higher than 212F (100C) should not be reached in any part of your potting soil during sterilization.
Why? Because this will cause the release of minerals like manganese, typically locked in the soil to become available. If this happens, your herbs might quickly die after repotting due to manganese poisoning, as discussed in this Cornell University study as well in this article. Moreover, temperatures higher than 212F can harm your due organic compounds and salts.
Hence, the “godlike” zone is between 180 and 210F. This explains while everyone on the internet will tell (rightly) to keep the soil temperature at around 200F (it is a round number easy to remember). I suggest, if you can, to go at 180F for 30 minutes, as safer to dangerous chemicals. This will be suitable for most of the soil.
Moreover, remember that the soil should be as uniform as possible and free from any non-soil particle (plastic, metal) that you might need to remove manually before proceeding.
The potting mix is usually sold as sterile. Hence, there should be no need to sterilize the soil, right? Wrong.
Indeed, it is not uncommon for potting to present pathogens, fungi, or even insects in it that might harm your herbs. Do you not believe me?
Just have a quick look to the comments on retailer websites regarding the potting mix. If you do not have the time here a quick summary for you where I list the most common insects that gardeners have found in just opened potting mix. For fairness, each experience is referred to a different potting soil brand, just to remind that this is a common problem that might affect also “good quality brands in the sector.
- Fly eggs: this appeared in the potting mix as a tiny white pearl. These are entirely invisible to a quick naked eye examination making it extremely hard for the gardener to identify them in time;
- Tiny Black bugs: although the gardener did not specify this might thrips or aphids. Both of them very dangerous to indoor herbs
- Gnats and centipede: a gardener even found a “zoo” as himself defined of insects including gnats (quite a problem for herbs) and centipedes with other not identified insects.
Hence, if you want to avoid such conditions, sterilization is a must. This is even more important for seedlings, as they are even more susceptible to these conditions. Plant them in an infested soil, and the chance for them to succeed and give you fresh herbs goes almost to zero.
I do not recommend to dig your outdoor garden soil and use it for indoor application. Indeed soil (or better potting mix) for indoor herbs must be air and fluffy. Drainage is a key element for a medium used in a container.
However, if you want to go ahead with garden soil, the answer is yes, you must sterilize it. Indeed, if in case of potting mix (in theory) sterilization should not be necessary, this is a must for garden soil. Garden soil is thriving with a large variety of life forms. Some of them, however, can be pretty harmful to small starting herbs. So the best decision is to sterilize your garden soil before using it as a potting soil.
However, just to restate what I said before if you can avoid garden soil for indoor plants. Indeed, garden soil might lack the physical drainage features of the right potting mix. You can either create such mix inexpensively yourself if you like DIY as discussed here or buying a good quality one, as discussed in this article.
You have followed all the instructions carefully, and you had sterilized your soil. However, you might have forgotten that all the tools and containers that come in contact with your original soil carry the same identical bacteria and fungi.
Hence, if you touch your sterilized soil with one of them, your efforts are vain, as such lifeforms will tend to repopulate your soil quickly. You need to sterilize them.
Hence, how to sterilize your tools? To sterilize my gardening tools, I use a simple 4-step approach:
- Just wash them under normal water to remove any visible soil trace.
- Place in a bucket a solution of bleach and water (9 liters of water for each liter of bleach);
- Place the gardening tools, plant container included, in the solution totally submerged for around half an hour.
- Take them out and let them dry at the open air
In performing such an operation, remember to wear protective plastic gloves. Indeed, bleach is a corrosive substance and possible a mask to avoid inhalation of the gases produced.
Sterilization is a “brute force” approach as the very high temperatures target all living forms in the soil. No organisms/life form remains.
This is not good news as soil hosts also lifeforms beneficial for your herbs. For instance, a few “good” allies might prevent nasty fungi from spreading by competing with them. Others might breakdown organic matter producing nutrients for your herbs, carbon, oxygen, and even natural pesticides.
However, if all organisms are removed through sterilization, there is no competition left in the soil. So the first living organisms that will land on your soil after sterilization might easily take over the whole soil. You can guess what would happen to your herbs if such organisms are fungi or harmful bacteria! This is the main drawback of sterilization.
Pasteurization can be an answer to this problem. This technique simply heats the soil (following the same approaches described above, excluding the hydrogen peroxide one) but at a lower temperature of around 145°F (63°C), as discussed in this article from the University of Natan.
Are all potting mix soil in the market sterilized? No, some potting mix they are sold as “natural product” so they do not undergo any sterilization process.
Can you sterilize the soil in a greenhouse? There is a large variety of techniques also discussed here. The cheapest is the use of heat resistant plastic sheet that, once properly fixed to cover the soil, allow to trap heat (injected beneath the layer). This will “steam” the soil. Although it requires several hours and has a limit of depths of soil sterilized, it is very cheap and covers a large portion of the ground at once (depending on the size of the sheets).
Why change the soil of potted herbs – https://yourindoorherbs.com/why-and-when-change-the-soil-of-your-potted-herbs/
Best Potting Soil – Best Potting Soil
2 Aspects for the best potting soil and DIY recipe –https://yourindoorherbs.com/2-aspects-of-the-best-potting-soils-and-diy-recipe/
Tips to grow massive basil indoor – https://yourindoorherbs.com/21-easy-tips-to-grow-massive-basil-indoor/
The three soil types – https://yourindoorherbs.com/sand-silt-and-clay-differences-you-need-to-know/