Reusing the same potting or garden soil is a sure-fire way to see pests make themselves at home in your plants or flower beds and risks an increase of fungal spores and other nasties that your growing garden won’t thank you for.
Thankfully, there are many ways to treat and sterilize old soil (or store-bought soil if it fails to specify its sterile status). We’ll explain a bit about soil sterilization before taking you step-by-step through 8 methods you can try at home!
Soil sterilization is essentially the process of driving harmful organisms out of the soil in a controlled manner using evaporation, burning, or chemical methods.
The main aim of sterilizing soil is to eliminate harmful pests, weeds, and disease-causing pathogens with minimal interference to the beneficial properties in the potting soil/garden soil.
This soil “detox” provides a clean environment for seeds, seedlings, and vulnerable established plants.
Pros & Cons
- Helps prevent the loss of crops
- Can prevent some pest and disease-related damage
- Inexpensive compared with treating issues when they appear
- Many methods are nontoxic
- Easy to do at home
- Can kill good microorganisms in the soil
- Some methods can be time consuming and result in only subtle improvements
- There’s a risk that other plants could be harmed
When To Sterilize Soil
Potting soil for houseplants can be sterilized any time of year while garden soil is best sterilized in spring or as soon as the soil can be worked in your area.
You’ll know your plants may benefit from soil sterilization if you notice persistent bug infestations or signs of disease (wilting, discoloration, stunted growth, etc.) despite regular appropriate care.
Ideally, it’s best to sterilize soil before planting to create a wholly clean environment and prevent cross-contamination of unsterilized and sterilized soil — improving the health of your germinating seeds or young plants!
1. How To Sterilize Soil With Hydrogen Peroxide
Sterilizing with hydrogen peroxide involves making a diluted solution and spraying it onto the soil.
According to Anya Anthony of Indoor Plants for Beginners, this method is great if you have an aphid problem or your houseplant has signs of root rot.
- Ensure you’re in a well-ventilated area and wearing protective butyl gloves.
- Using a bottle of hydrogen peroxide labeled no higher than 3% concentrate, mix one of the following solutions in a bowl/jug depending on your needs (from small to medium batches of soil):
- 1½ tsp of HP in 1 cup of water
- 2 tbsp of HP in 1 quart of water
- ½ cup of HP in 1 gallon of water
- 2½ cups of HP in 5 gallons of water
- Transfer the mixture into a spray bottle, and spray into a pot/container of fresh potting soil until evenly moist. It’s best to do this late afternoon/early evening to prevent premature evaporation. Leave until the morning when it will have fully evaporated.
2. How To Sterilize Soil in the Oven
This method involves putting small batches of moist soil in oven-proof containers so the soil can “bake,” allowing harmful microbes to evaporate.
Oven sterilizing makes sense if you have small to medium amounts of soil to work with (for filling a few seedling pots or seed trays, for example).
- Preheat your oven to 200°F and add approximately 3 inches of soil to an oven-proof container.
- Mix enough water in with the soil to get a moist (not runny) mixture, and cover the container top with aluminum foil.
- Poke a small hole in the top of the foil lid, and place the container in the oven.
- Check when the soil has reached 180°F by placing a meat thermometer in the lid hole. Once it has reached this temp, bake the soil for a further 30 minutes.
- Once 30 minutes are up, remove the container from the oven, and allow the soil to cool down to room temperature so it is safe to use (around 68-74°F ).
3. How To Sterilize Soil in Microwave
This option is only appropriate for very small amounts of soil and entails filling a small semi-sealed Ziploc bag with moist soil and steaming it — much like a boil-in-the-bag meal!
- Fill a clean Ziploc bag with a couple of pounds of moist soil (moist so it forms a clump in your hand, not overly saturated).
- Seal the top of the bag halfway for ventilation, and heat the bag of soil in the microwave on high power for about 90-150 seconds (or until the soil temperature has reached 180-200°F)
- Check the soil with a meat thermometer – once it has reached the desired temperature, remove it from the microwave, and allow it to cool to room temperature before using it.
4. How To Sterilize Soil in the Sun
Solarizing soil involves covering large, thin amounts of soil with layers of plastic to soak up the sun’s rays.
This long-term technique is most appropriate if you need to sterilize bulk amounts in large, sunny gardens and works best with clay and loamy soils.
- Sometime between June and August, select a spot in your garden that gets at least 6-8 hours of direct sunlight a day.
- Lay down a layer of medium-thickness clear/transparent plastic sheeting on your designated area. Author of The Yard and Garden blog Ben Hilton advises buying the plastic in large single panels if possible to customize it to the area you’re covering.
- Break up clumps in the amount of garden soil you intend to use, and remove any rocks/plant matter.
- Spread the soil evenly over the plastic sheeting, making sure it lies fairly flat and that the soil is kept a few inches away from the edges of the sheeting.
- Place a layer of plastic sheeting over the soil, pulling it tightly over the soil surface, and secure down the edges with stones. (You can also bury the edges of plastic sheeting with soil to truly seal in the heat!)
- When done during the hottest period of the year, it should take 4-6 weeks for the sun to fully sterilize the soil.
5. How To Sterilize Soil With Boiling Water
This super simple method is done by simply boiling water on the stove before pouring it into a bowl of soil, and this is typically best for sterilizing small to medium batches of potting soil at a time.
- Fill a large heat-proof bowl about two-thirds with potting soil or soil mix, and set aside.
- Fill up a small or medium-sized pot (depending on the soil amount needed) with water, and bring to a boil on the stove.
- Once boiled, remove the pot of boiling water from the stove, and carefully pour the water into the bowl of soil just enough for the soil to appear moist (not soggy or runny). It may be easier and safer to do this step with the bowl of soil placed in the sink first.
- Use a spoon to work the boiled water evenly into the soil, and allow the soil to cool down to room temp before using it.
6. How To Sterilize Soil With Chemicals
Alongside hydrogen peroxide, a popular form of chemical soil sterilization is using a formalin solution (formaldehyde and water). This soil treatment involves fumigating the soil and is best used outdoors on large batches of garden soil. You’ll find formalin here at a reasonable price.
- Before trying this method, you’ll need to ensure that you are wearing appropriate PPE throughout the process and that temperatures are reliably high outdoors (June through to August) as lower temperatures will not allow the formalin to properly fumigate and create the necessary gas to penetrate the soil.
- When ready, dilute the formalin product in water at a ratio of 1:50 (formalin to water).
- Soak the soil amount in the solution. Five gallons per square yard is typically recommended.
- Allow around 20-40 days for the soil fumigation process to be completed.
If done correctly, the chemicals should no longer be present in the soil after the waiting period and should be safe to use with your plants. In case you’re concerned that the treatment has altered your soil’s properties, you can test your soil’s pH before use.
7. Sterilizing Soil With Fire
This is a good option for small to medium amounts of soil and involves heating a metal container of soil from below. Some also choose to burn large heaps of garden soil in a bonfire-style burning, but this is discouraged for obvious safety reasons!
Note that because soil has low thermal conductivity, fire may not penetrate any deeper than a couple of inches, so thick layers of soil won’t be sufficiently sterilized.
- Firstly, be sure to start this method on a dry day late in the evening so moisture levels are low enough for the fire’s heat to have maximum impact.
- In a large container, moisten the soil batch with water until damp, and add some compost and sterilized wood substrate (sawdust, wood chips, etc.) to the mix.
- In a fire pit, start a fire with newspaper, dry wood, and matches.
- Fill a metal kettle with no more than 4 inches of soil, and spray the soil with water to moisten it. Cover the kettle opening tightly with aluminum foil.
- Place a spit over the fire, and hang the kettle onto the spit. Check when the soil temperature has reached 180°F using an oven thermometer to pierce through the foil.
- Once the soil has reached 180°F, allow the soil to cook for approx 30 mins. Afterward, remove the kettle of soil from the spit, and allow it to cool with the foil lid left in place.
8. Sterilizing Soil With Steam
This method involves essentially steam-cleaning garden or potting soil using a steamer pot ((this involves two pots with handles – the smallest has perforations in the base which is placed over a larger vessel of water).
Steam sterilizing is most appropriate for handling small batches of soil at a time (for starter pots/trays).
- Place a few cups of water into the larger pot, and bring the water to a boil on the stove.
- Once the pot is sufficiently boiling and releasing steam, place the desired amount of soil into the smaller perforated pot, and place it on top of the boiling pot (this pot should lock snugly into place).
- Place a lid on the smaller top pot, and allow the soil to steam for 30 minutes.
- After 30 minutes of steaming time, take the entire steamer pot off the stove, and allow it to cool on a heat-proof mat until the soil is cooled and ready for use.
When it comes to deep cleaning your potting or garden soil, there are many methods to choose from — all that’s left to decide is how much soil you need to sterilize (a few seed trays or an entire garden’s worth!) and which is most appropriate for your resources and climate (indoor steamer pot or fire pit and long-term solarization?).
Of all the methods suggested, we’d urge caution against relying on chemical treatments as they produce a gas by-product, so this is probably best left to large-scale commercial uses or only if you’re unable to use more straightforward sterilization methods.