How to Sterilize Soil for Gardening

Soil sterilization is an effective way to kill soil-borne pathogens, including viruses, bacteria, fungi, weeds, and bugs.

These organisms can destroy everything you have in your garden because it leads to serious outbreaks. To eradicate harmful microorganisms, gardeners must know how to sterilize soil properly.

Why you should sterilize Potting Soil

Soil sterilization is important when it comes time to germinate seeds. In addition, sterilized soil can help the plants to propagate cuttings and transplantation of juvenile plants.

Commercial greenhouses and agricultural producers use sterilized soil because it might contain pathogenic microbes, which are known to destroy vulnerable plants.

Whether you garden in pots or containers or you plant vegetables in a raised bed, potting soil often contains harmful pathogens, like bacteria, fungus, and viruses.

These can infect the seeds sown and ruin the germination process.

Unsterilized potting media also affects young plants and overall plant growth. If you are looking forward to growing mature plants, however, sterilization becomes less important.

Related post: Best Seed Starting Mix For Your Garden

how to sterilize soil in a pot

Plant growth potential depends on fertile soil mix but it also depends on diseases present.

Mahmood et al (2014) showed that sterilization increased water-soluble carbon and manganese, and seedlings in sterilized soils had greater root biomass than in unsterilized soils.

A similar study by Moreno-Velandia et al., (2019) revealed that sterilization can control the concentration of pathogens and antagonists.

Before we get into the various methods of how to sterilize soil, if you are looking for a general potting soil that is good for all types of plants, we recommend this potting soil. It’s been our go-to general potting soil for houseplants to vegetable growing – plus it’s organic.

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Espoma AP8 8-Quart Organic Potting Mix - 2 Pack
  • Compositions: Sphagnum Peat Moss Peat Humus Perlite
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How to Sterilize Soil for Gardening

Generally, gardeners sterilize the potting soil to grow seasonal vegetables in potting containers.

Nevertheless, sterilizing potting soil of an entire area is crucial, more so when you want to grow crops in a large area.

The four methods to sterilize soil are as follows:

1. Solarization Method of Sterilization

This a common and cheap procedure to sterilize garden soil. In this method, the sun sterilizes the potting soil because it drastically heats the pile.

The basic principle of solarization is to gradually introduce light and use solar energy for sterilization. It is a well-known method for destroying weed seeds.

  • Plastic
  • Water and
  • Full sun

If you wish to sterilize a small amount, a transparent plastic bag will work fine. In this case, you need to fill the bags with soil and keep them under the full sun for 4-5 weeks.

You will require a large and thin layer of plastic to fit a large amount of soil. To do this, shovel the soil into thin layers and spread it inside the plastic bags.

Make sure there are no adequate ventilation holes in the bag because these allow the microbes to enter.

To sterilize a huge area like the entire garden, you need to cover the area with plastic wrap. Clear plastic will heat the area and induce the sterilization process.

This process reduces the unpleasant odor because it heats the soil continuously.

2. Sterilizing Soil with a Microwave

This is an excellent option to sterilize potting soil. However, quantity becomes a major drawback in this process.

You cannot eliminate harmful bacteria in a large area, which is feasible in solarization and chemical treatments. If the quantity of the mixture is small, a gardener can obtain a sterilized potting mix.

how to sterilize soil with a microwave

You will obviously need access to a microwave oven.

This method could destroy fungus gnats of potting soil. To do this, place around 2 lbs of the moist soil in a plastic bag (zip-top).

The mixes should be damp but not wet. You don’t need to close the zip of the bag, instead, leave it open and place the bag in the microwave.

You should heat the potting medium and monitor the oven temperature until the internal temperature of the soil reaches around 180-200-degree Fahrenheit.

You then measure the temperature using a meat thermometer.

Finally, remove the sterilized potting soil from the oven and close the zip. Leave it in a cooler place and wait until it reaches room temperature.

3. Sterilize Potting Soil by Steaming

This is an effective method to sterilize the soil. One could use a pressure cooker and use its steam for sterilization. One could also use boiling water as the sterilizer.

You will require the following items:

  • Pressure cooker
  • Rack
  • Heat proof containers
  • Aluminum foil
  • Water

For the pressure cooker process, add a few cups of water and then place around four inches of soil in the container over the water.

Close the lid and turn on the cooker. It will create steam. Leave the steam valve open. The pressure cooker must be set at 10 lbs of pressure for around 15 to 30 minutes.

For the non-pressure cooker method, add an inch of water in the pot and place the rack. Fill the oven-safe container with mix and cover it with aluminum foil.

Close the lid of the pot and allow the steam to leave. Heat the pot till the water comes to boiling point. Once the boiling point is reached, boil the water for 30 minutes.

Finally, allow the soil to cool down. Dletrich et al., (2020) found that microbial biomass faced a 27% reduction after steam sterilization, nematodes were eliminated, and phosphorus concentration increased.

Though, after the sterilization process, potassium content decreased.

4. Chemical Treatments to Sterilize Soil

Chemical applications are widely used for sterilizing potting soil. Also, if you have a large land full of unsterilized soil, you could adopt chemical treatments to sterilize your soil.

Chemical treatments are successful if the gardener applies the pesticides in a correct manner. Wrong application might make the remaining pathogens resistant to that specific substance.

how to sterilize soil using chemical treatments

Some common and effective sterilizers for removing pathogens are:

  • Formaldehyde
  • Phenols
  • Cresylic acid
  • D-D Fumigant
  • Chloropicrin and Methyl Bromide
  • Dazomet

Remember that these chemicals can also kill beneficial microorganisms, affect overall plant health, and cause water pollution.

So out of the four methods, I’d suggest not using chemicals.

FAQ

Does Sterilizing Potting Soil Remove Nutrients?

No, sterilization only kills pathogens because it increases the soil temperature.

How to Sterilize Soil to Increase the Germination Rate?

Seed germination begins when all the factors are in favorable condition. Sterilization increases the chances of a successful germination rate and reduces the risk of soil-borne diseases.

What Is the Best Time for Sterilizing Soil?

The best time to sterilize your soil is before sowing seeds.

Do I Need to Sterilize Potting Soil?

Yes. It is always safe to sterilize potting soil (even fresh potting soil and store-bought soil) before seeds are sown or transplanting.

You should also purchase newly sterilized potting soil and a sterilizing container.

Conclusion

Hopefully, the instructions above give you an introduction to how and why sterilizing potting soil is important.

Among the four sterilization methods, solarization is regarded as the best one because it is environmentally friendly.

You could also disinfect enough potting mixes for your garden.

The major drawback of the solarization process is the duration – it is a long process.

At the same time, we discourage using chemical treatments to sterilize the land, being since you never want to add unneeded chemicals, but it is still effective.

Lastly, remember if you are going to sterilize your soil, make sure you get a new pot or sterilize your containers and pots as wellcpo.

Along with some basic plant knowledge and you will have a better chance of growing whatever you wish for!

Related post: Best Potting Soil Brands for Herbs, Potted Vegetables & Indoor Plants

References

1. Mahmood, T.; Mehnaz, S.; Fleischmann, F.; Ali, R.; Hashmi, Z.H. and Iqbal, Z. 2014. Soil sterilization effects on root growth and formation of rhizosheaths in wheat seedlings. Pedobiologia, 57(3): 123-130.

https://www.sciencedirect.com/science/article/abs/pii/S0031405614000079?via%3Dihub

2. Dletrich, P.; Cesarz, S.; Eisenhauer, N. and Roscher, C. 2020. Effects of steam sterilization on soil abiotic and biotic properties. Soil Organisms, 92(2): 99-108.

http://soil-organisms.org/index.php/SO/article/view/125

3. Moreno-Velandia, C.A.; Izquierdo-García, L.F.; Ongena M.; Kloepper, J.W. and Cotes, A.M. Soil sterilization, pathogen and antagonist concentration affect biological control of Fusarium wilt of Cape gooseberry by Bacillus velezensis Bs006. Plant and Soil, 435: 39-55.

https://link.springer.com/article/10.1007/s11104-018-3866-4

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