How to Sterilize Soil for Planting – Clean Dirt
There Are Several Methods
I will present you with a few options to try to see what works best for you. Oven and microwave sterilization is often the easiest for the home gardener. In addition, we will look at a couple other examples as well. You can never have too many options!
Dirt is Supposed to be Dirty – Why Sterilize Soil?
If you are new to gardening the idea of cleaning dirt may seem like I am making a joke. After all clean dirt is an oxymoron, contradictory.
There are several reasons to sterilize soil the big ones being pests and disease.
Pests and Harmful Nematodes
A big one for me is … Fungus Gnats! If you have never had them then you are the luckiest gardener I know. These annoying little things are the main reason I ever started sterilizing the soil. It seemed that no matter what brand or store I shopped I got gnats.
Insects and their larvae can be present in the soil. Even fresh purchased soil from the stores. It is not the fault of the soil companies. This is something that is often misunderstood and ends up costing a company reputation.
Soil packages need to be breathable or they will build up fungus and molds. They have holes in the bags for air. These holes that allow for air, also allow for pests.
These pesky little critters see this delicious looking awesome soil and think YUMMY! They want to make it a home. Who could blame them, you would too if you were a bug. They lay their eggs or set up camp after the soil is packaged.
Pathogens and Harmful Bacteria!
I could go on to list a plethora of plant diseases here. Some you may or may not have heard of. A common one you may have heard of is blight found often in tomatoes. Perhaps the dreaded Fusarium Wilt, or even the ever frustrating dampening off disease.
If you have not heard of them it is OK. They are BAD, that is all that is important.
They are all caused by harmful bacteria and pathogens in the soil. Bacteria and pathogens can live and remain and the soil long after plants have been removed. Every time a new plant is grown in the soil it can grab the disease from the previous planting.
Sterilization kills these pathogens that can cause issues. In the same fashion as a surgeon prepares for surgery, you prepare your plant for transplant. In a nice clean sterile environment.
The Bad Side of Sterilization – Replenish
There are ALSO beneficial bacteria in soil. Unfortunately, sterilization does remove those as well. You can not remove one without the other.
Re-nourish your soil with good organic fertilizers. I am a huge fan of worm castings. It is loaded with “goodies”. The results in my garden have always been the most compelling evidence I have needed to love worm castings. ⇒ I wrote an article about the castings I use. They are amazing. You can check it out by following this link Simple Grow Soil Builder – Worm Castings
Adding in mycorrhizal fungi is another great way to reintroduce the good bacteria. It helps with healthy root growth and counteracts soil deficiencies. It is an organic product that acts as a soil inoculant. Like a healthy vaccine.
“Unplanted Territory” – When to Sterilize Soil for Planting
Clearly sterilizing soil when you have a growing plant in a pot is a no-no. It will kill your plant. So, when do you perform soil sterilization:
Seed starting – if your seed mix is not pre-sterilized, sterilization is a great idea. Your seedlings will be more susceptible to harmful pests and disease-causing bacteria.
Cuttings – Like your seed starts your fresh cuttings have newly developing roots. This makes them more vulnerable to the baddies.
Transplanting Seedlings – Young plants do not have the root system strength to fend off attacks. Giving them a clean start will ensure ample time to develop.
Past Issues – We will all run into trouble in the garden now and again. Bouts of mold, fungus or pests that caused issues. The soil can be saved with some sterilization and fertilization.
Have a pest issue in existing potted plants. In this article, You can find some Super bonus pest prevention and Awesome ways to rid Aphids. It has a strong emphasis on the importance of soil health and healthy plants to prevent and treat infestations.
How to Sterilize Soil in the Microwave – Zap Them Critters!
What you need:
⇒ Glass or plastic container able to hold about 2lbs of soil
alternative: microwave safe polypropylene bag
⇒ Recommended: Meat thermometer or Infrared Thermometer
- Premoisten soil and break down clumps
- Place in microwave-safe dish or bag
- Leave uncovered or open for venting
- Microwave on high 2 to 2 ½ minutes (650 Watt) or 1 ½ minutes (1000 Watt)
- Check temperature. This is easily done with an Infrared Thermometer. In this case, a meat thermometer is less accurate when used in microwaved products. Should be 160 to 180 degrees Fahrenheit.
- Remove thermometer and microwave for 30-second intervals if more time is needed.
Baked Goods – Sterilize Soil in Oven
What you need:
⇒ Disposable aluminum roasting pan or old pan that will hold about 3 to 4 inches of soil
⇒ Aluminum Foil
⇒ A meat thermometer or Infrared Thermometer
⇒ Preheat Oven to 200 Degrees
- Premoisten soil and break down clumps
- Spread soil in the pan to about 3 to 4 inches in depth loosely
- Cover tightly with aluminum foil
- If using meat thermometer place through the center of foil. Leaving only a gap wide enough for the thermometer.
- Place in preheated 200-degree oven (important that your oven temp is accurate. Too hot an oven can cause overheating in the soil and burning. Burnt soil becomes toxic and unusable)
- Begin monitoring the temperature after 30 minutes and continue to check until the soil reaches 160 to 180 degrees. You do not want to exceed 200 degrees.
- When the soil has reached temp shut off oven and leave inside to cool down slowly.
The last part has a bit of controversy. I know several other gardeners who sterilize at a steady temp for 30 minutes to 2 hours. I find that overkill and a risky venture for killing your soil completely. Nothing will live after it has reached the proper temperature. In addition, the remaining time in the oven to cool ensures heat remains long enough to destroy bacteria and pests.
Different Ways to Clean Soil – Steam, Ice, and Boil
Even farms and Greenhouse operations need to worry about soil contamination. Large crop productions can easily be damaged by outbreaks of bacteria and fungi. Pests and nematodes can launch mass destruction and cause food crisis.
Learn how farmers sterilize their soil here in this Soil Steam sterilization article on Wikipedia.
Although large farms have heavy equipment to do steaming sterilization. Small home gardeners do not need to make that type of investment. Steaming can be done in small batches indoors. I do not personally use this method because I find the smell to be far more vial. Also, the temperature was difficult for me to regulate properly. It may be an alternative method for you to try if the oven and microwave are not ideal for your needs.
The linked video to the right focuses on two simple methods for seed starting mix sterilization as an illustration. In “how to Stop Fungus Gnats and Kill Insect Eggs by using boiling water”. It also instructs on Freezing methods and how to combine both.
A Little Prep Now Better Results Later
Soil sterilization can have a smell. Crack a window and get some fans buzzing if it is too much for you. Often times the richer the soil the stronger the smell. Try to plan for a nice day, so you can open things up. I have even brought the microwave outdoors on sunny days.
If you are not going to use your sterilized soil right away store it indoors. Try to keep it from becoming re-infected with pests. Store in a slightly vented container or bag. If you close off all air flow you could get mold from the dampness.
I would love to hear your dirt! I mean about your dirt. Drop me a comment below. Also, If you have any questions about how to sterilize soil for planting, please ask. I am always here to help!
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