How to Sterilize Soil for Planting – Clean Dirt

by | Mar 17, 2018 | Guides, HowTo's, Tips & Tricks | 11 comments

You can easily learn how to sterilize soil for planting in a few steps with items you likely already have at home. Sterilization does not need to be intimidating or costly. You can spend a lot of money buying new sterile potting mixes every time you go to plant something new. However, that can add up and become quite expensive!

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 castingsI 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

  1. Premoisten soil and break down clumps
  2. Place in microwave-safe dish or bag
  3. Leave uncovered or open for venting
  4. Microwave on high 2 to 2 ½ minutes (650 Watt) or 1 ½ minutes (1000 Watt)
  5. 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.
  6. 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

  1. Premoisten soil and break down clumps
  2. Spread soil in the pan to about 3 to 4 inches in depth loosely
  3. Cover tightly with aluminum foil
  4. If using meat thermometer place through the center of foil. Leaving only a gap wide enough for the thermometer.
  5. 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)
  6. 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.
  7. 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.

red arrow down rightI 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!

Happy Gardening!

Christina insideherbgardens.com

Have you been searching for a great soil?

My Top Potting Soil Recommendation!

Get the ultimate blend of the Earth and Sea and start your plants off right! Fox Farm Ocean Forest Soil is a premium blend earthworm casting, bat guano, hummus, kelp, fish and crab meal.

With the right soil, everyone has a green thumb!

11 Comments

  1. Miranda

    I do not have a “green thumb” AT ALL and I had no idea you were supposed to sterilize the soil first! I suppose that’s why I can never keep a plant alive. I’m glad I read this and I will pass this along to my mother-in-law as well, because she has the same troubles lol. I am going to bookmark this page so I can come back to it when I start all over again. Thank you for posting!!

    Reply
    • Christina

      Hi Miranda,
      I am glad my post was able to help you.
      I will let you know a secret. A green thumb is a myth 🙂
      Succesful gardening just comes from having all the right ingredients and knowledge in place. Your soil is an important base layer for getting things growing. A strong healthy soil will get you started off right.
      If you run into any other trouble or have any questions at all just ask. I will be happy to help. I love to see everyone succeed!
      At the bottom of the home page, there is a subscribe form that can keep you updated when a new post is published if you are interested. A great way to stay on top of new things.
      Happy Growing!
      Christina

      Reply
  2. Michelle

    OH Wow…who would have thought!
    I have learned so much from this article, thank for sharing Christina. I am guilty of just running to the store to get some potting soil and then planting. This is ideal reading as Spring is in the air! I will have to try out one of these methods – my curiosity is getting the better of me!
    Thanks so much.
    Michelle

    Reply
    • Christina

      Hi Michelle,
      Sounds like you are a passionate gardener too! You have those itchy spring thumbs! I can relate haha
      Just remember not all soils need to be sterilized. I was telling another do a glass jar test first. Sterilizing soil removes good stuff too. So if you’re not replanting in old soil or have an issue it is not necessary. Take a handful of soil and place it in a glass jar. Dampen it and seel. Give it 4 days to a week. If you see any wiggly pests or flying gnats then sterilization is a must. If all is clear, the soil is good for planting!
      Try to use good grade soils. Although any soil can have pests. Cheaper brands are more likely to carry unwanted pathogens and bad bacteria. I recently wrote an article on a soil that I highly recommend. The Fox farm ocean Forest.
      If sterilization on your other soil becomes necessary don’t forget to replenish! I love worm castings. This is the brand I use and wrote about. Simple Grow Soil Builder – Worm Castings
      Happy Growing, I look forward to hearing more!
      Christina

      Reply
  3. Melissa

    Wow I have never heard of sterilizing soil before. But with all you explained of why it is a good idea makes complete sense. I will have to give this a try next time. I am with you on those gnats. They are annoying. Thank you for the wonderful insight.

    Reply
    • Christina

      Hi Melissa,
      Yes, Gnats and Aphids are my enemies lol. I do not like any pests. Those two seem to be the most annoying to deal with. Sadly most often when I have an issue with either of them it is due to new plant introductions. I can not resist adding to my garden. I should be banned from nurseries and greenhouses! Well, I would be if my husband had his way!
      If you are anything like me and bring in stray plants. Keep them separated from your other plants for a few days and make sure they may not be your gnat issues too.
      Glad I was able to help!
      Christina

      Reply
  4. DianneBee

    I wish I’d had this info before I lost several new potted plants last year! I thought I’d never buy organic soil mix again. But now I know what to do. Thanks! Very enlightening.

    Reply
    • Christina

      Hi DianneBee,
      Not every soil you buy will need to be sterilized right away. Sometimes they will be perfectly fine. I like to do a test when I get a new bag. Take a small handful and place it in a glass jar. Moisten in and seel. Wait 4 days to a week and inspect the surface of the soil. if you can see little wiggly pests or flying insects in the jar you will need to sterilize for gnats. If you see nothing, the bag is likely fine for planting.
      My favorite soil is Fox farm ocean forest. I recently wrote a review about it here. If you are looking for a great organic soil, this is a good choice!
      Happy growing and better success to come!
      Christina

      Reply
  5. John

    Christina,
    Never thought that soil that is in a bag would be so full of bad things. Baking dirt does sound a little strange to me at first. I just planted my seeds for spring with soil that I purchased last fall and was sitting in the garage. I think I will test some of it to see if it has any pest in it, the plants have not sprouted yet.
    Thanks for the information.
    John

    Reply
    • Christina

      Hi John,
      In many cases, the bagged soil will be ok. Testing it is a great idea.
      As gardeners we have to keep in mind we often buy Great Dirt! Appetizing and attractive to pests. Those little holes in the bag are like open doors inviting in guests for dinner and to make homes. They are a necessary part of the bag construction. Many times we get lucky and no issues. other times, however, we will get a bag and it is loaded with gnats. I have been guilty myself before of quickly blaming a company.
      Years later in a similar case that you have right now, I realized the fault was “nature”. I had a vented tub in my garden shed. it was left over soil I had used the year before with zero issues. I transplanted several indoor plants in the soil and BOOM! Fungus gnat explosion hit the house. On examining the transplants I could see the larva in one of the pots. 🙁 I know they did not sit there dormant for a year, they had to infect the soil as it sat in my shed.
      That is when I adopted soil sterilization. It has helped me a lot, especially with gnat issues.
      Let me know how your test goes.
      Christina

      Reply
  6. Donnis Schroeder

    I live in Sun City, AZ in the winter months. I have large pots, with no drainage, that I plant flowers in each season. By March of each year aphids appear. When I leave for summer could I dump soil out on tarps in the hot summer sun in AZ. Would it do the same as doing the oven thing. I have too much soil to put in the oven. Thank you for your advice. Donnis

    Reply

Submit a Comment

Your email address will not be published. Required fields are marked *

Stay Up To Date On New Articles & More!

Simple Opt-in, No Spam!

Pin It on Pinterest

Share This