How To Sterilize Soil & Organic Potting Mix Recipes

Potting soil is sterilized to give your plants the best environment possible by killing weed seeds or disease organisms that might be lingering in the soil. Most commercial products have already done this but if you want to use that rich dark gold right from your garden or reuse what you have, here are a few different ways you can do it yourself.

You’ll also find a few different recipes for making your own organic potting soil (found at the bottom of this page).

Garden Gear

Oven Method: (small batches)

  • Fill an ovenproof container about 3 inches deep with soil, mix in a generous amount of water (not enough to make it runny or soupy but thoroughly wet) then cover with aluminum foil.
  • Bake in a preheated oven (200°F) until the temperature of the center reaches 180°F (use a meat thermometer to measure). Once the temperature reaches 180°F, bake for 30 minutes.
  • Do not overheat or overbake since it can release toxins harmful to plants as well as kill beneficial organisms.
  • It can smell quite foul when baking, this is normal.

Microwave Method: (small batch)

  • To use the microwave, put about 2 pounds of moist soil in a thick, plastic bag. Leave the top open and place it in the center of the microwave.
  • Heat it for two to five minutes on full power, checking the temperature in the middle of the dirt with a thermometer. When the target is reached (180°F to 200°F), close the bag carefully and put in a cooler to hold the heat in.
  • Allow to cool. Source:

Sun Method: (large batches)

  • Choose a spot in the yard that receives at least 6 hours of sun during the day (8 hours a day is best).
  • Lay out clear plastic sheeting and cover with a layer of dirt about 4 inches deep. Spray generously with water (not so much that it becomes runny muck).
  • Cover with another sheet of clear plastic and secure the plastic in place by laying a border of rocks all along the edges of the plastic.
  • Bake in the sun for at least 4 weeks in hot, sunny weather and up to 6 or 8 weeks in cooler weather (this technique is only good for summer).
  • Tip: Rake up the dirt each week to make sure the heat reaches all of it.


  • Reusing potting soil is fine when using with mature plants, though new seedlings or bedding plants require sterilized to have the best chance to thrive.
  • Don’t use dirt straight from the garden for a potting medium (alone), mix it with other ingredients to make it lighter and more beneficial for your plants (see below for a few recipes).
  • After completing one of the methods above, the dirt will likely be hard and clumpy. Break it down first before mixing with other ingredients.
  • Make sure that potting containers themselves are clean since they can also harbor disease organisms. You can wash the containers in a bleach and water solution or see this page for more suggestions.
  • You can use these same methods for used potting soil and sand.

SeedlingIf you want to make your own organic potting mix, you still have to avoid using any prohibited ingredients and that means checking out all the individual ingredients for their organic acceptability.

It may surprise you to learn that products like peat moss or limestone are sometimes treated with prohibited materials such as wetting or anti-caking agents, so don’t rely on assumptions about purity.

In addition to meeting certification requirements, your final product will also need to provide plant roots with the right amount of air, water and nutrients.

(Source: Potting Mixes For Organic Growers).

Classic Soil-Based Recipe

1/3 mature compost or leaf mold, screened
1/3 garden topsoil
1/3 sharp sand

For Styrofoam Seedling Flats

2 parts compost
2 parts peat moss
1 part vermiculite, pre-wet

For Sowing

5 parts compost
4 parts soil
1-2 parts sand
1-2 parts leaf mold, if available
1 part peat moss, pre-wet and sifted

Note: All ingredients are sifted through a 1/4-inch screen. For every shovelful of peat, add two tablespoons of lime to offset the acidity.

For Pots and Baskets

30 percent topsoil
60 percent peat
10 percent perlite
5 pounds lime per cubic yard
3 pounds dolomitic lime per cubic yard

Note: The handling of this pot mix is the same as for pack mix.

Bedding Plant Recipe

25% compost
50% peat moss
25% perlite or vermiculite

*Source: National Sustainable Agriculture Information Services (first published February 28, 2007 and moved to this page for better organization)

Looking for a diy seed-starting mix? Try this recipe (Source:

4 parts screened compost
1 part perlite
1 part vermiculite
2 pars sphagnum peat moss (or coir)

Controlled Release Recipe:

  • Add 1 TBS blood meal, 1 TBS kelp, 1 TBS greensand, 2 TBS bone meal to 1 gallon of potting mix. Mix well. Source:

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    • Janet Breault

    I put my soil into an oven roasting bag to cut Dow on the smell. Just remember to poke holes in the bag to prevent the bag from exploding

      • Lynn Gregory

      What a good idea Janet, I wish I had thought of that. I will be doing it in future, thanks.

    • Tim Auld

    Sterilising your material does not give your plants the best possible growing environment. There was a short reference beneficial organisms in the article, but it’s not clear how sterilisation kills the bad ones and not the good ones. Has anyone verified these proceedures with a microscope?

    A much better, albeit slower, alternative to sterilisation is to hot compost your material. This will not only destroy weed seeds and disease causing organisms, but breed the beneficial microbes your plant depends on.

    If you don’t want to wait so long you could add enough (well made) compost and/or actively aerated compost tea to the material so the aerobic microbes can compete with any disease causing ones.

    Check the work of Dr. Elaine Ingham, soil biologist.

    • Cindi

    Id like to know how to get rid of Gnats in my house plants…

      • Apple

      Here is a great gnat trap: about 2” of apple cider vinegar with a little bit of dish soap in a container. I use a glass jar. The gnats love the smell of the vinegar, but the soap breaks the surface tension, so they sink to the bottom and drown. Muahahaha!!!

      • Brenda

      I hang fly strips in areas where they are a problem.

    • Jane

    Cindi, I’ve had great success using plain old vinegar to catch and trap ‘fruit flies’…wouldn’t hurt to give it a try for those pesky gnats! Just pour about a 1” depth into a coffee cup or other small container. Cover tightly with plastic wrap (I secure mine with a rubber band), then poke several holes (5-6) into the plastic wrap. I use a tooth pick…wiggle it around a little to enlarge the hole a tiny bit. Make it large enough for the critters to get in – but not out! Then set the cup near where you’ve seen insects…and wait. I usually set out several cups…2 or 3 in the kitchen, 1 in each bathroom, etc, and I leave them there for several days. The fruit flies are drawn to scent of the vinegar…they crawl in and drown. (Hint: if you see them sitting on top of plastic but not crawling in, enlarge the holes a little). Hope this helps with your gnat problem.

    • Melvin Yoder

    Put 1 tablespoon of castor oil in 1 gallon water of,mix and water your plants as usual

    • Melvin Yoder

    Put 1 tablespoon of castor oil in 1 gallon water mix and water your plants as usual

    • Mary

    I tried the vinegar method first. Some gnats were caught, but it did not get rid of all of them. I read that several inches of sand on the top of your soil will get rid of the gnats. That worked like a charm.

    • Margaret Kalton

    Growing Pitcher plants, and other insect catching bog plants such as sundew and others in a kitchen window is a particularly successful method of controlling fruit flies when summer fruit comes in. A south facing kitchen window provides the bonus of the sun illuminating the various colors of the pitchers.

    • TheAbstractAnomaly

    When using vinegar use a small bit of antibacterial soap in the mixture so that the vinegars nutrients will stick to the roots instead of floating in the medium

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