Skip to main content

How to Sterlize Potting Soil

Stephanie is an avid gardener who loves to get creative in the kitchen.

Sterilizing potting soil will increase the chances of growing healthy plants, starting healthy seeds and propagating cuttings. Many small plants and seedlings require sterilized soil to avoid diseases and parasites that may harm their tender parts and wounds on cuttings.

Sterilizing potting soil is quite easy and doesn't take long, depending on the method you choose. Below are a few different methods to do so, starting with the safest and easiest way.

This guide will share three different ways of sterilizing your potting soil.

This guide will share three different ways of sterilizing your potting soil.

How to Sterilize Potting Soil the Safe Way

The easiest, safest way to sterilize potting soil is to use boiling water. You can either sterilize the potting soil in the container you plan on using, or your can sterilize a batch using a large container with holes for drainage.

Start by boiling enough water to saturate the amount of soil you are using. For instance, if you are going to sterilize a 1-gallon pot or seed tray of potting soil, then you will only need a small boiler of water. But if you are sterilizing a large Rubbermaid container, then you will want a large stock pot or maybe even two. Either way, you want to have enough water to fully saturate the amount of potting soil and have excess draining out.

Once you have your water on to boil, prepare your soil in the containers you choose to use (this can be done ahead of time if need be). Once the water comes to a rumbling boil, slowly pour it in to the container filled with potting soil. Avoid splashing or allowing the hot water to drain onto any plants, as it can burn them—and you too!

Allow the soil to cool completely before planting any cuttings, seedlings or plants to avoid killing them.

Why Do You Need to Sterlize Potting Soil?

Sterilizing potting soil is usually related to growing seeds. Seeds are susceptible to thousands of types of bacteria and viruses that grow in the soil. By sterilizing the soil prior to planting them, you are giving your seeds a better chance at a healthy start. If you have ever tried to grow a plant from seeds and failed, you may want to consider using sterilization this time around.

Sterilizing potting soil also helps to kill off tiny organisms and pests that can and will eat the roots, stalks and fruits of your plants.

How to Sterilize Potting Soil in the Microwave

Potting soil can be sterilized in the microwave in small amounts. Place 4 cups of soil in a plastic bag, such as a gallon bag or grocery bag, and add 1 cup of water.

If using a gallon bag, twist the top only one or two times so that it is still very loose. If you are using a gallon bag, seal the top, leaving one corner open. Place the potting soil in a microwave safe plate so that it is easier to handle when taking it out.

Microwave the soil and water for 2 minutes. DO NOT GO OVER TWO MINUTES! You will break down the contents of the soil, and it can cause harmful chemicals that are dangerous to you and your plants.

After two minutes, remove the plate and potting soil and set it outside to cool. It will omit a foul smell, so I highly recommend you take it outside immediately.

Leave the top closed on the bag until it begins to cool down. The hot steam is what will actually be sterilizing the soil, so allow it to stay in as long as possible.

Note: Be careful, as the steam will be hot!

Sterilizing Potting Soil in the Oven

Sterilizing potting soil in the oven requires a little bit more attention and has its limitations. It's not the most popular method, but it does work.

Start by preheating the oven to 190–200°F. Spread the potting soil evenly on an oven-safe baking dish. I would recommend using an aluminum disposable pan or an old one that you don't mind discarding or re-purposing in the greenhouse. I don't think I would want to cook in the same pan for safety reasons.

Once your soil is evenly spread, cover the pan with tin foil tightly. Place a thermometer through the tin foil and into the potting soil in the center of the pan. Ensure that the thermometer is in the center of the soil. Place the pan in the oven and keep an eye on the temperature of the soil. Allow the soil to reach 160–180°F for at least 30 minutes before taking it out.

Once the soil has met those temperatures for 30 minutes, carefully remove the pan from the oven and take it outside to cool.

Note that sterilizing potting soil in the oven also gives off a horrendous odor!

Leave the tin foil on until it cools, allowing it to continue to steam. Once it is cool, remove the tin foil and it is ready to use!

Caution: Potting soil will ignite at high temperatures and also create harmful gasses. Never allow the soil to get higher than 200°F.

What do you use your sterile potting soil for? Seeds, cuttings or all your plants? Share your thoughts on sterilizing potting soil.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Do You Use Sterile Potting Soil?

anonymous on August 13, 2013:

@anonymous: grew up on a farm and never heard tell of sterilizing soil. well, i am now an urbanite and growing dishpan gardens. i saw mushrooms sprouting where mushrooms shouldn't have been and assumed something needed taken care of. not being one for fungicides and other toxins, i boiled a cup of water and tossed on the villains who quickly succumbed.

i am now determined at the end of every season when i am ready to start new bags of soil, i will add last year's deforested soil, sterilized with boiling water. i don't relish the thought of putting dirt in my oven or nuke. the latter, i have no trust of, and the former i bake bread and coffee in. not too likely i'd want potting soil flavors and aromas in there.

anonymous on February 02, 2013:

never thought of doing this but my plants have not been doing well and i think it might be the soil wich i make my own and maybe i should do this to the soil i have mixed up couldn't hurt

anonymous on November 23, 2012:

Never even thought of sterilizing soil before! But will definitely do this for the rose cuttings I plan on trying to propagate!