Storing Potting Soil - Prevent Mold

If you have used potting soil that was stored for several months, you might have felt frustrated by growing mold or an increase in pest problems. It is important to remember that soil isn’t just a home for your intentional plantings but also for living organisms at many levels. Soil is living. While we can’t prevent every pest or spore from growing, we can do a few things to reduce their growth opportunities and ensure our potting soil is clean when we are ready to plant.

How to Store Potting Soil

• 1. Let it dry. Allowing the soil to dry before storage prevents unwanted mold, fungus, and pest growth. It does not necessarily need to be powder dry, but dry nonetheless. Storing soil in humid conditions or with a high moisture content feeds the organisms living within it. New bags can simply be left in the sunlight as they have perforations allowing for ventilation.
• 2. Either sanitize open bags now before storing, or plan to sanitize before use. Always store clean soil in a clean container.
• How to Clean Used Containers: If you will be using a used bin, be sure to scrub it before using for long-term soil storage. I recommend using a diluted bleach solution with a ratio of 1 bleach: 9 water. Soak the container for 30 minutes. Drain most of the solution away (not on the ground) and scrub the container well. Rinse well. Allow the container to completely dry.

When first learning how to garden, someone may have told you it is important to use clean soil. They are right, but what exactly does this mean? How do you know if the soil is clean or not? Clean soil is considered soil that does not have undesirable weed seeds, pests, mold, or fungus in it. Generally, fresh unopened bags of soil are clean. However, if you are wanting to use a bag of soil that has been sitting open outside all summer long, it has probably attracted some pests, mold, or fungus spores. Stored soil should be sterilized before use. If reusing soil from previous plants, always sterilize it. There are several ways to clean soil.

How to Sterilize Soil

Microwaving is the best method for small batches, especially if you will be storing it for later use. Steaming can also be used for small batches and is easier to do right before starting seeds or transplanting. Solarization can be used for large amounts of soil. Keep in mind it takes several weeks (6-8) during warm weather and is best for soil already on the ground. If you have had continual problems with pest infestations, this method may help kill off any eggs and larvae hiding just below the surface.

• Microwave - This method is probably the easiest and is used for small batches of soil. The soil should be moist enough to form a clump that can easily be broken apart, but not wet. Microwaving without enough moisture will prevent the needed temperature from being reached. Place your soil into a microwave-safe container, such as a glass dish. Or, add your soil to a microwave-safe resealable bag and place the bag into a microwave-safe dish with the bag open. Use a microwave with at least 600 watts of power. Microwave it for 90 seconds. Check the internal soil temperature with a meat thermometer. Once it reaches 180F, you are good to go. If it is still below 180F, continue in 30-second increments until the temperature is reached.

• Steaming - Add soil to a large bowl. Add enough boiling water to your seed-starting soil so that it starts to clump up and steam as you mix it with the water. Press a piece of foil over your soil and allow it to sit until cooled.

• Solarization - Place a heavy-duty sheet of plastic over exposed potting mix. If your soil is already being stored in heavy-duty clear plastic bags, you can simply lay the bags of soil out in direct sunlight. This is the same process used to help kill a cover crop without tilling the ground. The plastic creates a greenhouse effect allowing the sun’s energy to heat up the soil mix. This will kill mold and fungal spores that may be growing in your soil. Keep covered in direct sunlight during warm weather for 4-8 weeks. Keep in mind this method should only be used on your garden bed if needed. While it will kill unwanted pests, bacteria, and fungi, it will also kill beneficial microbes, insects, and worms. After using the solarization method, revitalize your soil by adding materials like fresh potting soil, compost, worm castings, and earthworms. This combination will help restore a healthy microbiome with natural soil-building processes.