By: Nikki Tilley, Author of The Bulb-o-licious Garden
Since soil can harbor pests, diseases, and weed seeds, it’s always a good idea to sterilize garden soil before planting to ensure the most optimal growth and health of your plants. While you can go out and purchase sterile potting mixes to meet your needs, you can also learn how to sterilize soil at home quickly and efficiently.
Methods for Sterilizing Soil for Seeds and Plants
There are several ways to sterilize garden soil at home. They include steaming (with or without a pressure cooker) and heating the soil in the oven or microwave.
Sterilizing Soil with Steam
Steaming is considered one of the best ways to sterilize potting soil and should be done for at least 30 minutes or until the temperature reaches 180 F. (82 C.). Steaming can be done with or without a pressure cooker.
If you’re using a pressure cooker, pour several cups of water into the cooker and place shallow pans of level soil (no more than 4 inches/10 cm. deep) over top of the rack. Cover each pan with foil. Close the lid but leave the steam valve should be left open just enough to allow the steam to escape, at which time it can be closed and heated at ten pounds pressure for 15 to 30 minutes.
Note: You should always practice extreme caution when using a pressure for sterilization of nitrate-rich soil, or manure, which has the potential of creating an explosive mix.
For those not using a pressure cooker, pour about an inch (2.5 cm.) or so of water into the sterilizing container, placing the soil-filled pans (covered with foil) on a rack over the water. Close the lid and bring to a boil, leaving it open just enough to prevent pressure from building up. Once the steam escapes, allow it to remain boiling for 30 minutes. Allow the soil to cool and then remove (for both methods). Keep foil on until ready to use.
Sterilizing Soil with an Oven
You can also use the oven to sterilize soil. For the oven, put some soil (about 4 inches deep) in an oven-safe container, like a glass or metal baking pan, covered with foil. Place a meat (or candy) thermometer into the center and bake at 180-200 F. (82-93 C.) for at least 30 minutes, or when soil temp reaches 180 F. Anything higher than that can produce toxins. Remove from oven and allow to cool, leaving the foil in place until ready to use.
Sterilizing Soil with a Microwave
Another option to sterilize soil is to use the microwave. For the microwave, fill clean microwave-safe containers with moist soil — quart size with lids are preferable (no foil). Add a few ventilation holes in the lid. Heat the soil for about 90 seconds per every couple pounds on full power. Note: Larger microwaves can generally accommodate several containers. Allow these to cool, placing tape over the vent holes, and leave until ready to use.
Alternatively, you can place two pounds (1 kg) of moist soil in a polypropylene bag. Put this in the microwave with the top left open for ventilation. Heat the soil for 2 to 2 1/2 minutes on full power (650 watt oven). Close the bag and allow it to cool before removing.
This article was last updated on
Read more about Soil, Fixes & Fertilizers
Slide the pan into the oven and check the temperature every 15 minutes. One option is to poke a meat or oven thermometer through the foil and into the soil. Otherwise, just use a cooking thermometer to check the temperature.
Baking soil gives off an odor that some people find unpleasant. To avoid filling the kitchen with this smell, bake the soil on a nice day so the kitchen windows and door can be left open. If a strong, sharp smell comes from the oven, check the temperature of the soil. Usually this means that the temperature has suddenly risen beyond 200 degrees Fahrenheit.
How to Reuse Potting Soil
It’s generally fine to reuse potting soil if whatever you were growing in it was healthy. But even if your plants seemed problem-free, or if you did notice pests or diseases popping up, it's best to sterilize the mix before reusing in it to avoid infecting next year’s plants. First, remove any roots, grubs, leaves and other debris from the old potting soil. Then, decide on the best method for banishing microbes and insects.
One technique for sterilizing soil is called solarizing. It involves putting old potting soil in lidded, five-gallon buckets ($5, The Home Depot) or black plastic bags that are tightly tied shut and leaving them in the sun for four to six weeks. The heat builds up inside the buckets or bags just enough to kill pathogens and bugs.
You can also sterilize old potting soil in your oven, by baking it in an oven-safe pan, covered with foil, at 180 to 200 degrees F for 30 minutes. (I tried this once, but didn't like the earthy smell it created.) It’s also important to check the soil temperature with a candy or meat thermometer ($20, Williams Sonoma) to make sure it stays below 200 degrees. Higher temperatures can release toxins. When it’s done, take the soil out of the oven and keep it covered until it cools.
Microwaving is another option. Put old, moistened potting soil in quart-sized, microwavable containers. Cover them with microwavable lids (never use foil) that you can poke ventilation holes in or can leave cracked to allow steam to escape. Heat at full power for about 90 seconds per two pounds of soil. Remove the containers, cover the vent holes with tape, and let the soil cool completely before using it.
Once your old potting soil has been sterilized, you'll need to replenish its nutrients. You can do this by combining equal parts of new potting soil with the old and adding a dose of slow-release fertilizer pellets according to package directions. Or, you can mix in one-part compost to three or four parts of your old potting soil. Besides adding nutrients that plants need, both the fresh potting soil and compost will help keep the mix from compacting.
How to Use Black Plastic to Sterilize Soil
You may think harsh chemicals must be used to sterilize soil, but plastic sheeting that captures heat from the sun can also kill weeds, fungi, bacteria and some nematodes in the soil. Clear plastic sheeting is often recommended because the sun's rays are able to penetrate through to heat the soil to hotter temperatures, but in areas with long, sunny days, black plastic is an effective method -- with the added benefit of suppressing weeds. Called soil solarization, this process isn't practical in foggy coastal areas but works well in areas with four to eight weeks of hot, sunny weather.
Till the top 8 to 12 inches of soil to break up any large clods. The area should be a minimum of 2 1/2 feet wide to build up enough heat to kill bacteria.
Rake the soil to remove debris and create a smooth, flat surface.
Dig a small trench approximately 3 inches deep around the perimeter of the area to be treated. A small shovel or garden hoe is all you need to dig this trench.
Saturate the soil to a depth of at least 1 foot so the soil is better able to conduct heat.
Stretch a sheet of ultraviolet-resistant black plastic over the soil, ensuring it is tight enough to come in direct contact with the soil across the entire sheet. Thinner plastic allows the soil to heat up better but also tears more easily. Use plastic with a thickness of 1 to 4 mils for best results.
Fill in the ditch around the perimeter with soil to hold the plastic in place. You may also wish to add rocks to the edges to help hold down the plastic.
Place crushed aluminum cans on top of the plastic to act as spacers for a second layer of plastic, if desired.
Stretch a second piece of black plastic over the first piece of plastic and hold it in place with additional soil or rocks around the perimeter. The second layer increases the heat in the soil to aid in solarization. A second sheet is recommended when using black plastic because the material doesn't generate as much heat as clear plastic.
Wait eight weeks to allow the soil to heat up and kill the fungi and bacteria. Leaving plastic on for longer than eight weeks may harm the soil structure, but the maximum time is best when using black plastic.
Remove the plastic and plant fall-season crops if possible in your area. Wait until the next growing season in areas with short growing seasons.
Sterilizing potting soil is crucial when planting seeds, cuttings, or seedlings. Soil naturally contains fungi, harmful bacteria, and pathogens that can damage or kill susceptible plants. Sterilization of potting soil is easy and beneficial to the plant’s health.
Your soil could also be having insects and larvae that are harmful to plants. Store-bought soil may also be contaminated via old shelf-life, bag punctures, or other reasons. You can easily and quickly sterilize your potting soil at home as follows
1. Large baking sheet/roasting pan