How To Sterilize Soil At Home

Garden Soil & Supplies

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

How To Sterilize Soil

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 the soil in a preheated oven (200°F) until the temperature of the center of the soil reaches 180°F (use a meat thermometer to measure). Once the soil temperature reaches 180°F, bake for 30 minutes. Do not overheat or overbake the soil since it can release toxins harmful to plants as well as kill beneficial organisms. Baking soil can smell quite foul, this is normal.

Microwave Method: (small batch)

• To use the microwave to sterilize soil, 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. Treat it for two to five minutes on full power, checking the temperature in the middle of the soil 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 the soil. Allow to cool. Source: Use Microwave To Sterilize Potting Soil.

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 the soil generously with water (not so much water 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 the soil 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 soil each week to make sure the heat reaches all the soil.

Tips

• Reusing potting soil without sterilizing it first is ok to use when planting mature plants, new seedlings or bedding plants require soil that is sterilized to have the best chance to thrive.
• Don’t use soil straight from the garden for potting soil (alone), mix it with other ingredients to make it lighter and more beneficial for your plants (see Make Your Own Organic Potting Soil) for a few recipes.
• After the soil is sterilized it will likely be hard and clumpy, break it down first before mixing with other ingredients when making potting soil.
• Before placing potting soil in pots, make sure that the pots themselves are sterilized since they can also harbor disease organisms. You can wash the pots in a bleach and water solution or see How To Pot A Plant: Gardening Tips for more suggestions.
• You can use these same methods to sterilize used potting soil and sand.

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