I found this on another gardening website. @Laurap
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. (82 C.). 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 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.
Read more at Gardening Know How: Tips To Sterilize Potting Soil, Garden Soil And Soil For Seeds Sterilizing Soil: How To Sterilize Soil