If you want to reuse organic soil but are afraid of carrying forward fungus, bacteria or seeds from unwanted weeds, you're going to have to "clean" the soil. To clean your organic soil and make it reusable, all you need is your oven and an afternoon. By baking your soil in the oven you can avoid adding any harsh chemicals to your soil and you'll be able to plant organic seeds or seedlings into it without having to wait for harmful herbicides or fungicides to wear away.
Water your soil to moisten it throughout. The soil should form a loose clump when squeezed but break apart easily when poked.
Add the soil to a roasting pan up to 4 inches deep. Cover with aluminum foil, pressing the sides of the foil to the pan to wrap up the pan.
Push a thermometer through the center of the foil and into the soil. Heat the oven to 180 degrees and put the covered pan into the oven.
Wait for the thermometer to reach 180 degrees. Set the timer for 30 minutes after the thermometer reads 180 degrees.
Take the pan out of the oven after 30 minutes. Allow the soil to cool before use.