Answer: It is mold or are the white particles perlite and the brown particles vermiculite? Both are often found in potting soil. They keep the soil light in weight and help hold moisture. Squish a few of these between your fingers. The perlite will be hard and will crumble when you apply pressure; vermiculite is spongy and lightweight. If that's what's in your soil, go ahead and use it right out of the bag. If the particles really are mold, all you need to do is spread the potting soil out and let it dry. Mold grows naturally on decomposing organic matter when there is no fresh air circulation, such as the conditions you would find in a plastic bag. After spreading it out in the sunshine, the mold will die off and you can safely use it for your plants.
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