Answer: What you've discovered is a natural fungus present when organic material decomposes. It won't hurt the roots of healthy plants because the fungus lives on decaying plant material. The fungus will die if you turn the soil and expose it to air and sunshine. Mushrooms growing above ground are an indication that something below is decomposing. Usually its an old piece of wood or a dead tree root, but in this case it's just a lot of decaying organic matter. The bed is perfectly safe to plant in and will probably produce lush, healthy plants because of all the organic matter it contains.
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