Mushroom compost can contain levels of soluble salts that can damage seedlings and salt-sensitive plants such as rhododendrons, azaleas, and blueberries, to name a few. Used with care, it is generally safe for most other things if you mix it in with the soil well. If you order mushroom compost in the fall and let it sit though the winter, it is a little safer because it has had a chance to cure a bit, thus reducing the salt content. Just remember not to store your mushroom compost pile next to your prize rhododendron!