If you have a backyard compost pile, it's obvious to many people that items like bones, meat and used cooking oil should never be tossed in, because they attract pests and can ruin a compost's moisture balance. But there are other never-add items that you may not know about: Sawdust, for example, comes from trees and seems compostable, but unless you know how the lumber was treated, you should not add it, since there may be chemical residue. Dead houseplants should also be avoided, because you could be adding disease to the compost. And finally, walnuts: They have a natural chemical called juglone that is toxic to plants. So adding walnuts to compost used in a garden could act like a herbicide over time.