Black walnut tree leaves or twigs:
Leaves or twigs from black walnut release substances that might prove harmful to plants.
Coal or charcoal ash:
Coal or ash from charcoal, such as the ash that builds up in the bottom of charcoal grills, may contain substances that are harmful to plants.
Dairy products: When added to compost piles, dairy products, including butter, milk, sour cream, yogurt and eggs, can be malodorous and may attract pests, including rodents and flies.
Diseased or insect-ridden plants:
The diseases and insects that plague plants may survive being transferred to compost piles. When the compost is ultimately distributed, these diseases and insects might then plague other plants.
Fats, grease, lard, or oils, meat or fish bones and scraps:
Like fats, grease, lard or oils, scraps from meat and fish and fish bones can smell unpleasant, potentially attracting rodents and flies.
Pet waste, including soiled cat litter, may contain parasites, bacteria, germs, pathogens, and viruses.
Grass trimmings treated with chemical pesticides:
Trimmings from grass that was treated with chemical pesticides can kill beneficial organisms produced within compost piles or bins.