Compostable items are designed to be composted in a compost heap only. Composting is a very specific process which does not occur in landfills. Microorganisms, carbon, water, oxygen and nitrogen are all essential parts of the compost process, and these factors need to be present in the right circumstances (such as in a compost pile) for composting to occur.  If compostable products are placed in an open landfill or dump where oxygen is available, they will decompose at a rate similar to other biodegradable materials in the same setting.