I read this and still don't fully understand the difference. Which decomposes faster? Do they mean biodegradables don't need "certain" conditions so they are good to go into a bin (or a hole in your backyard)? Or do they mean biodegradable plastic leave some non-soil residue behind?
Plastic that is compostable is biodegradable, but not every plastic that is biodegradable is compostable. Whereas biodegradable plastic may be engineered to biodegrade in soil or water, compostable plastic refers to biodegradation into soil conditioning material (i.e., compost) under a certain set of conditions. In order for a plastic to be labeled as commercially “compostable” it must able to be broken down by biological treatment at a commercial or industrial composting facility. Composting utilizes microorganisms, heat and humidity to yield carbon dioxide, water, inorganic compounds, and biomass that is similar in characteristic to the rest of the finished compost product. Decomposition of the plastic must occur at a rate similar to the other elements of the material being composted (within 6 months) and leave no toxic residue that would adversely impact the ability of the finished compost to support plant growth. ASTM Standards D6400 and D6868 outline the specifications that must be met in order to label a plastic as commercially “compostable”. There are currently no ASTM standard test methods in place for evaluating the ability of a plastic to compost in a home environment.