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The kitchen scraps and yard waste you toss onto the compost heap are, by definition, compostable. These organic materials break down and undergo chemical reactions that transform them from a heap of decaying food and shrub to nutrient rich compost. You use the compost to enrich the soil in your garden. As more products become "green", though, there may be more to learn about compostables.
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Decaying fruit and vegetables are perfect for your compost pile. Meat products, though, are not considered compostable items by backyard gardeners. Meat may break down into biological elements, but it is not an organic material that decomposes in a relatively short time an d turns into usable soil.
Meat and meat bone takes considerably longer than vegetable matter to break down, and a host of microbes will not aid in its decomposition. Meat then does not meet the vernacular definition of a compostable.
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Plastics That Can Be Composted
The definition of compostable goes beyond the lingo of home gardeners, though. The American Society for Testing Materials (ASTM) has put forth a document (ASTM D6400) that sets industry standards regarding compostable plastics. The ASTM defines such a material as one that decomposes due to biological processes, such as those that result from microbial activity, and the decomposition of such a product results in a non-toxic compost usable for soil enrichment. The plastic material must break down within a time frame relative to those of organic materials, such as kitchen scraps and yard waste.
Compostable plastics differ from petroleum based plastics. They're derived from starchy foods such as potatoes or corn. The foodstuff, through a manufacturing process, is turned into polylactic acid, or PLA. Due to the organic origins of the plastic, the material should theoretically act in a fashion similar to organic materials when put into a composting environment.
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How Well do They Break Down?
Polylactic acid is used to make items such as meat trays and salad containers you get at the deli, but if you put them in your backyard compost heap, you won't see them break down as quickly as your discarded orange peels.
The standard for defining compostables says the product has to decay within a specified time frame in a composting environment. That environment, though, is defined as one that maintains a temperature of over 140 degrees for at least 10 days, along with a laundry list of other factors. A backyard compost heap isn't likely to meet the criteria.
Organic based plastics don't decompose in landfills either. Modern landfills are designed to prevent sunlight and air from breaking down the rubbish to prevent toxins from leaching into the air and water. It also prevents the compostable materials in the landfill from composting.
Whether the growing use of PLA products expands the commercial composting industry in municipalities is yet to be seen. Creating standards for manufacturing compostable products may only be considered "green" if the material is actually composted. There's more to be learned about this important issue in years to come.