Composting is often a process learned by rote — worms hate citrus peels, oak leaves turn compost acidic, lime should be added to compost. These rules get passed along from one composter to another, without much thought as to whether they are true.
During a recent seminar at the annual Rhode Island Compost Conference & Trade Show, Robert Rafka, a chemist and a University of Rhode Island master composter, shared this thoughts. He cited personal experience, molecular science and old-fashioned common sense. Here is his rundown:
Woodchips and sawdust. Painted or treated wood should be avoided as it can contain toxins. Composite wood usually contains glue and triclosan, which prevents the growth of microbes and fungi integral to the composting process. Cherry contains cyanide. Untreated, unpainted, non-composite, non-cherry wood is a good source of browns for your compost pile.
Newspaper ink is soy based and a safe source of browns for a compost pile. Rafka avoids using glossy paper in his compost pile.
Cooked meat scraps can be composted during cold months in a secure bin. However, composting meat can attract animals. In warm weather decomposing meat will smell.
Citrus peels contain limonene, which worms dislike. Conveniently, the greenish blue mold that develops on old citrus peels selectively eats limonene. Once the mold has eaten enough of the limonene, the worms will eat the citrus peels.
Grass clippings treated with herbicides or pesticides shouldn’t be used in a compost pile. Rafka suggests keeping untreated, dried clippings on top of a compost pile to deter flies, rather than adding them to the pile where they will become “slimy.”
Gray water. Dishwater shouldn’t be used to wet a compost pile. Dish detergents often contain triclosan, which prevents microbial and fungal growth. Some detergents also contain molecules that break down into toxic byproducts.
Manure. Medicines and antibiotics administered to farm animals are present in manure. During the composting process they break down into copper sulfate, which is toxic to humans at high concentrations, according to Rafka.
Feather meal raises the nitrogen level of compost. Unfortunately, the feathers come mainly from commercially farmed chickens and contain residues from antibiotics, contraceptive hormones and Prozac.
Coffee grounds, walnut leaves and sunflower seed hulls. Each comes from plants that produce allelopathic chemicals, such as caffeine, that kill surrounding plants to eliminate competition. While there is no research about whether these chemicals survive the composting process, Rafka assumes they do and uses these items only in moderation.
Lime, seashells, wood ashes, eggshells and bones slow the composting process by raising the pH level of the pile. Compost piles function best when they are slightly acidic. These items also strip nitrogen from the finished compost. They don’t make the compost pile toxic.
Oak leaves decompose slowly because of their waxy coating, but don’t negatively affect the finished compost’s pH as is often rumored. Chopped leaves break down more quickly.
Rafka recognizes that, unlike himself, not all composters are scientists with a Ph.D. He suggests looking at a wide range of reputable sources when trying to determine the best composting practices. Rafka ranks universities among the most reputable sources and commercial operations as the least.
When all else fails, Rafka suggests approaching composting the same way you approach a lunch buffet. “Mix up your plate and you will be happier,” he says.