Composting is the natural process of recycling organic material into a nutrient-rich soil amendment by accelerating decomposition. This transformation is driven by microorganisms that require a balanced diet of carbon and nitrogen, along with adequate moisture and air. The quality of the resulting compost depends on selecting the right inputs. Understanding which materials support this microbial activity and which ones hinder it is key to creating a healthy compost system.
Essential Acceptable Materials (Greens and Browns)
A thriving compost pile requires a blend of two main categories of organic matter, known as “Greens” and “Browns.” Greens are rich in nitrogen, which microbes use to build proteins and reproduce, fueling decomposition. These items are typically fresh, moist, and break down quickly. Common examples include fruit and vegetable scraps, used coffee grounds, tea bags, and grass clippings.
Browns are high in carbon, serving as the energy source for microorganisms and providing structure and bulk to the pile. Their drier nature ensures good aeration, which is essential for rapid, odor-free composting. Standard Brown materials consist of dried leaves, shredded newspaper, cardboard, wood chips, and straw.
Materials That Must Be Excluded
Certain organic materials must be excluded from a standard home compost pile because they pose risks of attracting pests, spreading disease, or introducing contaminants. Animal products like meat, bones, fish, dairy products, and grease/oils are primary exclusions. They decompose slowly, emit strong odors that attract rodents and flies, and can harbor human pathogens. A typical backyard pile cannot consistently heat high enough to reliably kill these pathogens.
Diseased plants should not be composted because pathogens, such as fungal spores or viruses, can survive low temperatures and reinfect the garden. Similarly, pet waste from cats and dogs contains parasites and disease-causing organisms that require sustained high heat for eradication. Chemical contaminants like treated wood, coal ash, and materials containing pesticides or herbicides must also be avoided, as they introduce toxins that persist in the finished product.
Managing the Carbon-Nitrogen Balance
The functional relationship between Greens and Browns is expressed as the Carbon-to-Nitrogen (C:N) ratio, the most important factor in active composting. Microorganisms require a C:N ratio of approximately 30:1 for optimal metabolic activity and efficient decomposition. When this balance is upset, the composting process slows down significantly, and undesirable conditions emerge.
A pile with too much nitrogen (too many Greens) often becomes overly wet and anaerobic, leading to ammonia gas production and a foul odor. Conversely, a pile with too much carbon (too many Browns) will be dry, extremely slow to decompose, and may fail to heat up. Practical management involves adding more Browns—such as dry leaves or shredded paper—to correct a smelly, wet pile, or incorporating more Greens, like grass clippings or food scraps, to activate a sluggish pile.
Conditional Items and Preparatory Steps
Some common organic materials are conditional, meaning they can be composted successfully but require specific preparation or moderation. Sawdust, a high-carbon Brown, should only be added if it comes from untreated wood, and it must be balanced with extra nitrogen-rich Greens. Citrus peels and other acidic fruits can be included, but only in limited quantities, as excessive amounts may temporarily lower the pile’s pH and slow microbial activity.
Bread and baked goods should be buried deep within the pile and added sparingly to prevent attracting pests. Weeds that have gone to seed, or plants known to harbor persistent soil-borne diseases, should only be added if the compost pile can consistently reach the high temperatures necessary to kill seeds and pathogens. For most home composting systems, which rarely achieve this sustained heat, the safest practice is to dispose of these materials elsewhere.