Composting is the natural, aerobic process where microorganisms break down organic materials into a dark, rich, soil-like substance called humus. This process requires a balance of four components: carbon-rich “browns,” nitrogen-rich “greens,” moisture, and oxygen. While the goal is to create a nutrient-dense soil amendment, not all organic material is suitable for a home pile. Introducing the wrong items can disrupt decomposition, create odors, or contaminate the finished product with harmful pathogens or toxins. Understanding which materials to exclude is necessary for a successful composting experience and a safe garden.
Materials That Attract Pests and Pathogens
Meat, fish, poultry, and related scraps like bones must be kept out of backyard compost systems due to health and nuisance risks. These animal products decompose slowly, often creating dense, anaerobic pockets that lead to foul odors. The smell of putrefying animal protein attracts scavengers, including rodents, raccoons, and flies, which can contaminate the pile.
Fats, grease, and oils also present problems beyond attracting pests. The hydrophobic nature of these materials causes them to coat other organic matter, creating a water-resistant barrier that slows microbial activity. This coating reduces airflow and moisture absorption, hindering the aerobic decomposition necessary for odor-free composting. Additionally, the high nitrogen content in animal products can disrupt the ideal carbon-to-nitrogen ratio, potentially leading to the release of ammonia gas.
Dairy products, such as milk, cheese, and butter, are discouraged for reasons similar to meat. The dense proteins and lipids in dairy attract pests and can create anaerobic conditions as they decompose slowly. Both meat and dairy can also harbor dangerous human pathogens, including Salmonella and E. coli.
A typical home compost pile may reach temperatures between \(130^{\circ} \text{F}\) and \(160^{\circ} \text{F}\) in the center, but these temperatures are often not sustained or uniform. To reliably kill most harmful bacteria and viruses, the compost must be maintained at a minimum of \(131^{\circ} \text{F}\) (\(55^{\circ} \text{C}\)) for at least three consecutive days. Failure to meet this thermal sanitization standard means pathogens can survive and be reintroduced into the garden.
Pet waste, specifically dog and cat feces, must be excluded from any compost intended for food gardens due to the high risk of parasites. Cat feces can contain the protozoan Toxoplasma gondii, which causes toxoplasmosis and is dangerous to pregnant women and individuals with compromised immune systems. Dog waste often carries roundworms, such as ascarids, which can cause serious human conditions. The eggs of these parasites are highly resilient and can survive the variable temperatures of a home compost pile, persisting in the soil.
Items Introducing Harmful Chemicals and Contaminants
Materials containing synthetic chemicals or non-biodegradable elements compromise the safety and integrity of the final soil amendment, making them unsuitable for composting. Chemically treated wood, such as lumber used for decks or fences, is preserved with substances like chromated copper arsenate (CCA) or other copper-based compounds. When this wood breaks down, these heavy metals, including copper, chromium, and sometimes arsenic, leach directly into the compost.
These inorganic toxins do not decompose and will remain in the finished product, posing a risk of absorption by food crops. Similarly, ash from charcoal briquettes should never be added, as it contains sulfur and chemical residues from petroleum-based additives and binders. This is distinct from untreated wood ash, which can be added sparingly, as charcoal ash introduces undesirable chemicals and heavy metals.
Paper products featuring a glossy finish or colored inks should also be excluded, despite their organic origin. The glossy coating is often a non-biodegradable plastic or clay-based material, and the colored dyes and pigments used in printing inks can contain heavy metals like lead or cadmium. While black-and-white newspaper ink is typically safe, colored or shiny paper may contaminate the compost with substances that accumulate in the soil.
Textiles, plastics, glass, and metals must be kept out of the compost pile because they are non-organic or non-biodegradable. Synthetic fabrics, such as polyester or nylon, are petroleum-based and will not break down, persisting as microplastic contaminants. Even items that appear organic, like tea bags or coffee pods, may contain hidden plastics that fragment into the finished compost.
Yard waste exposed to certain chemicals poses a contamination risk that can ruin an entire batch of compost. Grass clippings or hay treated with persistent broadleaf herbicides, known as synthetic auxins, should be avoided. These chemicals are designed to survive environmental breakdown and can persist through the composting process for months or years. When the resulting compost is used, these residual herbicides can cause damage to sensitive garden plants, leading to stunted growth or deformed foliage.
Plant Matter That Should Be Excluded
Certain types of organic plant matter must be excluded from the compost pile because they can undermine the garden’s health. Plant material showing signs of disease, such as tomato blight, rose black spot, or powdery mildew, should never be composted at home. These fungal and bacterial pathogens often produce resilient spores that are not killed by the temperatures achieved in a typical backyard system. Reintroducing this infected compost will spread the disease back to susceptible plants, especially since home piles frequently fail to maintain the necessary minimum temperature of \(131^{\circ} \text{F}\) uniformly.
Aggressive weeds and plants that have gone to seed are problematic additions that should be managed separately. Weeds like thistle, bindweed, and quackgrass have tenacious root systems and rhizomes that are difficult to kill through low-temperature decomposition. Perennial roots can survive the process and begin growing once the finished compost is spread, effectively re-infesting the area.
Weed seeds are especially resilient, often requiring a sustained temperature of \(140^{\circ} \text{F}\) or higher for several days to be sterilized. Since the cooler outer layers of a home compost pile may not reach or maintain this temperature, the finished compost can become a carrier for viable weed seeds. Introducing these seeds back into the garden creates a cycle of persistent weed problems.