Composting is a natural biological process that recycles organic waste into a nutrient-rich soil amendment called humus. This decomposition relies on a delicate balance of carbon-rich “brown” materials, nitrogen-rich “green” materials, air, and moisture. Introducing the wrong items into a home compost system can disrupt this balance or contaminate the resulting product. Understanding which materials to exclude is fundamental to creating safe and successful compost for your garden.
Materials That Attract Pests and Cause Odor
Food scraps high in protein and fat are the primary materials that attract unwanted pests and create foul odors in a home compost pile. Meat, fish, bones, and animal fats should never be included because they decompose slowly and attract rodents, raccoons, and flies. When these materials break down without sufficient oxygen, they create anaerobic conditions that generate strong-smelling volatile organic compounds.
Dairy products, including milk, cheese, and yogurt, also fall into this category due to their fat and protein content. The breakdown of these fats and oils coats other compost materials, slowing down the overall microbial decomposition process. This delayed breakdown contributes to putrefaction, making the pile a prime target for scavengers and insect pests.
Grease and cooking oils should also be excluded. They create a water-repellent barrier in the compost pile, hindering proper moisture distribution and airflow. The resulting lack of oxygen encourages the growth of anaerobic bacteria, leading to the emission of sulfur-containing organic compounds that produce the rotten-egg smell. Avoiding these high-fat and high-protein items is the most effective way to prevent pest infestations and unpleasant smells.
Contaminants That Introduce Toxins or Pathogens
Certain materials introduce harmful elements that can compromise the safety and quality of the finished compost, especially if it is intended for use on food crops. Pet waste from dogs and cats is a significant source of concern because it can harbor human pathogens and parasites. These are not reliably destroyed in the lower temperatures of a typical backyard compost pile.
Diseased or insect-ridden plants should be kept out of the compost to prevent the spread of problems to your garden when the compost is applied. Chemically treated wood and its sawdust must be avoided due to the presence of toxic preservatives. Older pressure-treated lumber, for example, often contains Chromated Copper Arsenate (CCA), which can leach heavy metals like arsenic and chromium into the compost.
Coal or charcoal ash is another problematic additive, as it may contain sulfur, heavy metals, and persistent toxins, especially if it comes from briquettes containing additives. Coal ash is high in heavy metals and should never be used. Furthermore, household chemicals, pesticides, and herbicides should never be added, as their toxic residues can kill the beneficial microorganisms that drive the composting process and contaminate the final soil amendment.
Items That Will Not Break Down
A successful compost pile requires materials that organic microbes can efficiently consume, which excludes many manufactured items that are inorganic or overly resistant to microbial action. Metals, glass, and most plastics are inorganic materials that will not decompose and will remain as contaminants in the finished product. Even items labeled “biodegradable” often require the sustained high temperatures and specialized microbial colonies of a commercial composting facility to fully break down, leaving behind non-composted fragments in a home setting.
Synthetic fabrics, rubber, and treated paper products like glossy magazines or heavily inked paper resist decomposition. They introduce non-organic dyes and coatings into the mix, and often contain synthetic polymers or chemical additives that are not food sources for the composting microbes.
Perennial weeds that propagate aggressively, such as bindweed or couch grass, should also be excluded unless the pile is guaranteed to reach and maintain sustained high temperatures (above 145°F). The root fragments and seeds of these persistent weeds can survive lower temperatures, allowing them to sprout later when the compost is spread in the garden. To avoid reintroducing invasive plants, it is safer to dispose of these weeds through other methods.