Composting is a natural biological process that transforms organic waste materials into a stable, nutrient-rich soil amendment through controlled decomposition. This process is driven primarily by microorganisms like bacteria and fungi, which break down complex organic structures. The resulting product, often called humus or finished compost, significantly improves soil structure, water retention, and plant health. Understanding which materials support this microbial ecosystem is the foundation for successful home composting.
Acceptable Compost Materials (Greens and Browns)
Successful composting relies on providing a balanced diet for the microorganisms, achieved by mixing materials categorized as “Greens” and “Browns.” These categories refer not to color, but to the chemical contribution, specifically the ratio of carbon (C) to nitrogen (N). Carbon-rich Browns serve as the energy source for microbes and add necessary bulk and aeration to the pile.
Brown materials include dry, woody, or paper-based items that are high in carbon. Examples suitable for home compost include dry autumn leaves, shredded cardboard, untreated sawdust, straw, and non-glossy newspaper. These high-carbon materials decompose slowly but provide the structural matrix that allows oxygen to penetrate the pile, essential for aerobic decomposition. They must be balanced with nitrogen-rich items to prevent decomposition from stalling.
Greens are materials high in nitrogen, the protein source microorganisms need for growth and rapid decomposition. Common green materials include fresh fruit and vegetable scraps, coffee grounds, used tea bags, and fresh grass clippings. Manure from herbivores like cows, horses, and chickens is also an excellent nitrogen source, despite often appearing brown. Incorporating these nitrogen-dense items ensures the microbial population thrives, driving the compost pile temperature upward.
Materials That Must Be Excluded
Certain organic materials should never be introduced into a home composting system because they pose risks related to pests, pathogens, or chemical contamination. The heat generated by small-scale home piles often does not reach the sustained temperatures required to destroy harmful elements, unlike industrial facilities. Excluding these items protects both the compost quality and the safety of the user.
Meat, fish, bones, and dairy products, including milk, cheese, and oils, must be excluded from the pile. These materials are high in fats and proteins that break down slowly, attract pests like rodents and flies, and produce foul odors as they decompose anaerobically. Grease and other fatty foods will also coat compost materials, creating a water-repellent barrier that slows the entire process.
Pet waste, specifically dog and cat feces, must be avoided due to the high risk of transferring human pathogens and parasites. Cat litter, for example, can harbor the parasite responsible for toxoplasmosis, while dog waste may contain roundworms. Plants that are visibly diseased or infested with insects should also not be composted. If the pile does not become hot enough, these pathogens or pests can survive and spread to healthy plants when the finished compost is applied.
Materials contaminated with chemicals or preservatives require exclusion to prevent introducing toxins into the soil. This includes any wood that is painted, pressure-treated, or stained, as well as plants recently treated with pesticides or herbicides. Glossy or coated paper, such as magazines or receipts, should also be avoided because of the potential for heavy metals and inks to leach into the final product.
Optimizing Input Preparation and Ratios
Achieving high-quality compost quickly depends heavily on how accepted materials are prepared and layered. The efficiency of decomposition is directly related to the surface area available for microbial action. Therefore, it is beneficial to shred, chop, or break down all inputs, aiming for particle sizes no larger than one or two inches. Smaller pieces accelerate the work of the microorganisms by providing more edges for them to colonize.
Proper moisture content is necessary, with the ideal range being between 50 and 60 percent, or the consistency of a wrung-out sponge. If materials are too dry, microbial activity slows significantly; if too wet, the pile becomes compacted and oxygen-deprived, leading to slow, smelly anaerobic conditions. Dry brown materials should be moistened before or during addition, while overly wet greens should be balanced with dry browns.
The goal is to maintain a Carbon-to-Nitrogen (C:N) ratio near 30:1 for optimal microbial performance, though a range of 25:1 to 35:1 is acceptable. Since nitrogen-rich greens have a C:N ratio around 15:1 and carbon-rich browns can exceed 100:1, this ideal is achieved by using a significantly greater volume of brown materials than green materials. A common practical guideline is to layer the pile with roughly two to three times the volume of browns for every one part of greens. A nitrogen-heavy mix results in the loss of nitrogen as ammonia gas, producing an unpleasant odor, while a carbon-heavy mix causes decomposition to proceed very slowly.