Composting is the natural process of recycling organic matter into a dark, nutrient-rich soil amendment. This controlled decomposition transforms kitchen and yard waste into a valuable resource for gardening and landscaping. A successful home composting system relies on identifying and combining the correct inputs to feed the beneficial microorganisms responsible for the breakdown. This guide focuses on the specific materials that should be included or excluded to ensure the creation of a healthy, finished product.
Understanding Carbon and Nitrogen Sources
The successful decomposition of organic materials is governed by a balanced relationship between carbon and nitrogen. Microorganisms, primarily bacteria and fungi, require both elements to thrive, using carbon as their energy source and nitrogen for building proteins and reproducing cells. Carbon-rich materials are referred to as “Browns,” while nitrogen-rich inputs are known as “Greens.”
The speed and efficiency of the process depend on maintaining an ideal carbon-to-nitrogen (C:N) ratio, approximately 30 parts carbon to 1 part nitrogen by weight. When the ratio is too high, decomposition slows significantly because microbes lack sufficient nitrogen for growth. Conversely, a ratio too low means excess nitrogen is lost to the atmosphere as ammonia gas, resulting in unpleasant odors. The goal is to provide the microorganisms with the nutritional balance needed for rapid conversion.
Common Materials Safe to Compost
Materials safe to compost provide the necessary carbon and nitrogen to maintain the optimal C:N ratio and support microbial activity. These inputs are divided into high-nitrogen “Greens” (kitchen waste) and high-carbon “Browns” (yard or paper waste). Preparing these materials by chopping or shredding them into smaller pieces increases the surface area and accelerates their breakdown within the pile.
Kitchen Scraps (Greens)
Vegetable trimmings, fruit peels, and cores are excellent nitrogen sources. Coffee grounds (C:N ratio around 20:1) are a potent nitrogen addition and can be added directly along with their paper filters. Used tea bags, provided they are made of natural fibers and do not contain plastic staples, are also welcome contributors.
Eggshells are a valuable mineral source, primarily calcium carbonate, which helps enrich the final soil amendment. They should be crushed into a fine powder before being added to ensure they fully break down. Stale bread, pasta, or cereal not contaminated with grease or dairy can also be included, offering carbohydrates for the microbial population.
Yard & Paper Waste (Browns)
Dry autumn leaves are common and effective carbon inputs, providing structure and bulk to the compost pile. Shredded newspaper, which has a high carbon content, is an excellent way to balance the moisture and nitrogen from kitchen scraps. Unwaxed and untreated corrugated cardboard, torn into small pieces, also acts as a high-carbon bulking agent that helps maintain proper aeration.
Wood shavings, chips, and sawdust (from untreated wood only) are beneficial for their high carbon content and ability to absorb excess moisture. Hay and straw are commonly used carbon materials that introduce pockets of air into the pile. This aeration is necessary for the aerobic respiration of the decomposing organisms and helps speed up the composting process.
Essential Materials to Keep Out
Certain materials should never be included in a home composting system due to risks they pose to human health, their potential to attract pests, or their ability to contaminate the finished product. Excluding these items ensures the compost is safe and beneficial for garden use. Home piles rarely reach the sustained high temperatures required to neutralize risks that industrial facilities can handle.
Meat, fish, poultry, and dairy products must be excluded because they rapidly putrefy, creating strong odors that attract rodents and scavengers. These animal products introduce fats and oils, which form water-repellent barriers that slow decomposition and can become rancid. Grease and cooking oils should also be avoided as they coat organic materials, preventing water and air from reaching the microbes.
Dog and cat feces are a serious exclusion because they can harbor human pathogens, including Toxoplasma gondii and various species of roundworms. These parasites and bacteria may survive the fluctuating temperatures of a home compost pile, creating a health hazard when the compost is used in food gardens. Diseased or insect-infested plant matter should also not be added, as pathogens or pests may survive and later infect healthy plants.
Chemically treated wood, such as lumber preserved with copper, chromium, or arsenic, will leach toxic compounds into the compost. Persistent herbicides, which remain active through the composting process, may damage sensitive plants like tomatoes and beans when the compost is used. Glossy magazine paper or coal ash should also be avoided because they may contain heavy metals and toxic dyes that can leach into the soil.