Composting is a natural biological process that transforms organic waste into a dark, stable, and nutrient-rich soil amendment. This decomposition, driven by microorganisms, reduces household waste and creates a valuable material that enhances soil health and structure. Successful composting requires balancing different types of organic materials to provide the microbial community with ideal conditions. Knowing what to include and what to exclude is the first step toward a productive compost bin.
The Essential Ingredients: Defining Browns and Greens
The foundation of effective composting rests on achieving the correct balance between carbon-rich and nitrogen-rich materials, commonly referred to as “Browns” and “Greens.” Browns are dry, carbon-rich materials that serve as the primary energy source for microbes and add necessary bulk and air circulation to the pile. Greens are generally moist, fresh materials high in nitrogen, which microbes use as a building block for proteins and new cell growth.
The rate and efficiency of decomposition are governed by the Carbon-to-Nitrogen (C:N) ratio of the combined materials. Microorganisms function most effectively when this ratio is maintained within a range of approximately 25:1 to 35:1 by weight, with 30:1 often cited as the optimum target.
If the ratio is too high (excess carbon), decomposition slows because microbes lack sufficient nitrogen to reproduce. If the C:N ratio is too low (overabundance of nitrogen), excess nitrogen is released as ammonia gas, resulting in an unpleasant odor.
Acceptable Materials for the Compost Bin
A wide variety of household and yard waste can be safely added to a compost bin, provided they are correctly categorized and prepared. Carbon-rich Browns include materials that are typically dry and structural, such as shredded newspaper, untreated cardboard, sawdust, wood chips, and dried leaves. These items create air pockets within the pile, preventing compaction and ensuring aerobic decomposition. Nitrogen-rich Greens are generally fresh, moist, and soft, providing the necessary microbial fuel for heating the pile.
Common examples of Greens include:
- Fruit and vegetable scraps
- Coffee grounds and filters
- Tea bags without staples
- Fresh grass clippings
- Garden trimmings
Even materials that are brown in color, such as coffee grounds and manures, are classified as Greens due to their high nitrogen content. To accelerate the composting process, materials should be prepared before being added to the bin by chopping or shredding them into small pieces (0.5 to 1.5 inches). This increases the surface area for microbial contact, and adding a handful of finished compost or soil helps jump-start the decomposition.
Materials to Exclude (The Absolute No-List)
Certain organic materials should never be included in a home compost bin because they pose risks related to pests, pathogens, or chemical contamination. Meat, fish, bones, and dairy products are major exclusions because they attract scavengers and create foul odors as they decompose anaerobically. Fats, grease, and cooking oils also fall into this category, as they repel water, slow decomposition, and attract pests.
Pet waste from dogs or cats must be excluded because it can harbor harmful parasites and pathogens, such as Toxoplasma gondii and roundworm. Home compost piles typically do not reach the sustained high temperatures required to kill these pathogens. Similarly, diseased or insect-infested plants should be avoided to prevent the spread of problems when the finished compost is used.
Any wood that is pressure-treated, painted, or stained should not be added, as it can leach toxic chemicals into the final product. Glossy or coated paper, like magazines or certain food packaging, often contains non-biodegradable polymers or inks that contaminate the compost. Weeds that have already gone to seed are also discouraged, as the seeds may survive the composting process and sprout wherever the finished material is applied.