Composting is a natural biological process where microscopic organisms break down organic materials into a nutrient-rich soil amendment called compost. This transformation relies on providing the microbial workforce with the right resources: “greens” and “browns.” A proper mixture of these materials supplies the chemical elements necessary to fuel decomposition, ensuring the pile remains active, heats up sufficiently, and produces high-quality finished compost.
Defining the Components: Browns (The Carbon Source)
“Browns” are primarily the source of carbon (C) in a compost pile, serving as the main energy supply for decomposing microorganisms. Carbon is the foundational building block, making up a significant portion of microbial cells. These materials are typically dry, woody, or fibrous, decompose slowly, and have a generally brown or gray appearance.
The high-carbon content of browns provides structure and bulk, helping to maintain air pockets within the pile. This aeration is important because the microbes responsible for rapid decomposition require oxygen to thrive. Common examples include dried autumn leaves, shredded paper and cardboard, wood chips, sawdust, straw, and hay.
Browns create the “fluff” that prevents the pile from becoming dense and compacted. Their function is two-fold: providing long-lasting energy for microbial activity and ensuring the proper airflow needed for an aerobic, odor-free process. Without adequate carbon, the composting process slows down significantly and fails to generate the heat required for efficient breakdown.
Defining the Components: Greens (The Nitrogen Source)
“Greens” are characterized by their high nitrogen (N) content, which serves as the “protein” source for the microbial community. Nitrogen is necessary for the growth, reproduction, and synthesis of enzymes within the microorganisms. These materials are often moist, fresh, and tend to decompose quickly, though their color is not always green.
Non-green materials like coffee grounds, tea bags, and most animal manures are classified as greens due to their significant nitrogen contribution. Other examples include fresh grass clippings, fruit and vegetable scraps, and green garden trimmings.
The moisture and nutrient density in greens stimulate microbial activity, causing the compost pile to heat up quickly. However, too many greens can lead to problems due to their high moisture content and rapid breakdown. An overabundance of nitrogen causes the pile to become slimy and releases ammonia gas, resulting in an unpleasant odor.
The Critical Balance: Understanding the C:N Ratio
Achieving a balance between the carbon from the browns and the nitrogen from the greens is known as the Carbon-to-Nitrogen (C:N) ratio. This ratio dictates how efficiently microorganisms utilize available resources to break down organic matter. The optimal starting ratio for a mixed compost pile is approximately 30 parts Carbon to 1 part Nitrogen by weight (30:1).
The ideal C:N ratio ensures that microbes have enough carbon for energy and sufficient nitrogen to build their cell mass. If the ratio is too low (too many greens), excess nitrogen cannot be utilized and is lost to the atmosphere as ammonia gas, causing an odor.
Conversely, if the ratio is too high (too many browns), microbes consume the available nitrogen quickly and slow down their activity. This results in a compost pile that remains cool and takes a longer time to break down.
Home composters typically aim for a volume ratio of about two to three parts browns for every one part greens. If the pile becomes wet, slimy, and smells like ammonia, add more browns (e.g., shredded cardboard or dry leaves) to absorb moisture and increase carbon. If the pile is dry and decomposition has stalled, adding more greens (e.g., food scraps or grass clippings) and water will help reignite the process.