Composting is a controlled biological process that transforms organic waste into a rich, soil-like material. This natural cycle occurs within a bin or pile, acting as a specialized ecosystem. Understanding the contents involves looking beyond the initial scraps to the living organisms, chemical byproducts, and final stable matter that emerge. The process reveals a complex composition of diverse materials.
The Essential Input Materials
The foundation of a functioning compost bin relies on a balanced mixture of organic matter, categorized by its primary elemental content. Carbon-rich materials, or “Browns,” provide the energy source and structural bulk for the microbial workforce. Examples of Browns include:
- Dried leaves
- Shredded newspaper
- Cardboard
- Sawdust
- Straw
Nitrogen-rich materials, or “Greens,” supply the protein-building elements necessary for microbial growth and reproduction. This category includes fresh grass clippings, fruit and vegetable scraps, coffee grounds, and herbivore manure. A proper ratio of these two categories, usually favoring carbon by weight, ensures efficient decomposition.
The Microscopic and Macroscopic Workforce
The decomposition process is driven by a biological population within the compost bin. The primary agents are microscopic organisms, with bacteria making up the largest proportion. These organisms are responsible for the initial breakdown of simple compounds like sugars and starches. Fungi and actinomycetes, a type of filamentous bacteria, break down tougher, more complex plant materials. Fungi use enzymes to degrade resilient components like cellulose and lignin.
The macroscopic workforce includes larger invertebrates such as earthworms, mites, springtails, and beetles. These organisms physically shred the organic matter. This physical breakdown increases the surface area, making food sources more accessible for the microscopic decomposers.
The Intermediate and Finished Products
As the organisms consume and transform the raw inputs, several intermediate and finished products are created. The metabolism of the microorganisms generates heat, which can raise the internal temperature to 55–65 degrees Celsius (the thermophilic phase). This heat is a byproduct of carbon oxidation and destroys pathogens and weed seeds.
Aerobic decomposition releases gases, primarily carbon dioxide and water vapor, which reduces the overall mass and volume of the material. If the bin lacks oxygen, anaerobic conditions may occur, leading to odorous compounds like hydrogen sulfide and methane. The stable, dark end product is known as humus, a complex, nutrient-rich material that improves soil structure.
Unintended and Hazardous Discoveries
A compost bin can contain materials that are hazardous or impede decomposition. Non-compostable contaminants like glass, metal, and most plastics do not break down and can introduce physical dangers or chemical leaching. Items mistaken as compostable, such as glossy paper or chemically treated wood, can introduce inks, waxes, or heavy metals.
Improper additions, particularly meat, dairy products, and pet waste, pose a risk by introducing harmful pathogens. These materials also attract pests like rodents and flies. Furthermore, the fats and oils they contain decompose slowly, creating foul odors. Strict avoidance of these materials ensures the final product is safe for garden use.