Composting is a controlled biological process where microorganisms break down organic material, converting waste into a nutrient-rich soil amendment. When decomposition stalls, it indicates that the living organisms inside the pile are no longer thriving. A non-breaking down pile is common and usually points to an imbalance in the environment necessary for these microbes to work effectively. Stagnation typically falls into three categories: an imbalanced chemical mix, poor physical management, or the presence of inappropriate materials.
Incorrect Carbon-to-Nitrogen Ratio
The most common cause of a stalled pile is an incorrect balance between carbon and nitrogen, known as the C:N ratio. Microorganisms require carbon for energy and nitrogen for protein synthesis and reproduction, with an ideal ratio falling between 25-to-1 and 30-to-1 parts carbon to nitrogen. Carbon-rich materials, often called “Browns,” include dried leaves, shredded cardboard, and wood chips, which are slow to decompose. Nitrogen-rich materials, or “Greens,” are fast-decomposing items like fresh grass clippings, vegetable scraps, and manure.
If the pile contains too much carbon (excess Browns), microbes lack the necessary nitrogen to build their population, causing decomposition to slow or stop entirely, resulting in a cold pile. Conversely, a pile with too much nitrogen (excess Greens) often becomes slimy, dense, and emits a strong ammonia odor. This occurs because the excess nitrogen converts into ammonia gas and escapes into the atmosphere rather than being incorporated into microbial bodies.
To correct a pile that is too heavy in Browns, incorporate nitrogen sources like fresh manure, coffee grounds, or vegetable waste to reignite the microbial activity. For a pile that smells of ammonia and is too rich in Greens, the solution is to introduce dry, high-carbon materials such as shredded newspaper, dried leaves, or sawdust. Adjusting the ratio provides the organisms with the balanced “diet” they need to efficiently break down the material and generate heat.
Physical Stagnation from Lack of Air or Water
Composting is an aerobic process, meaning the microbes responsible for decomposition require a constant supply of oxygen. When a pile is not turned or aerated regularly, the internal structure compacts, cutting off the oxygen supply to the center. This lack of oxygen forces decomposition into an anaerobic state, which is a much slower process that produces foul-smelling organic acids rather than the high heat seen in aerobic composting.
The moisture level within the pile is equally important, as microbes live in the thin film of water surrounding the organic particles. The ideal moisture content is similar to a wrung-out sponge—damp but not dripping wet. A pile that is too dry causes the microbes to enter a dormant state.
If the pile is too wet, water fills the tiny air pockets (pore spaces) between materials, displacing oxygen and causing anaerobic stagnation. To fix a dry pile, water it thoroughly while turning to ensure even saturation. For a pile that is too wet and dense, adding coarse, dry carbon materials like wood chips or straw will help absorb excess moisture and restore air flow.
Material Size and Non-Compostable Items
The physical size of the materials added to a pile significantly affects the decomposition rate. Microbes and their enzymes can only access and break down organic matter on the surface area of the particles. Large items, such as whole fruit rinds, thick stalks, or unshredded paper, possess a low surface-area-to-volume ratio, which means the breakdown process is extremely slow.
To accelerate decomposition, all materials should ideally be reduced to pieces between 0.5 and 2 inches before being added. Shredding or chopping inputs, like leaves, grass, and kitchen scraps, provides the microbes with exponentially more surface area to colonize and consume, which can speed up the process by as much as two times.
The presence of non-compostable or harmful items can stall a pile by introducing pathogens, contaminants, or materials that resist breakdown. Items like meat, dairy products, grease, and cooking oils should be excluded because they decompose slowly, attract pests, and produce strong odors. Avoid adding pet waste from carnivorous animals, diseased plants, or persistent weeds like bindweed, as these can introduce harmful pathogens or spread undesirable species. Materials like treated wood, glossy magazines, and synthetic tea bags contain chemicals or non-biodegradable components that contaminate the final product.