Hot composting aims to reach the thermophilic stage, where temperatures rise to between 130°F and 160°F (55°C and 71°C). This heat is a direct byproduct of the metabolic activity of millions of microorganisms breaking down organic material. Achieving this high temperature accelerates the decomposition process, often yielding finished compost in a matter of weeks. The heat also pasteurizes the material by killing most weed seeds and harmful pathogens. When your compost pile fails to heat up, it indicates that one or more of the fundamental requirements for microbial activity—mass, food, water, or air—is missing or out of balance.
Too Little Thermal Mass
A common reason a compost pile remains cool is that it is too small to trap the heat being generated. Microorganisms are active, but the heat they produce escapes into the surrounding air faster than it can accumulate. The outer layer of a large pile acts as insulation, allowing the core to reach and maintain the necessary thermophilic temperatures.
The minimum size generally accepted for a reliably hot compost pile is one cubic yard (3 feet by 3 feet by 3 feet). Materials closer to the edges primarily serve to insulate the active decomposition zone within the center. If your pile is smaller than this recommended volume, it lacks the thermal mass required to prevent rapid heat loss, and the decomposition process reverts to slower, cold composting. The solution is to collect and add enough bulk material to reach the minimum size threshold.
Insufficient Nitrogen or Carbon Material
Microorganisms driving decomposition require a balanced diet of carbon and nitrogen, referred to as the Carbon-to-Nitrogen (C:N) ratio. Carbon (brown materials like dry leaves and shredded paper) provides the energy source, while nitrogen (green materials like grass clippings and food scraps) is used for microbial reproduction. The ideal starting ratio for rapid heating is between 25:1 and 30:1 (carbon to nitrogen).
Excess Carbon (High C:N Ratio)
If the ratio is too high (excess carbon), decomposition slows, and the pile remains cool. Microbes lack the nitrogen needed to build their populations and process the abundant carbon. The pile will appear dry and crumbly. To remedy this, turn in fresh, nitrogen-rich materials like manure or fresh grass clippings.
Excess Nitrogen (Low C:N Ratio)
If the ratio is too low (excess nitrogen), the pile also fails to heat up effectively and often produces a strong, unpleasant ammonia odor. Nitrogen is lost as ammonia gas because the microbes cannot utilize it quickly enough due to a lack of carbon. To correct this, add bulky, high-carbon materials such as sawdust, straw, or shredded cardboard to restore the proper C:N balance.
Lack of Aeration or Moisture Imbalance
Lack of Aeration
Heat-generating microbes are aerobic, meaning they require a steady supply of oxygen to survive and work efficiently. When a pile is too dense or has not been turned regularly, the oxygen supply is depleted, which slows or halts the thermophilic process. Turning the pile every few days, or when the temperature drops below 130°F, introduces fresh air. This re-energizes the microbial populations and often causes a rapid spike in temperature.
Moisture Imbalance
Moisture directly impacts microbial function and is closely linked to air flow. The optimal moisture content for active composting is between 45% and 60%. If the pile is too dry (below 40%), microbial activity is inhibited, and the breakdown process stalls.
If the pile is too wet (exceeding about 65%), water fills the pore spaces, pushing out the necessary oxygen. This leads to anaerobic conditions, causing aerobic microbes to die off and resulting in a putrid odor. The material should feel moist but release only a few drops of water when squeezed tightly (the “wrung-out sponge” test). If the pile is too dry, add water while turning; if too wet, turn the pile and add dry, bulky carbon material to absorb excess water and restore air pockets.