Composting is a process where microorganisms convert organic waste into a rich soil amendment. Bacteria and fungi break down materials like food scraps and yard waste into simpler compounds. The heat produced indicates microbial activity and is a central factor in monitoring the health and progress of the composting process.
The Ideal Temperature Range
The composting process moves through distinct phases defined by temperature and the types of microbes active. Initially, mesophilic organisms, which thrive in moderate temperatures, begin the breakdown, causing the pile’s temperature to rise rapidly, typically between 50°F and 115°F (10°C to 46°C).
Above 115°F (46°C), heat-loving thermophilic bacteria take over the work. The goal of “hot composting” is to maintain the thermophilic range of 131°F to 160°F (55°C to 71°C). Sustaining this heat for several days achieves thermal sanitization, effectively killing most common human pathogens like Salmonella and E. coli. Furthermore, this exposure to high temperatures sterilizes viable weed seeds, preventing them from sprouting when the finished compost is applied to a garden.
Consequences of Overheating
Compost can get too hot, and temperatures exceeding 160°F (71°C) are detrimental to the process. When the temperature rises above this point, the beneficial thermophilic microorganisms responsible for rapid decomposition begin to die off or become dormant. This heat-induced sterilization slows the entire composting process.
Excessive heat leads to the volatilization of nutrients, particularly nitrogen, which escapes into the atmosphere as ammonia gas. This nutrient loss reduces the overall fertility of the finished product and is often noticeable by a strong, unpleasant ammonia odor emanating from the pile. High heat also causes internal moisture to evaporate too quickly, creating pockets that lack the necessary water content for microbial function. If the pile cools without sufficient oxygen, these dry spots can become anaerobic, leading to putrefaction and foul smells.
Managing Excessive Heat in a Compost Pile
Controlling the internal temperature of a compost pile involves adjusting the balance of air, water, and material composition. The most immediate and effective action for a pile that is too hot is turning it, which releases trapped heat and introduces fresh oxygen, providing a temporary but significant cooling effect. This aeration also helps move material from the cooler edges into the hot core, ensuring all organic matter is exposed to the sanitizing temperatures.
If the pile is overheating, it may also be too dry, as low moisture can exacerbate rapid, uncontrolled heat spikes. Adding water to the pile while turning it helps to cool the material and provides the necessary moisture for microbial activity to continue at a more regulated pace. A long-term strategy for heat management involves adjusting the carbon-to-nitrogen (C:N) ratio, which is the fuel source for the microbes. A pile with too much nitrogen (green material) decomposes too rapidly, leading to overheating. Incorporating bulky, high-carbon materials like shredded paper, dry leaves, or wood chips can slow the decomposition rate and prevent future temperature spikes.