Temperature stages and microbial succession
Composting proceeds through distinct temperature phases. Mesophilic bacteria (37-41C range) initiate breakdown of readily available carbon. As organic matter heats from microbial respiration, thermophilic bacteria take over, raising temperature to 50-72C (peak heat).
At peak heat (50-72C), pathogens, weed seeds, and pests are killed by the temperature itself, not chemical action. High-heat composting (active management, regular turning) reaches peak temperature within 3-5 days. Passive piles take 2-3 weeks to reach peak. Duration at high temperature determines pathogen kill: 15 minutes at 63C kills most organisms.
Turning and curing for complete decomposition
Frequent turning (every 2-3 days) maintains aerobic conditions and accelerates decomposition to 3-4 weeks total. Less frequent turning (weekly) extends the process to 6-8 weeks. No turning produces cold compost in 6-12 months.
After the thermophilic peak, temperature declines as easily decomposable material is consumed. Curing (stable storage without turning) follows for 2-4 weeks, allowing mesophilic microbes to stabilize what remains. Mature compost is cool, dark, crumbly, and earthy-smelling, indicating humus formation. Immature compost is warm and still active, potentially depleting soil nitrogen if incorporated prematurely.
Carbon-nitrogen balance and feedstock quality
Optimal compost balances carbon (dry leaves, straw, sawdust) and nitrogen (grass clippings, manure, food waste). Ideal C:N ratio is 25-30:1. Too much carbon slows decomposition; too much nitrogen produces odors and nutrient loss as ammonia.
Finished compost (stable humus) improves soil structure, water retention, and microbial habitat. Adding compost to poor soils increases organic matter by 1-2% annually. Long-term compost use builds soil carbon, fertility, and resilience without synthetic inputs. Quality depends on feedstock and process: manure-based compost feeds soil biology; plant-based compost feeds structure and carbon.