Compost is created by billions of microbes in a natural exothermic process of biological oxidation, generating heat to make rich soil from organic matter. Bacteria, archaea and fungi do much of the work unseen.
The Exothermic Process: Benefits of Heat
Seed Sterilization: High temperatures help eliminate seeds and pathogens, yielding a cleaner end product beneficial for garden use.
Destruction of Harmful Microorganisms: The heat also reduces the presence of disease-causing microbes, lowering risk of plant disease.
Faster Decomposition: Warm temperatures increase the metabolic rates of microorganisms, speeding breakdown times.
The process of biological oxidation is simplified as follows:
Organic Matter + Oxygen → Carbon Dioxide + Water + Heat + Humus
Within this equation, complex biochemical reactions take place. The process can be divided into stages, each dominated by different groups of microorganisms.
Bacteria
Bacteria dine on organic materials like vegetable scraps, yard waste and manure, releasing nutrients into the soil. The process begins with aerobic or oxygen-loving bacteria.
The bacteria break down sugars, starches and proteins into organic byproducts including nutrients and energy as heat. Quick to reproduce, bacteria number millions per gram in healthy compost.
Bacillus species are especially known for their ability to degrade cellulose and other complex carbohydrates. They're even added to compost starter mixes.
Healthy compost temperature is between 54° - 71°C (130° - 160°F). As heat increases, so do activities of microorganisms like archaea and fungi. They decompose tough materials like the cellulose and lignin in plant cell walls.
Archaea
Archaea are single-celled organisms. Like a bacterium, an archaeon lacks a nucleus. Archaea include a vast number of extremophiles, who live in extreme environments, including high temperatures and pH levels.
Archaea are especially active during later stages of composting. Some archaea are methanogens.
They produce methane as a byproduct of metabolism. A greenhouse gas, methane can form in dangerous quantities in large-scale composting, mass livestock and industrial operations.
Micromycetes (Fungi)
This group includes filamentous fungi (molds) and yeasts. Filamentous fungi send out hyphae or thread-like filaments to penetrate and break down materials like lignin, cellulose, and chitin into simpler sugars.
This makes them accessible to bacteria and other sugar consumers like yeast. Fungi also create pathways for water and air to circulate. They're often seen as white, thread-like growth on decaying leaves or wood.
Yeasts are single-celled fungi best known for their ancient uses in baking and brewing. In nature they break down sugars like the simple carbohydrates glucose and fructose in grapes.
Exothermic Stages of Compost
Mesophilic Stage (Moderate Temperature)
This initial stage is marked by a rapid rise in temperature as mesophilic bacteria, who enjoy a moderate climate, rapidly break down available sugars and starches. Temperatures range from 20° - 40°C (68° - 104°F).
Thermophilic Stage (High Temperature)
As the temperature rises above 40°C (104°F), thermophilic bacteria and fungi take over. These organisms break down more complex materials like cellulose and proteins. Temperature can rise to 70°C (158°F) in this phase.
Cooling and Maturation Stage
As the supply of readily available organic matter diminishes, the temperature gradually decreases. Mesophilic organisms return.
The final stages of decomposition result in the formation of humus. This is the stable, nutrient-rich end product of the composting process.
Humus enhances soil fertility, water retention, CEC (cation exchange capacity), nutrient availability and overall soil health. In agricultural soils, humus content typically ranges from 2-10%
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