What is Compost?

The Chemistry and Physics of Composting

Of the many chemical elements required for microbial decomposition, carbon and nitrogen are the most important. While the initial ingredients in a compost pile might start out a bit acidic or basic, the finished compost will level off to a neutral pH.

The rate at which composting occurs depends on physical as well as chemical factors. Temperature is a key parameter determining the success of composting. Physical characteristics of the compost ingredients, including Moisture content and Particle size, affect the rate at which composting occurs. Other physical considerations include the Size and shape of the compost pile, which affect the type and rate of Aeration and the tendency of the compost to retain or dissipate the heat that is generated.

The Biology of Composting

Microorganisms of the compost pile

Microorganisms are the workhorses of the compost pile, breaking down organic matter to produce carbon dioxide, water, heat, and humus. Different communities of microorganisms predominate during the three composting phases; the moderate-temperature phase (mesophilic), the high-temperature phase (thermophilic), and the cooling and maturation phase.

Initial decomposition is carried out by mesophilic microorganisms, which rapidly break down the soluble, readily degradable compounds. In many compost piles, mesophillic microorganisms do most of the work. However, given optimal conditions, they may produce enough heat to kick in the real hot shots, the thermophilic, or heat-loving, microorganisms.

If the temperature rises above about 40°C (104°F), the mesophilic microorganisms are replaced by the thermophilic ones. If the pile gets hotter than 55°C (131°F), most human and plant pathogens are killed (a good thing). However, if the pile gets hotter than 65°C (150°F), most of the "good" microbes are also killed (a bad thing). Since these "good" microbes are needed to speed the decomposition process along, the ideal temperature during this phase is between 55°C and 65°C. These high temperatures also accelerate the breakdown of proteins, fats, and complex carbohydrates like cellulose and hemicellulose, the major structural molecules in plants.

As the supply of these high-energy compounds becomes exhausted, the compost temperature gradually decreases and mesophilic microorganisms once again take over for the final phase of "curing" or maturation of the remaining organic matter.

(Bacteria, Actinomycetes, Fungi)

Invertebrates of the compost pile

Larger than the more numerous microorganisms, invertebrates break down the organic material into smaller pieces (increasing the available surface area) and transport microorganisms throughout the compost pile. In short, they're the shredders and taxis of the composting community.

(Mites, Springtails, Sowbugs, Centipedes, Millipedes, Ants, Beetles, Green Fruit Beetle Larvae, Redworms)