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The Science of Composting

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Composting is the biological decomposition or breakdown of organic material by bacteria and other organisms.

Bacteria are the primary decomposing organism, but there are many others, including fungi, actinomycetes, worms and beetles. These organisms bring about decomposition by feeding on organic material. The result is compost or humus, a dark, nutrient-rich soil conditioner.

While our ancestors recognised that composting was useful in building soil and growing healthy plants, they didn't know how or why it worked. (See History of Composting)

Much of the how and why of composting has arisen out of the explosion of interest and research into composting over the last 50 years. This research has produced a body of information that we call the science of composting.

Any organic material in the natural environment will decompose over time. Composting, as a science, looks at the factors that enable decomposition to proceed more quickly and efficiently by managing or controlling these factors.

Organic Material

Any organic material can be added to the compost. For our purposes organic material is everything growing in the garden, on the allotment and most food waste, especially fruit and vegetable scraps. Weeds, leaves, plant trimmings, apple cores, tea bags and potato peels are organic. Wood and large twigs are organic, but may take too long to decompose to be of practical use in composting.

Decomposing Organisms

Decomposing organisms are all the micro-organisms and larger organisms involved in breaking down organic material. Bacteria are the primary decomposing micro-organism. They arrive with the organic material, and start the process by breaking down the organic material for their own food. Bacteria grow and multiply while conditions are right for them, and die off as they create conditions more favourable for others.

Bacteria, actinomycetes and fungi all consume waste directly and are known as first-level decomposers. They are assisted by larger organisms - earthworms, beetle mites, sow bugs, white worms, and flies, which also consume waste directly.

First-level decomposing micro-organisms are eaten by second-level decomposers such as springtails, mould mites, feather-winged beetles, protozoa and rotifers. Third- level decomposers eat both first and second-level decomposers and include centipedes, beetles, ants and predatory mites. Organisms at each level of the food web help keep populations of the lower levels in check.

Micro-Organisms - Chemical Decomposers

Abundant. There may be millions in a gram of soil; you would need 25,000 laid end to end on a ruler to make 2.54 cm (an inch). They exist on every piece of organic matter even though you can't see them. When exposed to organic tissue, bacteria "invade" - eating and digesting the tissue, breaking it down into simpler forms for other bacteria and organisms to consume.

As a group, bacteria are considered to be nutritionally diverse, which means that they can eat almost anything, living or dead. Bacteria require both nitrogen and carbon that come from organic materials. The more variety, the greater likelihood they will find a blend of essential nutrients. Bacteria use carbon (C) as a source of energy and, by oxidising carbon, generate heat and carbon dioxide (CO2). Nitrogen (N) is their main source of protein, which is needed for bodybuilding and population growth. Given the proper environmental conditions, bacteria reproduce very quickly by binary fission. Their nuclei split in two and new cell walls grow crosswise over the middle of the cells. Then they do it again and again. The life span of one generation of bacteria is as short as 20 to 30 minutes. With sufficient food and other favourable conditions, one gram of bacteria could become .4536 kg (a pound) in three hours and the size of the earth in one and a half days. Of course, these conditions never exist.

There are two types of composting processes: aerobic and anaerobic - and different species of bacteria occur in each process. Aerobic composting uses oxygen. Rapid, high-temperature composting is mostly aerobic. This is the recommended method for residential composting.

When there is little air and high moisture, anaerobic composting is likely to result. Anaerobic means without oxygen. In an anaerobic process fermentation results in the formation of ammonia-like substances and hydrogen sulphide, which smells like rotting eggs. The anaerobic process is not recommended for backyard composting.

Temperature is an important variable in composting. As temperatures rise and fall in the compost, different bacterial species will become more or less active.

Psychrophilic bacteria, mesophilic bacteria and thermophilic bacteria each operate best within specific temperature ranges. The psychrophiles are the first to go to work. They can work in temperatures below 0 degrees C (as low as -18 degrees C), but are most active around 13 degrees C. They are a cooler temperature aerobic bacteria that burn or oxidise carbon and generate some heat. Often they generate enough heat to make conditions tolerable for the next group of bacteria called mesophiles.

Most decomposition work is done by mesophilic bacteria. These are the midrange bacteria that operate in temperatures between 15 degrees and 40 degrees C, but thrive when temperatures are closest to 21 to 32 degrees C. Heat generated as a by-product of the mesophiles' work will raise the temperature in the pile even more, creating conditions suitable for thermophilic composting. The thermophiles do "hot" composting. They start to take over when temperatures reach 40 to 45 degrees C and will continue to work in temperatures up to about 70 degrees C when their numbers start to decline. Thermophiles work quickly and don't live long, three to five days at most. Turning the pile will provide oxygen (O2) and allow the thermophilic bacteria to continue their activity. As temperatures drop and thermophiles die off, the compost moves into a more mature stage. Mesophiles and psychrophiles, which may have been working in a reduced capacity around the cooler edges of the decomposing organic material, will become more active.

Actinomycetes
Actinomycetes are a higher form of bacteria, similar to fungi, and second in number to bacteria. They don't respond well to acidic conditions (below pH 5) or high moisture conditions, but operate best at medium temperature areas of the compost. Actinomycetes take over during the final stages of decomposition, often producing antibiotics that inhibit bacterial growth. They are likely to work on tough organic material and give compost its pleasant, earthy smell. They are especially important in the formation of humus. They liberate carbon (C), nitrate nitrogen (NO3) and ammonium nitrate (NH4), making nutrients available to plants.

Fungi
Fungi are smaller in number than bacteria or actinomycetes, but larger in body mass. Fungi are simple organisms that lack a photosynthetic pigment. The individual cells have a nucleus surrounded by a membrane and they may be linked together in long filaments. Fungi live on dead or dying material and obtain energy by breaking down organic material. Like actinomycetes, fungi are present during the early and final stages of composting, when the organic material has been changed to a more digestible form. Of the major micro-organisms, fungi function best under acidic conditions.

Macro-Organisms - Physical Decomposers

Macro-organisms are the visible organisms involved in transforming organic material into compost. They are more active in the later, mature stages of composting, when temperatures are dropping but decomposition isn't complete. Micro-organisms decompose chemically, while macro-organisms are higher up in the food chain and decompose physically - by digging, grinding, chewing, digesting, sucking and churning.

Ants

Ants are insects with six legs, a head, thorax and abdomen. Ants feed on a variety of materials in the compost. They may bring fungi and minerals such as potassium and phosphorous into the compost.

Millipedes
A millipede is thick-skinned with dark red segments. There are many segments, each with two pairs of legs, but not the thousand that "milli" implies. They eat soft decaying vegetation. They will roll up in a ball when they are in danger.

Snails and Slugs Snails and slugs are molluscs that travel in a creeping movement. Snails have a spiral shell with a distinct head and a foot that is retractable. Slugs are without a shell and somewhat bullet-shaped with antennae on their front section. Both feed on living plant material but you find them in the compost attacking plant debris.

Nematodes

Nematodes (roundworms) are the most abundant invertebrates in the soil. Some nematodes live on decaying organic matter, while others are predators on other nematodes, bacteria, algae, protozoa and fungal spores. There are also pest forms of nematodes that attack plant roots.

Fermentation mites
Fermentation mites, also called mould mites, are transparent-bodied creatures that feed on yeasts in fermenting masses of organic matter. These mites are able to withstand anaerobic conditions for moderate periods of time, and may be a good indicator of these conditions in the compost. They eat plant material, such as the soft tissue of leaves.

Springtails
Springtails are small insects distinguished by their ability to jump when disturbed. They are principally fungi-feeders, though they also eat moulds and nematodes and chew on decomposing plants. They vary in colour from white to blue to black.

Spiders
Spiders are eight-legged creatures and third-level consumers that feed on insects and small invertebrates.

Centipedes
Centipedes are flattened and segmented with one pair of legs in each segment. They have 15 or more pairs of legs. They are third-level consumers, feeding on soil invertebrates they're size or larger. This means they are unwelcome in a worm bin as they may attack and kill the worms.

Woodlice
Woodlice have a flat and oval body with distinct segments and ten pairs of legs. They are first-level consumers that feed on rotting woody materials and other durable tissues like leaf veins.

Flies
Flies are two-winged insects that feed on almost any kind of organic material. They also act as airborne carriers of bacteria. Wherever they land, they deposit bacteria. Aerate your compost once a week to control.

White worms
White worms (pot worms) or enchytraeids (en kee tray' id) are about an inch long. They help finish off compost by breaking particles of material down into smaller pieces.

Earthworms

Earthworms are the most important of all the larger decomposers in the compost pile. The one we most often find in compost or manure pile is the red wriggler (Eisenia foetida). They are about five cm. long with alternating segments of buff and maroon. They consume bacteria, fungi, protozoa and organic matter. As they digest organic material, they leave nutrient-rich castings in their path. Unlike other large decomposers, they break down material both physically and chemically. The earthworm will also visit your compost.

Key Factors That Affect Composting

Compost variables are the factors affecting the speed of composting. The organisms that make compost need food, air and water. Provide them with a favourable balance of food, air and water and they will make compost quickly. Other variables affecting the speed of composting include temperature, surface area and volume.

Food
Organic material is food for bacteria and other organisms. This organic material or food contains carbon and nitrogen. Bacteria use carbon (C) for energy and nitrogen (N) for protein to grow and reproduce. Carbon and nitrogen levels vary with each organic material. Carbon-rich materials tend to be brown and dry like leaves or straw. Nitrogen-rich materials tend to be green and wet like fresh grass clippings and food waste. Another way to think about organic material's carbon/nitrogen content is to remember that fresh, juicy materials are usually high in N and will decompose more quickly than older, drier and woodier tissues, which are high in C.

All organic material is composed of carbon and nitrogen. For most home composting, a ratio of between 20 and 30:1 is recommended - that is 20 to 30 parts carbon to one part nitrogen. When the ratio is between 20 and 30:1, composting proceeds most efficiently. When C content rises above 30, heat production drops and the rate of composting slows. When the ratio drops below 20:1, excess nitrogen is lost to the air as ammonia and there is a rise in pH level, which may be toxic to some micro- organisms. A blend of one part carbon-rich material and one part nitrogen-rich material is a general rule in composting.

Air
Proper aeration is a key environmental factor. Oxygen is required by many micro-organisms, especially aerobic bacteria. With sufficient oxygen they produce energy, grow quickly, consume more material and make nutrients available for plant growth. When oxygen is not available, aerobic bacteria die off and anaerobic bacteria take over. They will break down the material, but more slowly, and they produce an unpleasant odour.

Moisture
Aerobic bacteria require water to live. A sufficient quantity of water must be provided that will lightly coat the materials to be composted. Less than 40 per cent moisture in the compost and bacterial activity slows down. More than 60 per cent and water will replace air in the pockets amidst the organic material causing aerobic bacteria to drown and anaerobic bacteria to take over. The recommended water level is between 40 and 60 per cent but the ideal percentage will also depend on the organic material's structure. Straw and wood will need more moisture than leaves, while food wastes or lawn clippings may not need any added moisture. Water forms a film of moisture on the materials, allowing the bacteria to do their work. Simply, the compost should feel as moist as a wrung-out sponge.

Temperature
As temperatures rise in the compost, decomposition speeds up. As temperatures drop, composting slows down. Outside temperatures also play a role. Warmer outside temperatures in the summer months stimulate bacteria and speed up composting. Colder temperatures in the winter slow it down.

Surface Area and Particle Size

Smaller particles of organic material provide more surface area for microbes to attack and speed up composting. Material that is reduced to two inch pieces is ideal, exposing lots of area for the bacteria to work and allowing air spaces.

Volume
Volume is a factor in retaining heat in the backyard compost bin. The more volume of material in the bin, the more self-insulating it will become in retaining the heat. For general composting one cubic metre is the recommended volume for a compost bin. One cubic metre easily retains heat and moisture, but it is not so large that material will become too heavy and compacted, or too unwieldy for turning.

General Overview

Composting happens. Organic material breaks down over time. Making compost is a controlled or managed version of the natural process. By concentrating the activity in one place and balancing food, air and water - composting happens faster.

© copyright 1999, P. A. Owen

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