Compost Magazine

Composting tips, advice and science.

Man lifts spade with compost material from a compost bin.

An A-Z of Composting

You don’t need an in-depth understanding of composting terms in order to compost successfully.

But if you’re keen on developing your composting skills and knowledge, an understanding of key terms can quickly come in useful.

In this article we explain many of the terms used in more in-depth guides.

Do note we haven’t included bins, which are covered in our guide to choosing a compost bin.



A natural bacteria found in soil that can thrive in compost as it cools down. Actinomycetes help break down the compost material and are particularly good at breaking down woodier material that contains higher quantities of lignins. Their fine white filaments, which can look like a spider’s web, resemble fungal growth.

Activated charcoal

Activated charcoal is carbon that has been treated with oxygen. The process results in a huge surface area of up to 3000 meters per gram. Used as an odor control filter in some composting systems. Has also been used to neutralize certain chemicals in compost and soil. May assist with treating herbicide damage to soil.


A compost activator is an ingredient/s added to the compost heap in order to kick start or speed up the composting process.

Can be bought off the shelf or made at home from easily available ingredients. Also known as compost accelerators or starters.


Aeration refers to the process of introducing air into your compost.


Literally: with air. Aerobic composting refers to composting which utilizes bacteria that require oxygen.


A chemical weed killer that is used by some farmers and can be passed on to manure via animals. Has been found in some commercially produced compost, which is another reason to make your own compost!

See Manure In Compost: A Cautionary Tale


Literally means the absence of air. Anaerobic composting takes place in composting systems like Bokashi bins where the material is essentially fermented.

In open piles, it is generally not a good idea as it is slower than aerobic composting and can produce harmful gases such as methane.



Single-celled micro-organisms that do much of the work of breaking down organic materials into nutrients. See compost bacteria to learn more.


These are microscopic particles that include pollen and fungal spores. Dry and musty compost heaps can release these when turned. These can, in rare cases, cause allergic reactions. One of the worst is Aspergillus fumigatus, which can lead to a condition known as Farmer’s Lung. If a compost heap is dry, it is best to soak it before turning or wear a face mask.


Biochar is a charcoal that is used to improve soil. Can be used as an additive in compost. One study found that it could improve composting speed by 20%.

Biodynamic composting

Biodynamic gardeners and farmers prepare six plants, each in a different way, to be used in a compost heap. The plants are yarrow, chamomile, stinging nettle, oak, dandelion and valerian. Each plant is prepared in a different way.

For example, the flowers of the chamomile are encased in a cow’s intestine and buried over winter, while Yarrow flowers are put in a stag’s bladder, hung from a tree over the summer, and then buried over winter. For more information see


An anaerobic form of composting that uses fermentation to break down composting materials such as cooked food. See Bokashi Bins: A Complete Beginner’s Guide.


A good composting material, when fully unfurled a pile can be composted on its own with no further additions.


In composting, browns refer to carbon-rich materials. Examples include leaves, paper and cardboard. Confusingly, not all ‘browns’ are brown in color. Coffee grounds, for example, are rich in nitrogen and, in composting terms, is a ‘green’.

Bulking agents

Bulking agents are high-carbon materials that help create air pockets in your compost. They are an excellent way to introduce more air into the heap.



Carbon provides the energy source for the microbial cells that turn waste into compost. In the form of sources like wood chips, or sawdust it can also provide structure for the soil, while carbon-rich materials such as shredded paper can help absorb excess liquid.

Some sources argue that carbon which does not break down becomes humus, although this is now controversial. Some scientists believe that compost is an effective way of storing carbon in the soil.

Carbon-rich materials need to be mixed with nitrogen for effective composting.

Carbon piles

Piles of carbon-rich material, such as autumn leaves, which are stored until ready to be used in the compost pile.

Carbon: Nitrogen ratio

This refers to the ratio of carbon and nitrogen in a compost heap. Results from research vary slightly in their advice on optimum levels for composting, but it is in the region of 25:1 to 35:1.

Given the complexity of calculating the ratios, and the fact you can get good compost without getting the C: N ratio perfect, most gardeners don’t need to worry too much about it other than ensuring a good balance of greens and browns. See our article on C:N ratios for information.

Carbon Sequestration

The process of capturing and storing atmospheric carbon dioxide in organic matter, such as compost.

Cation exchange ratio

Cations are positively charged ions (a charged atom or molecule). The cation exchange ratio measures how many cations can be retained in the soil.

The humus produced by the composting process has a negative electrical charge, which attracts cations, making them available to plants. Many minerals and nutrients are cations.

A high cation exchange rate improves fertility by ensuring the soil provides important nutrients to plants such as calcium, magnesium and potassium to plants.

If it sounds complicated, all you need to remember is that the material created via composting can massively increase the fertility of your soil. You can further improve the cation exchange ratio of your soil by applying a no-dig method to your garden.


Comfrey is a herb that is often grown to be used as a fertilizer. It is used both as a compost activator and as a liquid feed.


Materials that can break down in a compost pile, such as food scraps, paper, and certain biodegradable plastics. See What Can I Compost? for examples.


The process of taking organic material and speeding up the decomposition process to turn it into a soil amendment or potting mix. See our Introduction to Composting for detail.

Compost leachate

A nutrient rich liquid that seeps from compost. Often collected and diluted for use as a fertilizer. For benefits, hazards and how to collect and use see Compost Leachate: Valuable Fertilizer or Potential Hazard?

Compost Maturity

The degree to which the composting process has been completed. Mature compost is stable and ready to use, while immature compost can harm plants if applied to soil. See 5 Reasons to Let Your Compost Mature for Longer for details.

Compost worms

Species of worms that are used in the composting process. The most well-known is the Red Wriggler (Eisenia fetida).

Cool Composting

Composting without heat. This takes place in smaller non-insulated bins and compost heaps where smaller amounts of material are not sufficient to generate the heat found in larger piles. While hot composting is faster, cool composting will still produce finished compost. See Hot v. Cold Composting for details.

Compost Tea

A liquid fertilizer made by steeping compost in water. Some recipes add additional ingredients to improve the compost. You can find a compost tea recipe on the Home Composting Made Easy site.


Dalek Bin

Mass-produced, plastic composter. While not the most effective system, as it’s cheap (and sometimes free) it can make a good entry-level introduction to composting. See our article on Dalek Bin for more information.


Organisms that break down organic material into simpler compounds, releasing nutrients back into the environment. These include bacteria, fungi, and invertebrates like earthworms and insects.


Systems like the Green Cone are embedded into the soil, allowing the soil to ‘digest’ food waste. Doesn’t produce finished compost but is an effective way to deal with kitchen waste. See types of compost bin for more information.

Dynamic accumulators

Dynamic accumulators are plants that produce above-average nutrients in their nutrients. Examples include clover, comfrey, and stinging nettles.


Compost duvets, whether bought or made, help insulate compost bins, even in winter.


Free Air Space

Free Air Space (FAS) refers to the air pockets created around individual particles in your compost heap.


Fungi are a natural part of the composting process and help break down compost materials.


Green Johanna

Insulated hot compost bin originating from Sweden.


Compost materials that are high in nitrogen. The term green refers to the nitrogen level, not the color. For example, used coffee grounds may be brown in color but they are high in nitrogen and are referred to as a “green”.


Hot Composting

A method of composting that relies on high temperatures to speed up the decomposition process. Hot composting requires a well-balanced mix of materials and regular turning to maintain optimal conditions for aerobic bacteria and fungi to rapidly decompose organic matter.

Humanure composting

The practice of composting human feces and urine. The term was created by Joseph Jenkins of The Humanure Handbook. See our guide to Humanure for more info.


Dark and spongy, humus consists of small compounds which surround soil particles and are made up of carbon, oxygen and hydrogen.

It may decay slowly over several years or persist for hundreds of years. It plays an important role in soil ecology as it can hold ions containing nutrients such as calcium, magnesium and phosphorus while its ability to maintain up to 90% of its weight in water means it has drought-resistant properties. Generally, only small amounts are found in finished compost.

Humus is also used, more colloquially, to refer to finished compost. In recent years, some scientists have argued that humus doesn’t actually exist.


Soil organisms that kill pathogens in maturing compost.


Indore Compost

A system of composting devised by Sir Albert Howard in India in the 1920s, which enabled highly effective composting without the addition of chemicals. See Sir Albert Howard’s Indore Compost Method on Permaculture Nusa for a fascinating account of his story.


A substance added to compost to introduce specific microorganisms, such as bacteria or fungi, that aid in the decomposition process.

In-vessel composting

A term that usually refers to large-scale, municipal composting in a large container or building.


Kitchen caddy bins

Small bins used to collect kitchen scraps for composting. Some contain charcoal filters to eliminate odor. May be combined with compostable bags or liners.



Refers to the leaching of nutrients from soil during rainfall. Compost is one of a number of ways to reduce leaching. Another method is to ensure that soil always has something growing in it – green manure plants are an effective way to cover soil in winter.

Leaf mold

Each autumn and winter trees deliver a veritable bonanza of leaves. When these get blown into drifts, you can collect sacks of them in no time at all.

These leaves can be gathered up and put in a wire frame, or just left in sacks, where they will eventually turn into an excellent compost called leaf mold (UK: leaf mould).

The process can take up to two years but can be speeded up by breaking up with a lawn mower. Good leaf mold is an excellent substitute for potting compost.


Lignin is a hard, carbon substance. It is more resistant to breakdown by microbes than other constituents of plants such as cellulose. Some lignin is broken down in the composting process, and the process is aided by white rot fungi and the fungus-like Actinomycetes.


Limestone can be added to compost in order to raise the PH of the compost pile.


Loam is a combination of clay, sand and silt which is considered excellent for growing.

Loam stacks

Turf covered with sacks and left to rot to produce compost. The process takes about a year.


Manure bottles

I’ve never read about this in a composting book or on the net, but this is an interesting technique used by a local farmer on his garden courgettes.

He takes a plastic bottle, cuts the bottom off, and inserts some manure before placing it, inverted, in the soil. When he waters the courgettes, he fills the bottle with water, allowing it to filter through the manure and fertilize the courgettes.

Learn more about how to use manure.


After the hot composting stage, compost is usually left to mature. Plants don’t seem to like fresh compost, and research has shown that microbial and fungal activity continues after the compost has been cooled down, hygienisers kill pathogens not removed by the hot composting process and worms continue to improve the compost.

Learn more: 5 Reasons You Should Leave Your Compost to Mature for Longer

Mesophilic bacteria

Bacteria that break down compost materials at lower temperatures. They work best at temperatures between 30 and 40 degrees Celius.


Microorganisms are microscopic organisms. There are different types of microorganisms that break down compost materials into compost, but the most numerous are bacteria.


Part of the organic material in compost is turned into minerals during the composting process. See this video on Compost Mineralisation by Jeff Gage from Green Mountains Technologies for more information.

Moisture control

Refers to controlling the amount of water in a compost pile. Research suggests that a moisture level of around 50-60% is ideal. For those of us who don’t have a soil moisture probe, a simple test is to squeeze a handful of compost – it should have the consistency of a wrung-out sponge.


A covering on soil and around plants. Mulch can take many forms, including finished compost, straw, seaweed or cardboard. Your climate may determine what type of mulch you use, as some mulches provide excellent hiding spaces for slugs.

Mycorrhizal Fungi

Mycorrhizal Fungi is a fungal network that shares a symbiotic relationship with plant roots. Mycorrhizas link plant roots with moisture, nutrients and other beneficial fungi, and also release chemicals which increase the availability of nutrients.

They’re also one reason why No Dig gardening is so successful, as over-cultivation can damage the network of Mycorrhizal Fungi in the soil.

Also see the Magic of Mycorrhizal Fungi.



An important nutrient utilized by plants. Nitrogen is also used to grow protein in the microorganisms in compost.

No Dig

No Dig refers to a system of composting in which the ground is not dug. Instead, the soil is covered with compost or mulch. Probably the best place to learn more is on Charles Dowding’s No Dig website.


Odor Control

Techniques used to minimize odors produced during the composting process, such as maintaining proper moisture levels, turning the pile, and using materials like activated charcoal or biofilters.


Passive Composting

A low-maintenance composting method that relies on the natural decomposition process without active management. This method takes longer than hot composting and may not reach high enough temperatures to kill pathogens or weed seeds.


Pathogens are microorganisms that can cause disease. The composting process is extremely good at killing pathogens, but it’s still a good idea to wash your hands after turning your compost.


PH measures how acidic or basic a substance is. Numbers below 7 indicate acidity, with 7 being neutral. Cornell University advises that compost microorganisms prefer neutral to acidic conditions, with a ph of 5.5 to 8.


Rock dust

Rock dust is sometimes added to soil in order to add missing minerals. While the use in soils is sometimes controversial, one study by Garcia-Gomez et al suggested that adding rock dust could speed up the composting process.


Sanitized compost

Compost that has been maintained at a high enough temperature to destroy pathogens. This term is usually used in commercial composting.


When scientists talk about salts in compost, they mean any substance that is formed of ions. See our guide to Salts in Compost for more info.

Sheet composting

A type of composting in which compost materials are spread flat on the ground above the soil.

Starch bags

Biodegradable bags are used to collect compostable waste.


A tool used to mix up or aerate compost. These can be very useful with tall compost bins.


Thermophilic microorganisms

Heat-loving bacteria that break down compost material. They work best at temperatures above 41 Degrees Celsius.


A type of compost bin that can be rotated in order to aerate the compost within.


Vertical composting units (VCU)

A vertical composting bin uses gravity to turn and mix the compost. An example is the Earthmaker composter.


Windrow Composting

A large-scale composting method in which long, narrow piles of organic material (windrows) are periodically turned to promote aerobic decomposition. This method is commonly used for commercial or municipal composting operations. See Industrial Composting Technologies for more info.


A form of composting that uses worms in a container (usually with multiple containers) to turn waste into compost. Also see Vermicomposting below.



A composting method that uses worms, typically red wigglers, to break down organic waste into nutrient-rich compost. This method is particularly suitable for smaller-scale composting systems, such as those used by households.