Compost cupped in a pair of hands.

What Is Compost? It’s Controversial…

Simple questions such as what is compost/composting should be easy to answer. 

Unfortunately, there doesn’t seem to be an accepted definition of either term. Indeed, explanations vary from book to book and from article to article. 

That makes life difficult for the serious composter. After all, our species’ very ability to communicate depends on having agreed meanings for words. 

Aerobic v. anaerobic composting

Wikipedia is often the first place people visit when they want to learn about a topic, and is also used to inform Googe’s knowledge graph. As a result, the site has a huge influence on our language. 

Wikipedia defines composting thus:

“Composting is an aerobic method (meaning that it requires the presence of air) of decomposing organic solid wastes.” 

This definition does not include anaerobic (‘without air’) composting.

Yet a quick search of Google Scholar shows there are over 90,000 results for the term anaerobic composting. On Google itself there are 2,470,000 results. 

That suggests both academics and the general population use a form of composting not included in a key definition. 

Aerobic composting is considered a superior form of composting. After all, it’s faster and it’s better for the environment.

But this isn’t about which form of compost is better, it’s about accurate use of language to inform communication. 

Hot v. Cold Composting

Roger Tim Haug, in The Practical Handbook of Compost Engineering, acknowledges that there is no agreed definition of what compost is. His definition, though, references hot composting:

“Composting is the biological and stabilization of organic substrates, under conditions that allow the development of Thermophilic temperatures as a result of biologically produced heat, to produce a final product that is stable, free of pathogens and plant seeds, and can be beneficially applied to land.”

The US Composting Council agrees, defining composting as:

“Compost – is the product manufactured through the controlled aerobic, biological decomposition of biodegradable materials. The product has undergone mesophilic and thermophilic temperatures, which significantly reduces the viability of pathogens and weed seeds (in accordance with EPA 40 CFR 503 standards) and stabilizes the carbon such that it is beneficial to plant growth. Compost is typically used as a soil amendment, but may also contribute plant nutrients.”

The mention of hot composting is deliberate, used to separate it from other methods used to turn organic materials into a soil improver.

Yet there are millions of gardeners who have a compost heap and get good compost, but never achieve Thermophilic temperatures (above 45 C). 

Read more: Hot v. Cold Composting: Which Is Right For You?


Some definitions say that composting either produces humus or a humus-like material. 

For example, Wikipedia states:

“The [composting] process involves decomposition of organic material into a humus-like material, known as compost, which is a good fertilizer for plants.”

Humus is a dark, spongy, carbon, organic material that may take hundreds of years to form. 

The problem is it’s also highly controversial. Some soil scientists don’t even believe humus exists. 

To study humus, scientists need to treat it with a strong alkaline solution. However, the alkaline solution completely changes the structure of the humus. 

Unfortunately, it is impossible to study the structure of a substance when you have to change that structure to do so. 

Robert Pavlis, in his book Soil Science for Gardeners, states that it is like: 

“…putting a variety of vegetables through a food processor and then trying to identify the original vegetables by looking at the results of the smoothie.”

As Humus is still not understood, and indeed may not exist, it is unhelpful to use it as part of a definition. 

A simple definition

When it comes to language, we can take two approaches. The first is prescriptive – experts or linguists define the rules and definitions, and the public is likely to follow them. 

There are two problems with that approach. 

First, the average person does not pay much attention when experts and linguists tell them what to say. 

The second is that language naturally evolves. If we followed the prescriptive system, we’d still be talking the way we did hundreds of years ago. 

Another approach is descriptive – which describes how people actually use language rather than stating how they should use language. 

The Cambridge dictionary definition is closer to what people understand by composting. It also has the definition of being simpler than other definitions and neatly gets around the hot/cold composting issue. It states that compost is:

“Decaying plant material that is added to soil to improve its quality.”

Its definition of composting also comes closer to capturing what people do when they compost. It describes compost as:

“…the act of collecting and storing plant material so it can decay and be added to soil to improve its quality.”

(By the time compost is added to the soil, it is a dark crumbly material that looks fully decomposed. However, actual decomposition continues for several years, slowly adding nutrients to the ground.)

Unfortunately, plant material is not the only thing that can be composted. Most substances that have recently been alive can be composted, including wool and manure. Perhaps a slightly more useful definition for compost could be achieved if you replaced plant material with the word ‘organic’.

In fact, No Dig expert Charles Dowding does just this

“Compost is organic matter that has decomposed, from leaves and manure to weeds, wood and paper. Compost feeds soil in a slow and steady manner, allowing soil to feed plants.” 

The process of composting could then be described thus:

“…the act of collecting and storing organic material so it can decay and be added to soil to improve its quality.”


The History of Compost

The history of compost is fascinating, from the Egyptian Queen who declared worms sacred to the use of human blood to improve compost, and from American Presidents who dedicated much time and thought to composting to British farmers who stole the bones of soldiers to fertilize their lands. Learn all about in our history of composting.