So you want to know more about compost?
You’ve come to the right place!
In this guide, you’ll find a brief overview of what compost is, along with lots of links to more in-depth guides to explore at your leisure.
(If you’d prefer to learn more about how compost is made, see our companion guide: What is composting?)
Let’s get started!
Table of Contents
Definition of Compost
Compost is partially decomposed organic matter which is used to improve soil.
Organic matter is anything that has recently been alive and can decompose.
It includes plant material, like leaves and grass clippings, and animal waste.
Benefits of Compost
Benefits for soil
Many people think that compost feeds plants.
Now it’s true that compost does provide some nutrients to plants.
However, its major benefits are more indirect.
Compost improves soil structure. For example, it can improve the ability of soil to both hold water and air.
As plants need both air and water, that helps them grow.
It’s also one reason why compost helps plants better withstand the adverse effects of global warming.
But the benefits don’t end there.
The microorganisms in the soil eat the decomposed organic material in compost.
Some of these microorganisms help plants.
For example, mycorrhiza fungi has a symbiotic relationship with plants, swapping nutrients for carbohydrates.
Benefits for the environment
Both making compost (in the right way) and using compost can help the environment.
When organic waste is sent to landfill, it produces methane, which is many times worse for the environment than carbon dioxide.
When that same food waste is turned into compost using an aerobic (with air) composting process, it produces far fewer methane emissions.
Using compost can also benefit the environment in other ways.
For example, compost can reduce erosion and help remove contaminants from soil.
Compost vs. Fertilizer: What’s the Difference?
Compost is quite different from fertilizer.
Fertilizers typically contain high levels of soluble nutrients such as nitrogen.
(Soluble means those nutrients can easily be dissolved in water.)
Because they are soluble, plants can easily take them up in their roots.
However, excessive use can also damage soil structure and cause damage to the environment.
However, while compost has some soluble and immediately available nutrients, it also has much higher levels of insoluble nutrients.
While these are not accessible to plant roots, they have longer-term benefits for the soil. What’s more, these insoluble nutrients do not leach out of the soil as easily.
Think of it like food. You can eat high-sugar food, which can you give you an instant energy boost but leave you feeling drained later on.
Or you can eat more wholesome food, which won’t give you the same immediate boost but will sustain you better over the long term!
Compost vs. Soil
People often ask how long it takes compost to turn into soil.
In fact, compost is not soil and can’t become soil.
Soil is made of sand, silt, clay, and organic matter.
However, that organic matter is typically only a small amount of the soil. Good soil may contain 5% organic matter, while sandy soil may only contain 1% organic matter.
Compost is made up of just organic matter.
In fact, one of the ways in which compost improves soil is by adding more organic matter. However, compost will never turn into sand or grit.
Types of Compost
There are many different types of compost. These include:
- General purpose compost: Compost designed for use in the soil or with mature plants.
- Seed or starter compost: Compost made for starting off young plants.
- Mushroom compost: An alkaline compost made for growing mushrooms, although it has also become popular for use in the garden.
- Ericaceous Compost: An acidic compost made for plants like Rhododendrons and Azaleas, which prefer more acidic soil.
See our full guide to types of compost for many more types!
How is Compost Made?
Aerobic v. Anaerobic
Compost can be made in many different ways.
At its simplest, these can be divided into two methods.
Aerobic (with air composting) uses microorganisms that need oxygen to break down organic material.
Anaerobic composting uses microorganisms that thrive in a low-oxygen environment.
Some composting methods also use larger creatures (physical decomposers) to turn organic material into a nutritious soil amendment.
Vermicomposting uses worms, although not everyone agrees this is composting, as the worms produce worm castings. Meanwhile, a lesser well-known method uses soldier fly larvae.
To learn more about composting methods, dive into the more specific guides below:
How to use compost
Traditionally, compost was often dug into the soil to improve it.
Nowadays, more and more people are using a No-Dig or No-Till system.
Instead of digging the compost into the soil, the compost is placed on top of the bed and left there.
Compost can also be used for seedlings and plants in pots. It is usually sieved first, and if used for seedlings may be mixed with other ingredients such as sand.
Commercial composting companies may also add fertilizer to the compost to provide more nutrients for plants.
Compost can also be used in many other ways, for example as a mulch and in compost socks.
You can learn more in our guide to using compost.
What is compost tea?
Compost can also be used to create ‘compost tea’.
Compost tea is quite a loose term. Sometimes it is used to describe the leachate (what drips out of compost) which is then diluted and used as a fertilizer.
However, it can also be used to describe a fertilizer that is brewed to encourage microorganisms.
The second is quite a controversial topic. Some composters and researchers believe it is beneficial, others treat it with scorn.
For more information, see our guide to compost tea.
What is humus?
If you research compost for a while, you’ll come across the word humus.
Many gardeners, composters, and even some researchers use the phrase humus.
Humus is supposed to be an element of compost that doesn’t break down for decades or even centuries.
However, in recent years some researchers have questioned the concept of humus, and whether it even exists.
Learn more with our guide to Humus in Compost.
Now you know what compost is, and its many benefits, why not start using it or making it yourself?
In a few minutes a day, you can do your bit for the environment and build up healthy soil your plants will love!
Cornell Composting: Cornell University maintains a very useful resource on composting which includes everything from fact sheets to resources for schools.
Definition of Compost: The US Composting Council provides an in-depth and technical definition of what compost is.