What is Finished Compost? The Answer Will Surprise You.

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Robert Pavlis

Gardeners have been making and using compost for hundreds of years and we talk a lot about “finished compost”, but what is it? When is compost really finished?

You can make so-called finished compost using a hot compost pile in a few months, but compost continues to decompose for many years. If it is not fully composted when it comes out of the compost pile is it really finished?

When I first asked this question I thought is was a simple one with a simple answer, but even science struggles to define finished compost.

What is Finished Compost? The Answer Will Surprise You.
What is Finished Compost? The Answer Will Surprise You, credit University of Colorado

What is Compost?

Before we can define finished compost it is important to understand compost. What is it?

Wikipedia defines compost as “a mixture of ingredients used to fertilize and improve the soil.” That is not right. Few gardeners would consider a bag of synthetic fertilizer to be compost and yet it is mixture of ingredients used to improve soil.

Compost Science for Gardeners by Robert Pavlis

The US EPA defines compost as “organic material that can be added to soil to help plants grow. ” So a fresh banana peel is compost? That’s not right.

Cornell University defines compost as “a dark, crumbly, and earthy-smelling form of decomposing organic matter.”

The University of Missouri Extension Office defines it as “partially decomposed organic matter. It is dark and easily crumbled and it does have an earthy aroma. It is created by biological processes in which soil-inhabiting organisms break down plant tissue.” So decomposed animal tissue does not qualify?

Combining these ideas we can define compost as organic matter that has undergone partial biological decomposition to an extent where it is black, crumbly and earthy-smelling.

An important part of this definition is the reference to partial decomposition and this leads to the next question, when is compost finished?

When is Compost Finished

From a gardeners perspective compost is finished when it is black, crumbly, has an earthy-smell and is mostly unrecognizable. The tomato plant and banana peel are no longer visible but some of the hard material like twigs and walnut shells may still be recognizable.

It is important to understand that this so-called finished compost is not finished. It might look finished to our macro eyes, but on a microscopic level this material still consists of large pieces of organic matter. We might not be able to see a banana peel, but under a microscope you can easily see many small pieces of banana.

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The compost will continue to decompose for years. The rate of decomposition depends on many things like temperature, soil type and level of oxygen, but as a general rule of thumb it will slowly decompose for another 5 years after it comes out of the compost pile. That is actually one of it’s most important properties – it continues to release plant nutrients for years.

Scientists Try to Assess Compost Maturity

Scientists have also been trying to define finished compost but it has turned out to be difficult. There are several lab tests for measuring the completeness of the process but none of them are perfect.

During the decomposition process sugars and other carbohydrates are digested and converted to CO2. The means the C:N ratio changes over time as carbon is lost and both the C:N ratio and the amount of CO2 produced can be used to monitor the decomposition process. The amount of water soluble carbon and humic substances increase during the process but measuring these is not a reliable way of measuring compost maturity.

A commercial test made by Solvita measures the production of carbon dioxide (CO2) and ammonia (NH3), and then uses these values to estimate compost maturity.

During the composting process, microbes use most of the available soluble nitrogen to help digest organic matter. As compost matures this activity slows down and the dying microbes release as much nitrogen as they use. At this point more nitrogen becomes available for plants. This knowledge has led to another test for maturity which is a seedling bioassay. Seeds are grown in a mixture of soil and compost. In the early part of the composting process seedlings don’t grow very well because of a lack of nitrogen. As the compost gets more mature, they start to grow better because they can now access nitrogen.

This seedling test is explained in more detain in the video below.

When the seedling test is compared to the measurement of other compost properties including the Solvita test, it is found that the seedling test is the most accurate way of measuring compost maturity.

One of the problems with measuring compost maturity is that all of the measured parameters vary depending on the source of the compost. A comparison of 9 different types of compost found that out of 29 different properties none were adequate for measuring maturity. At least three different parameters should be used, with the seedling test being one of them.

The bottom line is that even scientists don’t have a good way to determine when compost is finished.

YouTube video

Composting on a Molecular Level

What is composting? As a plants grow they take simple nutrients (nitrate, potassium, iron, sulfur etc.) and combines them with water and CO2 to form large molecules such as proteins, starchs, oils, carbohydrates, and thousands of different organic compounds. Composting is the process of taking these large molecules and breaking them down again into simple molecules. Composting is the reverse of plant and animal growth.

A common belief is that another class of large molecules are formed at the end of the composting process, called humus or humic substances. I have discussed this in a separate post, but current thinking is that humic substances do not exist in soil.

Decomposition is a multi-step process. For example, proteins are large chains of amino acids, so when they decompose they are first broken into smaller protein pieces. These smaller proteins are then broken into amino acids which are finally broken down into water, CO2, nitrate and sulfur. Some large molecules decompose faster than others. Lignin for example, is a complex polymer that gives plants their structural strength and is a main component in wood. It is also very hard for microbes to decompose it.

This process seems like a one way process where large molecules become smaller and smaller (mineralization), but that view is incorrect. Decomposition happens because microbes digest the organic matter but in order for them to do this they must live and replicate. In fact a hot compost pile experiences an explosion of microbe growth. As microbes grow, they take small molecules and build them into large ones, a process called Immobilization. They decompose the original organic material and at the same time produce their own organic material that will be again decomposed at some future time after they die.

When is composting finished? In one sense, it is never finished. That banana peel you threw into the garden will live on for hundreds of years in the form of other organisms.

Compost is never finished
Compost is never finished

When Can You Use Compost?

What gardeners really want to know is, when can compost be used?

Compost that is too fresh has some potential problems for plants. It can be physically hot, and it can have very low levels of nitrogen. At some point the decomposition process slows enough so that neither of these are a problem.

Gardeners define finished compost in terms of visual clues, black, crumby and earthy-smelling, but that really does not tell you very much. Scientists tend to define finished compost as material that is suitable for plant growth. The seedling test mentioned above is a good way to test compost to see if it is ready for use.

We can generalize the definition of finished compost as compost that is ready to use. This emphasizes the fact that the way in which it is used determines when it’s finished.

If compost is laid on top of the soil as a mulch, it does not need any decomposition to be “finished”. I use the cut and drop method of composting where I just drop raw organic matter on the soil. Wood chips that are used as a mulch don’t need any composting but when they are buried in soil they need a high degree of composting. Since mulch sits on top of the soil and does not come in contact with plant roots, it won’t harm plants.

Compost that is buried in a vegetable garden in fall does not need to be as finished as when it is buried in spring. The fall application gives compost time to mature in the ground before plants are added.

Adding compost to containers and potted plants means that it needs a high level of decomposition so it won’t harm plants.

In summary, finished compost does not exist. All a gardener can do is process the organic matter until it reaches a stage where it no longer harms plants. That degree of decomposition depends on how you use the material.

YouTube video

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

6 thoughts on “What is Finished Compost? The Answer Will Surprise You.”

  1. Thank you for your diligent research, as always, on a subject that vexes many a gardener. I have always found the claim of “black gold” as the end result of any particular composting pitch as nothing more than a marketing ploy. To conceptualize compost as a process with so many variables (types of organic material, temperature, moisture, air exposure, etc.) makes practical sense when determining how and when to use it.

  2. Sri lanka is a tropical country.It seems your observations are based on temperate conditions.Will it be applicable in our climate and soil ecology?Nevertheless your post educated me and thanks.

  3. Very illuminating! As a long time compost maker, I very much appreciate this and more fully entrenched in the practice with the now added knowledge.


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