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Compost–What Is Compost?

I was doing some reading about compost and compost myths and I asked myself, do I really know what compost is? I thought I knew. I had certainly read a lot about it, and I have been making it for over 40 years. I started by Goggling the definition of compost and it quickly became clear that the popular dictionaries on the net don’t agree on a definition. In fact they contradict one another. It became clear that this simple question had a more complicated answer.

The posts for the next several weeks will be dedicated to composting, making compost and composte‘, if you prefer that term. I’ll unravel the truth behind this black gold.

Compost - what is compost?

Compost – what is compost?

What is Compost?

To start with, we need to frame the discussion. In the UK and probably in other parts of the world, the word compost is used to describe potting soil. This is not what we are talking about here. When I use the word compost, it refers to the resulting product of the composting process, which consists of taking plant material and other compostable material, piling it up, and letting it decompose into a black friable material.

Is Compost Decomposed?

There are two basic definitions of compost:

  • Decayed organic material
  • Decomposing organic material

I have simplified the definitions to their very basic elements. The first definition is the most popular one and the one used by Wikipedia. They are usually right–but in this case they are wrong!

You might think that this is a silly academic discussion, but it’s not. Understanding this definition is fundamental to understanding the benefits of compost to your garden and to your plants.

Compost is Not Decomposed

When we look at compost it looks like black dirt. We can no longer see the plant parts that went into the compost pile. To us it looks completely decomposed, but it’s not.

The plant material has been broken down into small bits that, to our eye, are no longer visible. But to the small life in soil; the bugs, dew worms and microbes, compost still consists of big pieces of plant material. More importantly most of the complex organic molecules in the original plant material have not been broken down yet.

For soil life there is still lots of food to be digested. Under a microscope you will still see the plant particles.

Finished Compost

Finished, or mature compost is a term gardeners use to describe the product produced by the composting process. When it is ‘finished’, compost is black or dark brown, crumbly, and on close inspection there are no visible plant parts.

Finished compost is ready for the garden.

As mentioned above, on a microscopic level, finished compost still consists mostly of pieces plant material. It is still decomposing and will for some time which is important for the garden.

Is Compost Humus?

I have discussed Humus before in What is Humus?

Compost is NOT humus. Many web sites and gardening books confuse the two terms, but they are very different things. As organic matter decomposes over many years, it slowly turns into humus.

Reference (1) below suggests that finished compost might have 10% (+/-5%) humus. So anything from 0 to 15% humus. The exact number is not so important. What is important is to understand that most of the compost is not fully decomposed. When it is fully decomposed it will be 100% humus (not including the released nutrients).

Garden Compost

If compost contains almost no humus, is it still good for the garden?

Humus is good for the garden, but in the short term (next couple of years) compost is much more valuable for your garden.

To understand this you need to understand what happens to compost once you add it to your soil. The decomposition process continues. Microbes and other soil life  continue the decomposition process. Dew worms eat compost, and poop out digested compost. Bacteria live on the poop and digest it even more. This process continues over and over again for quite some time.

While this process is going on two important things happen.

  • Soil structure is improved.
  • Nutrients are slowly released.

I have discussed the development of soil structure in my post titled, What is the Real Value of Organic Fertilizer?

The slow release of nutrients is critical. The fact that compost is not humus yet, means that the large molecules in the compost still contain lots of nitrogen, phosphorus and other minerals. The continual decomposition process releases these slowly to the soil so that your plants have a steady supply of nutrients.

As decomposition continues, more and more of the compost gets converted to humus. As this happens, the amount of nutrients released goes down. That is why is is important to add some compost to your garden each year.

References:

1) Do I Have Humus in My Compost?: http://www.hotbincomposting.com/blog/do-i-have-humus-in-my-compost.html

2) Excellent Primer on Compost: http://aggie-horticulture.tamu.edu/earthkind/landscape/dont-bag-it/chapter-1-the-decomposition-process/

3) Image Source: University of Tennessee

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

8 Responses to 'Compost–What Is Compost?'

  1. Laura Allais says:

    Robert, you are a true gem for sharing all this info.
    I share many of your posts on our group’s wall. BUGGs group. Balcony and Urban Gardening Group of Arabia.
    If i ever visit Canada, i would LOVE to see your garden.
    Bless you!
    Laura

  2. You are very informative. I understand my compost heap a lot better now. What i need to know. Do you Just put the compost straight into the pots and put my my seeds or plants directly into the compost or do I need to mix something in with it??? I am a still learning new gardener.

  3. rogerbrook says:

    Great review Robert! There is so much tosh written about it and as to the rubbish you sometime buy in a bag!
    I some times suggest in the old days – very old days – garden compost and potting compost were almost the same when Victorian gardeners mixed their own potting compost using compost heap material exclusively or as an additive. In fact I know some folk who mix their own grow bag compost in this way.
    Humus is a mine field and you do right to correct the idea that it is synonymous with compost.
    In fact I rather doubt your figures about humus in old compost, it will eventually oxidise away too.
    Now what really fascinates me is humus hundreds of years old, a condition which almost only arises when clay particles intimately mix with very well decayed organic matter. (and as a no dig gardener had better add not turned over too much)

    • True humus is quite stable and should remain for many years, in part because it is not a food source for bacteria.

      I had a real hard time finding the one number I referenced about the amount of humus in compost. I doubt people test for this and hence the number is hard to determine. I suspect the 10% is high because humus forms very slowly. Would love a reference that reviews this process.

  4. Very clear, short explanation! There is certainly some confusion in general, regarding this subject. I wonder if what’s sold as ‘compost’ in bags is really compost? I had the chance to see, feel and incorporate real, organic compost last year, and it was something entirely different!

    • I am glad you enjoyed it. I had the same question about bagged compost and I asked the question on several forums and never got a satisfactory answer. The material seems as if it has a lot of soil in it. but I am not sure soil is less expensive than compost? With growing recycling a lot of compost is now made. It is a possible myth to investigate in the future.

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