Garden Myths - Learn the truth about gardening

What is Humus?

You have probably heard that humus is an important part of your soil, but few people know what it is and why it is important. There are many myths about humus that need to be cleared up.

It turns out that humus may be the most important thing in soil: more important that dew worms, and organic matter, but it gets so little attention. This post will have a closer look at humus to better understand how we should be gardening to create and maintain humus rich soil.

Humus soil

We are talking about humus, not hummus!

What is Humus?

Before I define humus, let’s look at some similar terms that add confusion to the story.

Humus Layer

This term is used to describe an upper level of soil – that dark black layer, such as in “that humusy layer of soil’. Although the dark color is probably due to humus, humus is not a layer in soil. There is no such thing as a ‘humus layer’.

Humus Soil

This term is floated around the net and it is not clear what it means. Is it soil with humus in it? Most soil has some humus so why not just call it soil? It is a term that should not be used.

Humus = Compost

In agriculture and gardening the term humus is sometimes used to describe well aged compost. You can buy bags of stuff labeled ‘humus’ at gardening centers, but this is just mislabeled compost. This is an incorrect use of the term. Compost is plant material that is slightly decomposed. Even aged, well-rotted compost is still only slightly decomposed. Have a look at this post for more on this topic; Compost – What is Compost. Once added to your garden compost will continue to decompose for several years. Compost is NOT humus.

Fulvic acid, Humic acid and Humin

These are terms referring to different sub-parts of humus. They have specific scientific definitions and should not be used in place of the word  humus. From the point of view of gardeners these terms should not be used.

Humification – The Process of Creating Humus

The best way to understand humus is to understand how it is formed. Dead plant and animal material consists of organic matter. Organic matter is a catch all phrase used to describe a wide range of molecules including starches, proteins, sugars, carbohydrates, amino acids etc. When organic matter starts to decompose these molecules are broken down into smaller and smaller molecules by the micro-organisms in the soil (mostly bacteria and fungi). This is a complex process and the gardener does not need to understand the details of the process. What is important is that most of the useable chemicals in the organic matter are extracted by the micro-organisms and are eventually made available to plants.

At some point, all of the good stuff in the organic matter is used up and some molecules remain that can’t be used by micro-organisms or plants. This remaining material is called humus. It consists mostly of carbon and so it is still organic, but micro-organisms just can decompose it any further. Humus is so stable that it can persist in the soil for hundreds of years.

Humus consists of very large complex carbon based molecules. More recent research suggests that it might actually consist of smaller molecules that are conglomerated into large complex systems. Scientists still don’t understand humus completely.

Humus is very important for your garden – I’ll explain why in the rest of this post. The gardeners job is to increase the amount of humus in soil.

Humus – The Secret to Great Soil

Think of humus as being a big sponge that can hold up to 90% of it’s weight in water.  This water holding capacity of humus is why humus rich soil will remain moist for weeks longer than soil without humus.

Humus has a negative charge which means that many of the nutrients plants require stick to humus, including ammonium (source of nitrogen), calcium, magnesium and phosphorous to name a few. The humus sponge holds onto these nutrients and prevents rain from washing them away. When a plant root comes in contact with it, the plant root is able to remove the nutrients from the humus sponge. The process is a bit more complicated than this, but you can think of humus as being a slow release source of fertilizer for your plants.

Perhaps the most important reason for having humus is that it is responsible for aggregation. Aggregation is what makes soil loose and very friable, improving the structure of soil. Better soil structure found in humus rich soil makes it easier for plant roots to grow by providing them with better access to nutrients, water and most importantly oxygen.

How do You Increase Humus?

Humus is left after organic matter decomposes. Each time you add organic matter to the soil, it will increase the amount of humus in the soil. It is a slow process but if organic matter is added each year, the amount of humus will continue to increase.

You can use any type of organic matter. I believe that the best organic matter to use is the one that costs the least. This is not strictly true, but a low cost usually means that the material has not been overly processed and it has been trucked a shorter distance. Both of these are good for the environment. Use the material that is locally available.

Manure, compost and wood chips are great choices. Just add your organic matter as a mulch and let nature incorporate it into the soil. Never rototill or dig it into the soil since this practice destroys soil structure.

As far as I know you can’t buy humus. Every product that I have looked that calls itself humus, is just some form of compost. I guess someone might be able to buy soil from a forest that has been in place for 100 years. That soil will certainly contain humus–but it is not just humus.

Can You Have Too Much Humus?

Healthy soils contain 2.5 to 5% organic matter, by weight (5 -10% by volume). This number does not include the humus amount. Too much organic matter can be a problem for soils so adding huge amounts of organic matter in order to build humus quickly is not a good idea.

In gardens like shrub boarders and flower beds where you are not harvesting crops, a small annual addition of organic matter, say a 2″ layer, is all that is required. In vegetable gardens where you are harvesting crops and taking nutrients away from the garden, you can add a bit more but not huge amounts. You can add too much organic matter which will cause all kinds of problems.

I have not really answered the question–can you have too much humus? I am not sure. Since humus is created very slowly, I would not be too concerned about having too much.

Does Humus Exist?

This section was added March 2016.

I wrote  the above in 2013, and at the time it was the latest information available. In December of 2015, a new study was published that drastically changes our understanding of humus. It concludes that humus does not really exist. Humus is created when soil is treated with a pH solution, but it never occurs in soil.

For a detailed review of this finding, have a look at Humus Does Not Exist – Says a New Study.

References:

1) Photo Source: Middle East Delights

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.

24 Responses to 'What is Humus?'

  1. The notes are just too supet. Ive learned a lot. Its great and very good explanations

  2. Tim says:

    Firstly, Thank you for sharing your knowledge, sir:)

    I build closed terrariums. I use organic, no chemical or time release potting soil. Is there much humus in such soil? Do I want humus in the soil?

    Sometimes you don’t necessarily want the plants to grow. I make some mostly full, stalks and stems as big as wanted. And some with seeds or seedlings that one does want to grow but to a limit.

    I use tropicals to zone 5 like ivys, ferns… And also Orchids! They do not grow in soil, Do they have access to humus naturally!?

    A product you may be interested in or already know of: Nutro-Gro? I have come into possession of a bottle with a homemade but detailed label. Claims to be humus. Unfortunately the label is faded in places but it looks like instructions for garden beds of 2-3 ml per L of water and a gallons per acre dosage? I dont know where it came from but Im in Edmonton, AB. Any knowledge of this product.

    Tx again for sharing, it is a gift. Peace to All

    election eve – GOOD LUCK AMERICA!

    • Any sold product that claims to contain humus does not contain it. In fact humus does not even exist – see a newer post; Does Humus Exist?

      Epiphitic orchids do not have access to humus.

      Anything labeled as “2-3 ml per L of water and a gallons per acre dosage” is snake oil. Fertilizing an acre takes pounds of nutrients, not a couple of ml.

  3. Thomas Fisher says:

    I have been reading The Living Soil, 1942, and this has led me to a wealth of older works, which has shown me that the truth has been ignored by those who were supposed to be looking out for our health.

    Here is some of them:
    – first in response to humus. Humus is the byproduct of microorganisms. It can be manufactured by making compost the correct way, which must include animal manureally and urine with vegetable wastes. Howard wrote, The Manufacturerof Humus by the Indore Process. Search for a copy of his pdf.
    – This type of compost is humus, and it does far more then we have been led to believe.
    – It gives plants that are, mycorrhiza forming, what it needs to establish the myccorrhizal relationship which is amazing. Learn about it. You’ll be glad you did.

    Anyone who wants to discuss more, email me.

    Tom

    • The material you have at the end of composting is not humus. It will take much more time and degradation by microbes to become, what people consider to be humus.

      Never heard that mycorrhiza need humus present to connect with plant roots and I don’t believe it is true. These relationships can be developed in straight peat moss – which does not contain humus.

      Unfortunately, it looks as if many of our ideas about humus are wrong. I’ve written on this recently in Humus Does Not Exist – Says New Study.

  4. Thank you Robert, for publishing this interesting article – it’s a very enjoyable read! We sheet compost in our food forest in Southern California, and we noticed a layer of organic material had formed a crust on top of the soil. This seems to be where the alkaline clay soil and the organic material are interacting and transforming.

    Keep up the good work!

  5. Linda long says:

    Now that everyone is more confused than before I had questions I guess I shd stick to my protocol of Pacific rim minerals mushroom compost cow manure decomposed comfrey in water all the scrap from my table coffee grounds etc n continue to have people come back yr after yr n say I NEVER HAD PLANTS GROW LIKE THIS WHAT R U DOING!!!

  6. aloysius kingston says:

    Hello I live in northern new Brunswick i have a wood lot and on that wood lot there is about 3 ac. and 3 ft deep of hums(leaf mold ) i was wondering what that would worth road side if i mixed with lime and bag it, or is any value ? thanks in advance ak kingston

  7. Robert. Thanks for your comments.
    I have nutrient deficient beach sand in South Australia.
    What do suggest is the best compost
    Composition to eventually create Humus

    • My rule of thumb is to use what ever you can get locally. Local means it is not transported very far, which means it is much more environmentally friendly.

      Any organic mater works.

  8. Dea says:

    Is Leaf Mold considered Humus?

    • Robert Pavlis says:

      No. Leaf mold is a living organism–a fungi. It lives on leaves and in soil and helps degrade organic material.

      The term is also (incorrectly) used to describe the resulting material after fungi digest the leaves. It is best to think of this type of leaf mold as a form of compost. Organic material that has been broken down to a certain extent, but which still contains lots of nutrients for plants.

      • Dea says:

        I was talking about the finished product that comes from rotting down leaves, like what is found on the forest floor. I’ve always been told that that is leaf mold and considered the best component to add to your garden as it has no nutrient value to speak of yet improves soil condition and structure…

        • Dea says:

          Also, Wiki illudes to Leaf Mold being “humic”…
          http://en.wikipedia.org/wiki/Leaf_mold

          • Robert Pavlis says:

            If you look in Wiki under Leaf Mold, you will see the term ‘humic’ matter highlighted. This will take you to the definition of Humus. There it says “humus is sometimes also used to describe mature, or natural compost extracted from a forest”. This was the main point of my post. The word humus is used incorrectly in a lot of places. Even though some people refer to leaf mold as humus–it is not the correct definition.

        • Robert Pavlis says:

          If you collect leaf mold from the forest then it is not yet completely broken down. It is basically compost.
          If you are talking about the soil in the forest, then it will contain leaf mold from the current year and humus that is a result of decomposing organic matter from past years.

          The former might be useful in the garden. The later would also be good for the garden, but it should not be removed from the forest for environmental reasons. Secondly, it would be better for the garden to use compost or your leaf mold that you are making. when people talk about making leaf mold for the garden, they are essentially making a pile of leaves and waiting until they start to decompose. This adds nutrients to soil in the short term, and humus in the long term.

  9. Donna says:

    With a view to preserving soil aggregation, low till or no till gardening has been suggested. The concept makes sense to me, but am unsure about planting seeds in what seems to be compacted top soils-would you loosen the row where you’re planting?

    • Robert Pavlis says:

      I would loosen the top inch or so of the soil before planting seed. They have difficulty sending their initial root into compacted soil.

  10. garcanad says:

    It is interesting to learn that humus is basically some form of carbon. I vaguely remember reading an article in one issue of the Rhodo Society publication which described the research on some large areas in Argentina with very fertile soil right next to extremely poor soil area, and the only difference they found was the presence of carbon in the fertile area. I am curious whether crashing wood BBQ charcoal into soil can in some way enhances such ‘carbonization’.

    • Robert Pavlis says:

      Keep in mind that carbon is not nutritious to plants. Humus “consists mostly of carbon” – it is not really a form of carbon. Sugar and starch also consists mostly of carbon, but they are not forms of carbon. The real value of the humus is in conditioning the soil and holding on to nutrients. Carbon also has a holding property for nutrients – it is used in aquarium filters to remove toxins from the water, and it can be used in home water filters for the same reason.

      Adding crushed charcoal to soil may help a bit to hold nutrients, but I doubt that it would have the same soil conditioning affect as humus. It also does not add nutrients. It is better to add organics.

  11. Robert, I’ve noticed that when I put a mulch layer of shredded leaves on my veggie beds for the winter, worms (so it seems) go to town and come spring the result is a whole bunch of “crumbs” (plus leftover shredded leaves on top). I had read the term “crumb” in gardening books as something to promote in soil, but I had no idea the term was being used so literally in terms of visuals and texture–these crumbs really do look and feel like dead ringers for crumbs from a chocolate cake. Is there any reason to believe that there is much humus as part of these crumbs? This is material that has only been through one decomposer (the worm, if I’m right) so presumably has more cycles of decomposition to go through. On the other hand, the stuff looks just like what I imagine plants have in mind when they daydream about what they want their roots to growth in.

    • Robert Pavlis says:

      The ‘crumbs’ are probably worm castings – or more directly – worm poop. It is a long way from being humus, but it is on it’s way to decomposing and great for the garden.

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