What is Humus?

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Robert Pavlis

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.

Soil Science for Gardeners book by Robert Pavlis

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’t 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

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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!

45 thoughts on “What is Humus?”

  1. I want to plant a mahonia, and it says to plant in cool soil with a lot of humus. I don’t have a compost bin/pile/heap. It’s been stated that it cannot be “bought” so how can I give the plant what it needs to start it’s healthy long life?

    Reply
  2. Nitro-Humous is human shit, obtained from sludge plants. I once was broke and frozen toilet. So collected my “refuse” in a bucket and turned that into my experimental corn patch at 9000 feet. The comment from others was: how is your corn so dark green and so tall. I just answered green thumb and brown dirt.

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  3. Check out Forest Floor Soil in Creswell, Supposedly harvested from peel piles of old mill sites. They called it humus. We brought home a yard of it today to make a 2” top layer on the garden. Looks pretty good to me!

    Reply

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