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.
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’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.
I bought a bag marked humus if it’s not humus what is it and should I add it to my indoor plants
No idea. Manufacturers can use the term on anything they want to put into the bag.
I collect soil from decomposing logs whenever I can come across them. I have found this strategy to be extremely useful in my mixture for any horticulture botany experiment. Mix it with worm castings, peat moss, and perlite and you are on the right step to success. I have been practicing with general horticulture for some time, I’ve had a lot of success using mushroom compost and bokashi fermented compost. I live in Missouri which is often a harsh and unforgiving climate.. Winters are cold summers are unreasonably hot and humid, and there are plenty of vermin and pests to prey on everything in the garden. I relish the challenge and focus on both guerilla gardening and indoor and backyard setups. I have thoroughly enjoyed reading the comments and information on this site. I have constantly been struggling with this deep forming desire that grows in me everyday to promote and flourish a magnitude of life to help push the diversity around me.. Gardening is the best outlet for this struggle. Can I ask a question , you seem like an experienced gardener, what kind of indoor grow lights do you prefer? Do you have a source for cheap grow lights?
How is humus important and beneficial to people?
Tastes good
No wonder I’m having so much trouble understanding exactly what humus is. It apparently doesn’t even exist.
I stumbled across this web page while trying to find out where humus goes when it’s depleted from the soil, since it’s supposed to be very stable and doesn’t decompose any more. If it doesn’t exist in the first place, that would explain it 🙂
That is correct, Leonardite is not Humus. I like the explanations and definitions by Dr. Robert Faust of BIOAG. (http://www.bioag.com)
“Leonardite–a soft brown coal-like deposit usually found in conjunction with deposits of lignite.”
It’s origin is from prehistoric plants, “salt water reed/sedge based”
The definition of loenardite may be correct, but much of the statements on your reference about the important of humic acid are not.
TOP TWO OR THREE INCHES OF FORREST FLOOR IS FULL OF MYCHORRIZAE ….PUT THREE OR FOUR HANDFULS INTO PLANTING HOLE……TAKE A LITTLE HERE AND THERE…..
I agree.
Hi, if you want to make the soil more healthy/fertile do you need to add only humus, or you need to apply compost too?
Compost becomes humus in the soil – think of them as the same thing. Besides, you can’t buy humus – bags marked as humus are not humus – just false advertising.
There is two places I know of in the u.s that mine humus from the earth which is found 20 feet under the soil. Leonardite products in Williston ND is one of them.
Thanks for posting this. If you go to the Leonardite website, they never say they are mining humus. They are mining something called leonardite, which is some type of organic matter formed from coal and contains humic acid. This is not humus.
Besides humus does not exist; https://www.gardenmyths.com/humus-does-not-exist-says-new-study/
Questions ::
Which soil contain minimum humus rate?
I am not sure what the question is. If you are asking which type of soil has low humus levels – it is not soil dependent. It depends on what is growing in the soil. soil from a desert will have less humus than soil from the woods, because deserts produce less organic matter.
As a general statement, sandy soil usually has less humus than clay soils.
The notes are just too supet. Ive learned a lot. Its great and very good explanations
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.