Today I Define Humus

Define humus: The organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms.

That’s according to Google and it’s basically accurate, although if we’re getting nit-picky:

  • Humus isn’t the only “organic component of soil”
  • It’s not just leaves and plant material (it’s anything that was once living)
  • Insects and animals are involved in this decomposition process, too.

Today I want to define humus and why it’s important in organic gardening. I got an email the other day from someone asking “what is humus?” and so today I am going to talk about it.

Now humus is often use interchangeably with compost/organic matter but they really aren’t the same thing, although they’re derived from the same thing.

What humus is is basically organic matter that has been broken down by microorganisms, even starting with insects, fungi and bacteria, all different hosts of functional groups of microbes and insects until it really doesn’t get broken down much anymore.

The microbes have kind of done what they want to do with it. They’ll eat it if they have to, they’ll keep working on it if there’s no other food but really they have done what they want to do so really its basically described as biologically active carbon chains.

It’s a very complex molecule because all these things have been working on it. It’s very active in the soil. So that’s what it is and why it is important in soil is for a number of reasons.

Why Humus Is Important

One, it’s extremely good at holding onto nutrients. So like our fertilizers or even just nutrients in organic matter.

It keeps those in the soil and in fact apparently it’s especially important for calcium because calcium apparently sinks down lower into the soil profile so if you have that humus up there it keeps that calcium up there, which is so important for so many plant processes.

Another thing would be water – extremely good at holding water. It can hold many times its own weight in water. And so you know especially if you have like a sandier soil, that the water drains quickly you get some humus in there it’s gonna hold a ton of water.

Those are the main ones that I often think of. Another one would be toxins. It can help buffer toxins, hold onto them and buffer them so that they not available to the plant. I don’t really know the chemistry behind how that works but that’s one thing that I often read in the soil books.

Another one would be air. Air in the soil, just having that humus in there creates soil structure. I could say air/structure. You know it just helps create really nice soil structure whether you have a sandy soil or the other extreme, clay soil. Those are the main ones I can think of, I guess the other one would be microbes.

I know the microbes have created it but it’s also like a home for them. It creates a nice environment for them to live in.

So, that’s what humus is. It’s organic matter whether it’s compost or mulch or any kind of organic matter that’s been shredded by insects, broken down by fungi and bacteria until it’s just this very complex and yet it’s this very complex carbon chain that looks kind of like compost, but it’s not discernible. It’s just so broken down you can’t tell what it was in the beginning.

And then it does these things, you know holds nutrients, holds water, creates spaces, buffers toxins and makes home for microbes. It’s super important to have in an organic vegetable garden.

So that is all. I just wanted to define humus. Questions?


  1. Ron on February 18, 2012 at 1:38 pm

    if you wanted to top-dress your garden with carbon materials such as wood chips in a layer only 2” thick, does it take too long for them to break down for them to have any real impact as being beneficial to your garden. 

    • Phil on February 18, 2012 at 10:50 pm

      Takes a heck of a long time to become humus, but there are many other benefits – weed control, moisture conservation, soil temperature moderation. There can also be downsides to wood chips, potential nitrogen deficiency in the soil being the main one.

      • Bigjim10235 on April 10, 2012 at 7:00 pm

        Phil, I can get “unfinished compost” free from a local Public Works yard. It’s black, and is loaded with remnants of the parent material, like twigs and wood chips. I thought in one year the pile would mature and become “finished compost.” How long might a large pile take to become humus?

        • Phil on April 12, 2012 at 12:15 am

          If you turn it regularly (e.g. weekly), you can do a good job of composting it in a month or 2. The twigs and wood chips will still remain, but it can be used in gardens after that time. As for becoming humus, that mostly happens in the soil over a matter of years, but I think you’re really just looking for a usable product.

  2. Susan on February 18, 2012 at 1:44 pm

    Hi Phil,I enjoy you blogs and am waiting for your ”garden University?’ to become available.A couple thoughts on the use of a marker board:1. I suggest placing the board on a simple easel. When holding it, it moves around and produces random flashing reflections that are distracting and can at times obliterate what you are writing.2. You seemed to struggle a bit with your descriptive discussion. Though video can be efficacious in teaching, it’s a much different process than is simple ‘conversation. A lot of prep is needed–often as much as the cumbersome writing process you mentioned, to become comfortable and smooth in your presentation. Persistent pauses to order your thoughts with the accompanying ‘uh, uh, ums…’, which are time fillers, can be very distracting, the more so over time to the point of annoyance.I learned these things the hard way over a 25-year career as a corporate trainer.

  3. Steve on February 18, 2012 at 1:59 pm

    Hey Susan, I love the way Phil presents! He’s down to earth and not canned. That’s what makes him so good and unpretentious.Too many people are trained and end up putting you to sleep.

    • Ruminantia on February 18, 2012 at 2:24 pm

      Yes, Phil, Don’t change!  I agree with everything Steve said.  Susan:  learn to praise in public and criticize in private.

  4. Carrie on February 18, 2012 at 5:00 pm

     I agree with Steve and Ruminantia – Phil we love your unpretentiousness, but I think Susan’s suggestion of the easel is a good one. It would eliminate the awkwardness of talking and holding the white board. As far as the second suggestion on preparation, I think it’s also a good one. I really like this way to present information, and I am sure Phil welcomes the feedback since this is a relatively new medium of presentation for him. 

  5. Zuni53 on February 18, 2012 at 6:25 pm

    Phil, would it be correct to say that peat is humus? My undertanding is that while peat is organic (derived from moss, etc.) it is almost completely broken down so that it has little or no nutrients. What remains is primarily lignins, which is what I believe is also a primary ingredient of humus. Can you comment?

    • Phil on February 18, 2012 at 10:47 pm

      They’re quite different. Peat is organic matter that is very slowly broken down in very wet, anaerobic conditions by probably a small diversity of anaerobic organisms. Some of it has decent nutrition and some of it has very little. Most of it, like sphagnum, is not particular useful in gardening, definitely overused. I’m not aware of it being primarily lignins. It’s plants and a whole host of organisms.Humus is the result of more of an aerobic breakdown of organic matter by tends of thousands of different species of organisms, highly beneficial. These aren’t highly technical explanations, but it’s how I think of them.

  6. Milenegk on February 18, 2012 at 7:49 pm

    Is that the same humus made with chick peas?

    • Phil on February 18, 2012 at 10:48 pm

      Nope, that’s hummus. Much tastier.

  7. Phil on February 18, 2012 at 10:30 pm

    Thanks for the feedback Susan, Steve, Ruminantia and Carrie. I agree with all of Susan’s points. In the Smiling Gardener Academy that will be available in March, I’ve planned things out better before each video. These videos here are just a way to get something interesting up for people each week, but I can improve for sure.I’ve recorded a few videos that will go out over the next couple of weeks, but then hopefully the next batch after that will get better. I had challenged myself to record/edit/transcribe 8 videos in one day, so that’s also why they’re a bit rough around the edges.I like the easel idea. I’m moving in a couple of months, but will get one this spring. I say ‘um’ a lot, but I say ‘you know’ even more. It’s amazing how often that creeps in there!

  8. Deighton on February 19, 2012 at 2:23 am

    Hi Phil,Enjoyed your presentation on humus. in fact, I really enjoy and learn quite a lot from all your blogs.I have been experimenting over the winter, by digging a hole the width of the garden, and burying the kitchen waste, and when that hole is full, start another hole right next to it, till I eventually cover the whole garden. My theory is that this should add lots of good humus in preparation for spring.On another note, I will be trying my hand with a few Earth Boxes in spring.  The growing medium for the Earth Box is potting-mix, can that be classified as humus?  I would like to get your thoughts on the Earth Box.Deighton

    • yardenlb on February 20, 2012 at 5:18 am

      I love your idea of making your garden into a giant compost pile! I wish i had enough space to do just that. Do you have problems with pests digging up your food scraps? For how long does this practice put your garden out of growing commission?Grow on!

    • Phil on February 20, 2012 at 1:50 pm

      As long as no critters are digging up the kitchen waste, it’ll be great. And I’m sure the earth box will work – I’ve never seen one. It’s not a system to grow really nutrient-dense food, but perhaps its benefit is making the whole process easy. Potting mix has a bunch of stuff in it, probably not much humus.

  9. Mcs818 on May 30, 2012 at 1:21 pm

    Phil:  We collect leaves from around our neighborhood.  I try to ask whether or not people have used pesticides and herbicides, and avoid those piles.  But if they did, and are not telling, how long should I let the pile sit to sufficiently break down those chemicals so they are rendered harmless?  Is that even possible?

    • Phil on May 31, 2012 at 12:04 pm

      It’s so hard to say for sure. Composting them will help break down many of the chemicals, and a proper compost pile can be done in as little as a few weeks if you turn it every 3-7 days, and then it’s nice to cure it for a few more weeks after it’s done. Or you could pile them up over winter and put them on the garden in the spring.

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