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?