What is Humus?

What is Humus?

Humus is dark, organic material that forms in soil when plant and animal matter decays.

When plants drop leaves, twigs, and other material to the ground, it piles up. This material is called leaf litter. When animals die, their remains add to the litter. Over time, all this litter decomposes. This means it decays, or breaks down, into its most basic chemical elements. Many of these chemicals are important nutrients for the soil and organisms that depend on soil for life, such as plants.

The thick brown or black substance that remains after most of the organic litter has decomposed is called humus. Earthworms often help mix humus with minerals in the soil.

Humus contains many useful nutrients for healthy soil. One of the most important is nitrogen. Nitrogen is a key nutrient for most plants. Agriculture depends on nitrogen and other nutrients found in humus.

Some experts think humus makes soil more fertile. Others say humus helps prevent disease in plants and food crops.

When humus is in soil, the soil will crumble. Air and water move easily through the loose soil, and oxygen can reach the roots of plants.

Source:  National Geographic

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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 sometimes 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.  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.

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