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.

Humus can be produced naturally or through a process called composting. When people compost, they collect decaying organic material, such as food and garden scraps, that will be turned into soil.

humus
This leaf litter will eventually decompose and become nutrient-rich humus.

Compost and Humus
Compost, like humus, is made of decomposed organic material. Compost usually refers to material created by people from leftover foods and yard waste. Humus usually refers to the natural decay of material such as leaves in the soil's top layer.

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

compost
Noun

mixture of decaying organic material, such as food waste and plants.

decay
Verb

to rot or decompose.

decompose
Verb

to decay or break down.

fertile
Adjective

able to produce crops or sustain agriculture.

Noun

material that forms when plant and animal matter decays.

leaf litter
Noun

dead plant material such as leaves, twigs, pine needles, and bark that accumulates on the ground. Also known as plant litter and tree litter.

mineral
Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

nitrogen
Noun

chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

Noun

substance an organism needs for energy, growth, and life.

organic
Adjective

composed of living or once-living material.

oxygen
Noun

chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

remains
Noun

materials left from a dead or absent organism.

soil
Noun

top layer of the Earth's surface where plants can grow.