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.