• 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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    agriculture Noun

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

    Encyclopedic Entry: agriculture
    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.

    humus Noun

    material that forms when plant and animal matter decays.

    Encyclopedic Entry: humus
    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.

    nutrient Noun

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

    Encyclopedic Entry: nutrient
    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.