Humus
Humus is the dark, stable organic matter in soil formed from the decay of plant and animal remains, which holds water and nutrients.
Definition
Humus is the dark, crumbly part of soil made from dead leaves, plants, and animals that have rotted away, and it makes soil rich and helps plants grow. It is the stable organic component that remains after bacteria, fungi, and invertebrates decompose plant and animal residues; it improves soil structure, holds several times its weight in water, stores nutrients such as nitrogen and phosphorus that are released slowly to plants, and is concentrated in the A horizon. Chemically, humus consists of complex, partly decomposed organic compounds that resist further microbial breakdown, historically grouped as humic acids, fulvic acids, and humin. Its high density of carboxyl and phenolic groups gives it a cation exchange capacity of several hundred centimoles per kilogram, far above that of most clays, and it buffers pH and chelates metals. Humus turnover times range from decades to centuries, and its formation depends on litter quality, moisture, temperature, and clay mineralogy, which protects organic matter by adsorption.
Example
The black, soft material at the bottom of a compost pile is humus, and forest floors are dark because of it. A hardwood forest in the North Carolina mountains builds a thick humus layer from fallen leaves each autumn, while Piedmont farm fields that were plowed for cotton in the 1800s lost most of their humus and topsoil to erosion. Cold, wet peat bogs accumulate organic matter because decomposition nearly stops, whereas warm tropical forests recycle litter within months and keep little humus. Adding compost to Piedmont clay raises water-holding capacity and loosens structure by binding clay particles into aggregates.
Key Insight
Humus is what makes topsoil dark; pale soil is usually low in humus and less fertile. Humus is not the same as leaf litter, since litter is still recognizable while humus is the finished, decomposed product that no longer looks like anything. Humus is the largest active carbon store on land. Practices that build it (cover crops, reduced tillage) pull carbon from the atmosphere, while practices that burn it off (bare fallow, over-tillage) do the reverse.