humus

Humus is the dark, organic component of soil that forms from the decomposition of plant and animal matter through microbial activity. It is a stable form of organic matter that resists rapid decay and helps to maintain soil fertility and structure.

It develops as plant litter, roots, and other materials break down, passing through stages of decay. Soil organisms such as bacteria, fungi, earthworms, and other invertebrates assist in transforming matter into humus. In agricultural practice, composting, mulching, and residue management promote humus formation.

Humus has a complex, colloidal structure that improves soil properties: it increases aggregation, enhances water retention, reduces erosion, and helps retain nutrients by increasing cation exchange capacity. It also darkens the soil, promoting heat absorption and biological activity, and can buffer soil pH fluctuations.

In modern soil science, humus refers to the stable fraction of soil organic matter and may include humic substances, such as humic acids, fulvic acids, and humin. These substances differ in solubility and stability but together contribute to nutrient availability and soil structure.

The amount of humus in a soil depends on climate, vegetation, soil texture, and management. Undisturbed soils tend to accumulate humus, while intensive tillage and erosion can deplete it. Humus is a key indicator of soil health and soil organic matter content.

See also: compost, soil organic matter, soil fertility.