Humus Accumulation

A horizon formation

Within a few years of plants colonizing a new deposit of alluvium, the surface soil may begin to darken as plant residues decompose and humus accumulates to form an A horizon.

In this section, you can investigate the process of humus accumulation in soil.

There is more material on soil organic matter in the module of that name in the "Nutrient Cycling" section.

Carbon fluxes

The illustration shows a simplified carbon cycle through plants, soil, and the atmosphere. The amount of soil organic matter that accumulates in the soil depends on the relative significance of each of the fluxes.

By understanding the processes that affect humus accumulation in soil, you can predict how soil organic matter levels will change with environment and management. Test your understanding in the revision questions.

Refer to the image above as you find out more about how each flux affects organic matter accumulation.

Plant residues - Humus

When plant parts like roots and leaves die, they become plant residues. Fresh plant residues are attacked by soil microorganisms and rapidly (within weeks to months) are converted to a more stable material called humus, which can persist for years. Humus is a dark-colored complex mixture of organic compounds and is the main component of soil organic matter.

The amount of humus in the soil is thus related to the level of residue production.

HUMUS --> CO₂

The carbon in humus is slowly converted to CO2 as microorganisms respire. Like most biochemical reactions, the rate of microbial respiration increases as temperature rises, so humus is decomposed more rapidly in warm climates. In very wet soil environments, when soil aeration is restricted, respiration is slowed due to a lack of oxygen.

CO₂ --> PLANT

The carbon in humus is ultimately derived from CO2 fixed by plants, so the amount of humus in the soil is related to the amount of carbon fixed by vegetation.

For example, in slash-and-burn agriculture, tropical forests are cleared for food production. Humus formation from residue decomposition is curtailed, and soil organic matter levels decline. After several years, when the soil is no longer fertile, it is left to be reclaimed by the forest, where humus levels will rise once more.