What is the primary characteristic of humus?

It undergoes decomposition at an extremely slow rate.

The correct answer is Option (1) -  It undergoes decomposition at an extremely slow rate.

Humification and mineralisation occur during decomposition in the soil. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients.

The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.

Unlike less decomposed organic matter, which is more readily broken down by microorganisms, humus is highly resistant to further microbial action. Its resistance to decomposition is due to its complex molecular structure and chemical composition, which make it difficult for microbes to break down.

Humus does contain nutrients, it is not specifically rich in nitrogen and water-soluble substances. Its nutrient content varies depending on the composition of the original organic matter from which it formed.

In summary, the primary characteristic of humus is its slow decomposition rate, making it highly stable and resistant to further microbial breakdown.