What is the primary characteristic of humus?

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

Decomposition breaks dead plant and animal matter down in steps: fragmentation, leaching, catabolism, humification and mineralisation. In humification, a dark, amorphous substance called humus builds up in the soil.

Option (1) It undergoes decomposition at an extremely slow rate. — Correct. Humus resists microbial action, so it breaks down extremely slowly. It therefore stays in the soil for a long time and, being colloidal, acts as a store of nutrients.

Option (2) It is highly soluble in water. — Incorrect. Humus is a colloidal substance, not one that dissolves readily. It is the water-soluble inorganic nutrients that go down into the soil during leaching.

Option (3) It is broken down easily by microbial action. — Incorrect. This is the opposite of the truth. Humus is highly resistant to microbial action.

Option (4) It is rich in nitrogen and water-soluble substances. — Incorrect. Detritus that is rich in nitrogen and water-soluble substances, such as sugars, decomposes quickly. Humus decomposes very slowly, so this description does not fit it.

NCERT Extract:
Decomposition – “… 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. … In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.”