The accumulation of a dark coloured amorphous substance that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate is called:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Humification
Decomposition goes through five steps: fragmentation, leaching, catabolism, humification and mineralisation. The question describes the step that makes humus.
(1) Mineralisation — Incorrect. This comes after humification. Some microbes break down the humus further and release inorganic nutrients into the soil.
(2) Humification — Correct. It builds up humus, a dark, amorphous (shapeless) substance. Humus is highly resistant to microbial action and decomposes at an extremely slow rate. Being colloidal, it works as a store of nutrients.
(3) Fragmentation — Incorrect. Detritivores such as earthworms break the detritus into smaller particles. No humus is formed at this step.
(4) Leaching — Incorrect. Water-soluble inorganic nutrients move down into the soil and get fixed as salts that plants cannot use. It does not form humus.
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.”
