Consider the following statements about the "humus" formation process: 1. It releases heat, making the decaying heap warm. 2. It is a chemical process carried out by sunlight. 3. It involves the synthesis of new plant tissues from inorganic matter. Which of the statements given above is/are correct?

The correct option is (a). Explanation Humus formation, or humification, is a critical phase in the decomposition process where complex organic matter is broken down into a dark, amorphous, and stable substance known as humus. This process is primarily driven by the metabolic activities of soil microorganisms. Statement-wise Analysis: - Statement 1 is Correct: Decomposition is an exothermic process. As microorganisms (bacteria and fungi) break down organic bonds through respiration to derive energy, a significant amount of heat is released. This metabolic heat generation causes decaying heaps, such as compost piles, to become warm. - Statement 2 is Incorrect: Humus formation is a biological process carried out by decomposers (detritivores and microorganisms) using enzymes. It is not a chemical process driven directly by sunlight. While sunlight can cause physical degradation (photodegradation) of surface litter, the formation of humus within the soil ecosystem is biotic. - Statement 3 is Incorrect: This statement describes photosynthesis (primary production), where plants synthesize new tissues from inorganic matter (carbon dioxide and water). In contrast, humus formation involves the breakdown (catabolism) of organic matter into simpler substances, rather than the synthesis of new plant tissues. Key Takeaway: Humification is a biological, heat-releasing (exothermic) decomposition process driven by microbial activity, distinct from the light-driven, anabolic process of photosynthesis.

NCERT Class 7 Environment & Ecology — Chapter 1: Forests: our lifeline

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Consider the following statements about the "humus" formation process:

It releases heat, making the decaying heap warm.
It is a chemical process carried out by sunlight.
It involves the synthesis of new plant tissues from inorganic matter.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 only
B2 only
C1 and 3 only
D1, 2 and 3

Did you get this right?

Solution

The correct option is (a).

Explanation

Humus formation, or humification, is a critical phase in the decomposition process where complex organic matter is broken down into a dark, amorphous, and stable substance known as humus. This process is primarily driven by the metabolic activities of soil microorganisms.

Statement-wise Analysis:

Statement 1 is Correct: Decomposition is an exothermic process. As microorganisms (bacteria and fungi) break down organic bonds through respiration to derive energy, a significant amount of heat is released. This metabolic heat generation causes decaying heaps, such as compost piles, to become warm.
Statement 2 is Incorrect: Humus formation is a biological process carried out by decomposers (detritivores and microorganisms) using enzymes. It is not a chemical process driven directly by sunlight. While sunlight can cause physical degradation (photodegradation) of surface litter, the formation of humus within the soil ecosystem is biotic.
Statement 3 is Incorrect: This statement describes photosynthesis (primary production), where plants synthesize new tissues from inorganic matter (carbon dioxide and water). In contrast, humus formation involves the breakdown (catabolism) of organic matter into simpler substances, rather than the synthesis of new plant tissues.

Key Takeaway:
Humification is a biological, heat-releasing (exothermic) decomposition process driven by microbial activity, distinct from the light-driven, anabolic process of photosynthesis.

Your note

More from Chapter 1: Forests: our lifeline

With reference to the structure of a forest, consider the following statements: 1. The branchy part of a tree above the stem is known as the crown. 2. The roof-like structure formed by the branches is called the root system. 3. The forest floor is the only layer found in a forest. Which of the statements given above is/are correct?

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Consider the following statements regarding the vegetation layers in a forest: 1. Giant and tall trees constitute the top layer. 2. Shrubs and tall grasses form the lowest layer of the forest. 3. Herbs form the lowest layer of vegetation in the forest. Which of the statements given above is/are correct?

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In a dense forest ecosystem, the forest floor receives minimal sunlight primarily due to:

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With reference to the role of forests in the water cycle, consider the following statements: 1. Trees release water vapour into the air through the process of condensation. 2. The forest canopy intercepts raindrops, preventing them from hitting the ground directly. 3. Forests facilitate the infiltration of water into the ground. Which of the statements given above is/are correct?

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Consider the following statements about the "forest floor": 1. The forest floor is generally dark due to the dense canopy above. 2. The forest floor is completely devoid of any dead or decaying matter. 3. The forest floor is sterile and supports no living organisms. Which of the statements given above is/are correct?

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The correct option is (a).

Explanation

Statement-wise Analysis:

Statement 1 is Correct: Decomposition is an exothermic process. As microorganisms (bacteria and fungi) break down organic bonds through respiration to derive energy, a significant amount of heat is released. This metabolic heat generation causes decaying heaps, such as compost piles, to become warm.
Statement 2 is Incorrect: Humus formation is a biological process carried out by decomposers (detritivores and microorganisms) using enzymes. It is not a chemical process driven directly by sunlight. While sunlight can cause physical degradation (photodegradation) of surface litter, the formation of humus within the soil ecosystem is biotic.
Statement 3 is Incorrect: This statement describes photosynthesis (primary production), where plants synthesize new tissues from inorganic matter (carbon dioxide and water). In contrast, humus formation involves the breakdown (catabolism) of organic matter into simpler substances, rather than the synthesis of new plant tissues.

Key Takeaway:
Humification is a biological, heat-releasing (exothermic) decomposition process driven by microbial activity, distinct from the light-driven, anabolic process of photosynthesis.