Comparatively stable fraction of soil organic matter left after decomposition is called

Correct option is C

The correct answer is (C) Humus

Explanation:
• When plant and animal residues are added to the soil, they are decomposed by microorganisms. The stable, dark-colored, amorphous fraction that remains after the initial rapid decomposition is called Humus.
• Humus is highly resistant to further microbial breakdown and can persist in the soil for hundreds of years.
• It is a complex mixture of humic acid, fulvic acid, and humin. Humus is vital for soil fertility because it has a very high cation exchange capacity (CEC), improves soil structure (aggregation), and stores nutrients like nitrogen and phosphorus.

Information Booster:
• The C:N ratio of stable soil humus is typically around 10:1.
• Humus helps in "buffering" the soil, preventing rapid changes in pH.
• It gives soil its characteristic dark brown or black color.

Additional Knowledge:
• Podzol (Option A): A type of soil formed under cool, coniferous forests, not a fraction of organic matter.
• Mineralization (Option B): The process where organic matter is converted into inorganic forms (like $NH_4^+$) that plants can use.
• Immobilization (Option D): The process where microbes consume inorganic nutrients for their own growth, making them temporarily unavailable to plants.