The correct option is Highly resistant to microbial action and colloidal in nature
Explanation
Humus is a dark-coloured, amorphous substance formed during the process of decomposition (humification). It represents the stable fraction of soil organic matter that remains after the major portion of plant and animal residues have decomposed.
- Highly resistant to microbial action and colloidal in nature is Correct: Humus is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It is colloidal in nature, which allows it to hold water and serve as a reservoir of nutrients for plants.
- Rapidly decomposed and poor in nutrient-holding capacity is Incorrect: Unlike fresh detritus, humus does not decompose rapidly. It has a high nutrient-holding capacity due to its negative charge and high surface area (colloidal properties).
- Gas-forming and easily volatilised under heat is Incorrect: Humus is a stable organic complex. It is not characterized as a gas-forming substance nor does it easily volatilise under standard soil temperature conditions.
- Forms only under strict anaerobic conditions is Incorrect: Decomposition, including humification, is largely an oxygen-requiring (aerobic) process. While organic matter accumulation (like peat) occurs in anaerobic conditions, humus formation is not restricted to strict anaerobic environments.
Key Takeaway: Humus is defined by its amorphous structure, colloidal nature, high resistance to microbial decomposition, and function as a nutrient reservoir in the soil ecosystem.