The correct option is Degradation of humus to release inorganic nutrients.
Explanation
Decomposition is the process of breaking down complex organic matter into simpler inorganic substances like carbon dioxide, water, and nutrients. This process is critical for nutrient cycling and occurs in specific sequential steps: fragmentation, leaching, catabolism, humification, and mineralisation.
- Humification: This process leads to the accumulation of a dark-coloured, amorphous substance called humus. Humus is highly resistant to microbial action and decomposes at an extremely slow rate. It serves as a reservoir of nutrients.
- Mineralisation: This is the process where humus is further degraded by some microbes, resulting in the release of inorganic nutrients (minerals) back into the soil pool.
Analysis of Options:
- Degradation of humus to release inorganic nutrients is Correct: It correctly defines mineralisation as the degradation of humus to release inorganic nutrients.
- Conversion of organic detritus to humus is Incorrect: The conversion of organic detritus (dead plant and animal remains) into humus is termed humification.
- Polymerisation of sugars to lignin is Incorrect: This refers to biological synthesis processes (such as lignification in cell walls), not the decomposition of soil organic matter.
- Conversion of inorganic salts to organic residues is Incorrect: The conversion of inorganic salts into organic residues refers to immobilisation or assimilation by soil microbes, where nutrients are temporarily locked inside microbial biomass.
Key Takeaway: Mineralisation is the final stage of decomposition that converts organic nutrients stored in humus into inorganic forms available for uptake by plants.