Here's why:

* Physical Changes:

* Decomposition: The physical structure of the wood and leaves breaks down into smaller pieces.

* Fragmentation: Organisms like fungi and bacteria physically break down the material.

* Chemical Changes:

* Biodegradation: Microorganisms (fungi and bacteria) release enzymes that chemically break down the complex molecules in wood and leaves into simpler compounds.

* Humification: The breakdown products are further transformed into humus, a stable, dark, organic matter rich in nutrients.

Therefore, humus formation involves a complex interplay of physical and chemical processes.