Here's why:

* Soil Formation Process: Soil develops in layers, each with unique characteristics. The process is called pedogenesis.

* O Horizon: This is the uppermost layer and consists primarily of organic matter. It's made up of decomposing leaves, twigs, and other plant and animal residues.

* Other Layers: The O horizon forms on top of the A horizon (topsoil), which is rich in minerals and organic matter. Below that are the B horizon (subsoil) and C horizon (parent material).

Important Note: The O horizon is not always present. Its development depends on the climate, vegetation, and other environmental factors.

Let me know if you'd like to learn more about soil layers!