Here's why:

* Organic Matter: This layer is primarily composed of decomposing plant and animal matter (leaves, twigs, roots, etc.). It's rich in nutrients and provides food for soil organisms.

* Color: The O horizon is usually dark brown or black due to the presence of decomposed organic matter.

* Texture: It is often loose and spongy, with a high water-holding capacity.

It's important to note that not all soils have a distinct O horizon. In some cases, the topsoil (the uppermost layer) might be directly the A horizon, which is a mineral layer.