The topmost soil horizon is the O horizon, also known as the organic horizon.

Here's why:

* It's the uppermost layer: It sits directly on top of the mineral soil layers.

* It's rich in organic matter: This includes decomposing leaves, twigs, roots, and other plant and animal residues.

* It's typically dark in color: Due to the high content of organic matter.

* It's crucial for soil health: It provides nutrients, improves soil structure, and helps retain moisture.

While the O horizon is the topmost layer, it's not always present. In some soils, particularly in areas with high erosion or limited organic matter, the O horizon might be thin or absent.