Here's why:

* A Horizon (Topsoil): This layer is the most fertile and contains the highest concentration of organic matter. This organic matter, derived from decaying plant and animal material, is what gives the A horizon its dark color. It's also where most of the biological activity occurs.

* Other Horizons:

* O Horizon (Litter Layer): Found above the A horizon, this layer is composed of freshly fallen organic matter that hasn't fully decomposed yet. It might be brown or reddish depending on the composition of the litter.

* B Horizon (Subsoil): This layer has less organic matter than the A horizon, making it lighter in color. It's also where mineral accumulation often happens.

* C Horizon (Parent Material): This layer is composed of weathered rock fragments, and it doesn't have much organic matter. The color will depend on the parent rock.

* R Horizon (Bedrock): This is solid, unweathered rock and doesn't contribute to soil formation.

In summary: The dark-rich soil you're looking for is typically found in the topsoil layer (A horizon) due to its high concentration of decomposed organic matter.