Final answer:
The infertile layer of soil is the C-horizon, which is the bottommost layer of the soil profile.
Step-by-step explanation:
The layer of soil that is considered infertile is the C-horizon. The C-horizon is the bottommost layer of the soil profile and contains the parent material, which is the original rock that has been weathered and broken down over time to form soil. It is typically less fertile because it lacks organic matter and nutrients that are found in higher concentrations in the upper horizons such as the O and A horizons.