Hardpan is most commonly found in which soil horizon?

• A. A horizon
• B. B horizon
• C. C horizon
• D. O horizon

Answer: (B)

Hardpan forms where materials moving downward in the soil become concentrated and cemented, creating a dense, durable layer that roots and water have trouble penetrating. This process happens mostly in the subsurface zone where illuviation occurs—the B horizon. Here clay, iron, and aluminum oxides accumulate and can bond together, producing the hard, compact layer we recognize as a hardpan. The surface A horizon is the topsoil and is usually softer and richer in organic matter; the O horizon is the organic litter layer above mineral soil; the C horizon is largely unweathered parent material. Since the cementation and accumulation driving hardpan development are characteristic of the B horizon, that is where hardpan is most commonly found.