5.1.16 (HL)—Soil horizons

O is organic litter, A is topsoil mixed with minerals, B is a zone of accumulation, and C is weathered parent material.

Use the sequence to explain farming loss: erosion and tillage can remove O/A material while leaving deeper B/C horizons, reducing roots, humus and biological activity.

A field with only B and C near the surface has lost much of its organic topsoil, even if mineral soil remains.

Erosion usually exports the O and A horizons first, removing litter, humus-rich topsoil, roots and biological activity before the deeper B and C horizons.

Horizon letters describe material and process, not fixed universal thicknesses.