Dig a deep pit in a field and you will see bands of colour and texture stacked on top of each other. Each band is a soil horizon, a layer that formed over centuries through the slow work of weather, roots, worms, and water.

Soil scientists label them with letters. O is the surface litter of leaves and stems. A is topsoil, dark with organic matter and alive with microbes. E is a pale leached layer. B is subsoil, enriched with clay and iron washed down from above. C is weathered parent material, and R is solid bedrock.

Why the sequence matters

• Rooting depth depends on how thick the A and B horizons are
• Drainage is set by texture and structure of each layer
• Nutrient supply comes mostly from the A horizon
• Erosion strips the A layer first, and it takes decades to rebuild

Soil surveys map horizons across whole regions. For a farmer, the practical question is simple: how deep does the good soil go, and what happens when a hardpan or clay layer stops roots from reaching further?