Soil Horizon
Definition
A roughly horizontal layer of soil with distinct physical, chemical or biological properties. The standard sequence from surface down is O (organic), A (topsoil), B (subsoil) and C (parent material). Forensic comparison rarely matches across horizons.
- O horizon
- Organic surface layer
- A horizon
- Topsoil
- B horizon
- Subsoil
- C horizon
- Parent material
Common questions
Why does forensic soil comparison rarely match across different horizons?+
Each horizon forms through distinct weathering, biological, and depositional processes, giving it a characteristic mineral and organic profile. A sample taken from the A horizon at one site will generally not match a sample from the B or C horizon at another, even if both sites are otherwise similar, so comparisons are made horizon to horizon.
How does horizon identification help interpret soil found on a suspect's shoes or vehicle?+
Identifying which horizon a trace sample came from can indicate how it was deposited, for example a subsoil (B or C horizon) trace suggests digging or disturbance rather than simply walking across a surface, which is relevant context in cases involving burial or excavation.
Does every location have all four horizons clearly developed?+
No. Horizon development varies with climate, vegetation, and time; some soils have a thin or absent O horizon, and horizon boundaries can be gradual rather than sharp, which is part of why trained soil examination rather than casual observation is needed for reliable comparison.
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