A Horizon
Definition
The uppermost mineral soil horizon, darkened by organic matter accumulation and the layer most commonly transferred by footwear and vehicle contact.
- Position
- Uppermost mineral soil horizon
- Colour
- Darkened by organic matter
- Forensic relevance
- Most commonly transferred by footwear and vehicles
Common questions
How is the A horizon different from the O horizon above it?+
The O horizon is mostly organic litter, such as leaves and decomposing plant matter, sitting on the surface. The A horizon beneath it is mineral soil with organic matter mixed in, which is what typically adheres to shoes and tyres.
Why does its composition matter for comparing soil samples?+
Because it is the layer most often picked up during contact with the ground, its mineral content, organic content and colour, often recorded using the Munsell system, give investigators a comparable profile between scene and suspect samples.
Related terms
- Chroma
- The saturation or colourfulness of the hue, from 0 (neutral grey) outward in whole-number steps. Strongly weathered, iron-oxide-rich soils have high chroma;...
- Clay Mineral
- A secondary phyllosilicate mineral produced by chemical weathering; includes kaolinite, illite, smectite, and chlorite, all diagnostic of parent material and weathering intensity.
- Hue
- The dominant wavelength family of a colour, expressed as a position on the Munsell colour wheel. Soil hues run from 10R (red)...
- Loss on Ignition (LOI)
- Weight loss after heating dried soil to 550 degrees Celsius. Approximates the organic matter content. A simple, cheap, high-yield discriminator in field...
- Munsell Chip Comparison
- The visual procedure of matching a soil sample against printed colour chips in the Munsell Soil Colour Chart under standard daylight conditions,...
- Munsell Notation
- A three-component colour code (Hue-Value/Chroma) standardised under ASTM D1535 for describing soil colour from the Munsell Soil Colour Chart.
- Particle-Size Distribution
- The quantitative measurement of the proportions of gravel, sand, silt, and clay in a soil sample, determined by sieving and hydrometer or...
- Soil Texture
- The relative proportions of sand, silt, and clay in the mineral fraction of a soil, expressed as a USDA or ISSS texture...
- USDA Soil Taxonomy
- A hierarchical classification of soils into 12 orders based on pedogenic processes and measurable diagnostic properties; the primary reference for US forensic...
- Value
- The lightness dimension of the Munsell system, scaled from 0 (theoretical black) to 10 (theoretical white). High-organic soils tend to have low...
- World Reference Base (WRB)
- The FAO-IUSS international soil classification standard, with 32 Reference Soil Groups; mandatory vocabulary for ENFSI forensic soil evidence reports.