Using Soil to Solve Crimes

Definition of soil for forensics: Any disintegrated surface material, both natural and artificial, that lies on or near the earth’s surface Physical trace evidence Includes naturally occurring rocks, minerals, vegetation, and animal matter; as well as glass, paint chips, asphalt, brick fragments, cinders

Usefulness of soil evidence: Prevalent at crime scenes Unique to a particular location High transferability between the scene and the criminal Can link a suspect or object to the scene

Collection methods: Collect samples at various intervals within a 100-yard radius of the crime scene Collect samples at all possible alibi locations the suspect gives Collect only the surface layer in most cases Because there is variation of soil even at a particular location, it is important to collect standard/reference samples at various intervals. Need a representative sample of the soil.

Preservation methods: Soil collected at a site: Packaged in individual containers Soil collected from an object: Individually wrapped in paper Clumps of soil must remain intact Individual containers such as a plastic vial Vials need to be marked with the location where the sample was made Soil should remain intact on object Paper bags may be used for objects – collects any soil that may accidentally fall off in transport Clumps of soil in the case of a car shows order in which the soil adhered to the car (layering effect)

Analysis methods: Comparison of suspect/ object to the crime scene Need 1-2 tablespoons of sample Sample must be dry Samples must be dried under identical laboratory conditions

Analysis methods: Geological maps Using the characteristics of the sample and with knowledge of local geography geologists can direct police to the general vicinity where the soil originates

Analysis methods: Visual comparison of color & texture Munsell soil color chart books Nearly 1100 distinguishable soil colors The percentage of sand, silt, and clay will determine the soil texture. Can give students samples of clay, sand, and silt to feel the difference in texture.

Analysis methods: Microscopic examination Low power: Animal & plant material, artificial debris High power: Minerals and rocks Can provide the most varied and significant points of comparison between soil samples Minerals: naturally occurring crystals: physical properties (color, geometric shape, density, and refractive index) most useful for identification 2200 minerals known to exist

Analysis methods: Density-gradient tube Glass tube Filled with 6-10 layers of liquids with different densities Soil particles sink to portion of tube with similar density Ask students what a gradient is Useful in combination with other tests since many soils collected from different locations yield similar density distribution patterns