Soil is the broad term used to describe the uppermost layer of unconsolidated material at the earth’s surface. It is the thin layer of material that develops where bedrock meets the biosphere, atmosphere and hydrosphere. So by its nature, soil is a heterogeneous substance – it contains both the inorganic remains of eroded rocks and minerals in various states of physical and chemical breakdown (weathering), as well as organic material in various states of decomposition. The transport of soil components from one place to another by erosion creates even more variability among soils. Human activity can lead to an anthropogenic component to soils, which can range from chemical pollutants to other kinds of trace evidence like building materials, glass and paint.

The diverse and heterogeneous nature of soils raises two issues in forensic science. First, it requires representative sampling of known soils to ensure a valid comparison. It is impossible to individualize a soil sample. A forensic soil comparison asks whether a questioned soil sample could have come from the source in question, which is a particular area at the earth’s surface. Because soil has components that vary both laterally over the earth’s surface and vertically into the ground, a questioned sample might be falsely excluded from a source area if it is compared to known samples that do not reflect the variability of soil in the area.

Secondly, the diversity of soil components presents a forensic examiner with a variety of techniques for comparing samples. Microscopic techniques like stereoscopic and polarizing light microscopy (PLM) are the most important tool for comparing the bulk composition of the soils and for identifying the minerals present. For representative samples, mineralogical comparisons are often the best first screening tool to eliminate different soils. When samples cannot be excluded by a mineralogical comparison, the organic and inorganic fractions may then be separated an examined using a number of tools. The organic fraction can be extracted and analyzed by high performance liquid chromatography (HPLC). The mineralogical fraction can be separated and compared by density, X-ray diffraction (XRD), X-Ray fluorescence, inductively coupled plasma mass spectrometry (ICP-MS), and/or ion chromatography (IC).

Good soil comparisons therefore require an understanding of the geological nature of soil heterogeneity and constructing valid tests with this heterogeneity in mind. Our company employs scientists with a variety of expertise, such as biological and geological sciences, to ensure quality analysis of all evidence.