Soil as evidence

DEFINITION: Earth’s outer crust, which consists of rocks and humus, serves as ground for vegetation, and houses a wide array of materials, including glass and metals.

SIGNIFICANCE: Soil is the structural matrix or home for bacteria, plants, fungi, and nematodes, all of which are living organisms and all of which exist in the soils of different areas in varying identifiable combinations. Because soil is ubiquitous material and is easily transferred from one place to another, soil evidence often plays a role in criminal investigations. By comparing soil samples, forensic scientists can link persons and objects to crime scenes.

Soil has been used as material in investigations since the 1890s. For many years, basic microscopy and morphological analyses were the primary means of soil comparison, but increasingly sophisticated techniques have greatly enhanced forensic scientists’ ability to compare the contents of soil samples. Depending on the type of case and the other types of evidence available, physical examination of soil alone might provide the complementary information needed. Soils can be classified into different types based on their physical characteristics. Geologists, for example, classify soils according to particle size distribution, pH, color, and moisture content as well as other physical features. The analysis of soils for forensic purposes, however, often requires more detail than simple physical examination can provide. Forensic scientists look at soil not as an isolated material but as a group of materials, including any particles and any organisms that are part of a given sample.

Chemical Analyses

The quantities of soils found at crime scenes are not necessarily abundant, and small samples often limit the techniques forensic scientists can use to perform some physical analyses. Small sample size is not an impediment to analysis of soil’s content, however. Scientists can chemically analyze soils for trace elements and metals using techniques such as mass spectrometry (MS), which establishes a relationship between the mass and the ratio of the elements in a sample. MS technology is often coupled with other, more sensitive technologies to elucidate the elemental composition of a wide array of samples, ranging from the simplest to the most complicated matrices, including, but not limited to, drugs, chemical warfare agents, and environmental samples. Some of the technologies used in combination with MS are inductively coupled plasma (ICP-MS), gas (GC-MS), liquid chromatography (LC-MS), glow discharge (GD-MS), and capillary (CE-MS).

Other analysis methods that do not involve can provide similar results, such as inductively coupled plasma-optical emission spectrometry (ICP-OES) and atomic absorption spectroscopy (AAS). These various techniques provide different separation matrices and principles, and analysts must decide which should be used based on the type of sample being analyzed, the limit of detection, and the output resolution requirement.

Environmental samples have to be digested before being introduced into the instrument of choice. Once they are introduced either as a liquid (slurry) or a microspray, the ions are separated on the provided separation matrix based on their mass-to-charge ratio. The number of ions produced for a specific mass is assumed to be proportional to the amount present in the sample; these data are constantly transferred to a computer and analyzed by software that produces a mass spectrum. The masses are then compared to those in reference libraries or in the literature to determine the different elements present in the sample. These methods provide relatively fast, highly specific, and sensitive multielemental analytical information. Technological advances in the first two decades of the twenty-first century have also made analyzing soil samples taken from footwear much easier and led to an increase in its use in forensic science.

Molecular Analyses

When the amount of soil recovered at a crime scene is sufficient for both physical and chemical analyses, a more specific soil profile can be obtained, and this can help establish soil uniqueness. In some instances, however, the recovered amount of soil is too minute to allow either physical or chemical analysis. In such cases, information on soil content may be obtained through (deoxyribonucleic acid) analysis of microbial, fungal, and plant genomes present in the soil. Recent studies have shown that such analysis can provide unique information about the organism or material in question. Novel molecular techniques coupled with separation technologies used for human DNA analysis have been able to provide unique soil “fingerprints” that can be compared with known samples.

Specific markers exist in the DNA of every organism. Plants have sequences repeated in tandem, as is the case with humans. Microbes and fungi contain conserved and variable regions throughout their genomes; the differences encountered in the variable regions are what give each organism its unique identity. Ribosomal ribonucleic acid (rRNA) has been the marker of choice in the analysis of microbial communities because, unlike protein markers, rRNA is ubiquitous.

Terminal restriction fragment length polymorphism (TRFLP) and amplicon length heterogeneity (ALH) have both proven successful in determining the microbial community composition of soils. The first uses labeled primers that will bind to the ends of specific primer sequences to be amplified using Polymerase chain reaction (PCR). The PCR product is then cut with restriction enzymes; these molecular scissors recognize specific sequences and cut the DNA wherever a specific site is recognized. Because the sequences of the organisms are different, different patterns are obtained. ALH uses fluorescently labeled universal primers to amplify the variable regions within the genome. Both techniques use high-resolution genetic analyzers to separate the obtained fragments. The results are recorded by a camera and transferred to a computer, which makes the pattern available to the analyst for subsequent comparisons.

Bibliography

Conklin, Alfred R. Introduction to Soil Chemistry: Analysis and Instrumentation. Hoboken, N.J.: John Wiley & Sons, 2005.

Heath, Lorraine E., and Venetia A. Saunders. “Assessing the Potential of Bacterial DNA Profiling for Forensic Soil Comparisons.” Journal of Forensic Sciences 51, no. 5 (2006): 1062-1068.

Moreno, Lilliana I., et al. “Microbial Metagenome Profiling Using Amplicon Length Heterogeneity-Polymerase Chain Reaction Proves More Effective than Elemental Analysis in Discriminating Soil Specimens.” Journal of Forensic Sciences 51, no. 6 (2006): 1315-1322.

Ogilvie, Rhilynn H., and Igor K. Lednev. "Soil From Footwear Is a Newly Rediscovered Type of Forensic Evidence Due to the Application of Modern Analytical Techniques: A Review." TrAC Trends in Analytical Chemistry, vol. 163, June 2023, doi.org/10.1016/j.trac.2023.117081. Accessed 19 Aug. 2024.

Petraco, Nicholas, and Thomas Kubic. “A Density Gradient Technique for Use in Forensic Soil Analysis.” Journal of Forensic Sciences 45, no. 4 (2000): 872-873.

Pye, Kenneth. Geological and Soil Evidence: Forensic Applications. Boca Raton, Fla.: CRC Press, 2007.

Ruffell, Alastair, and Jennifer McKinley. “Forensic Geoscience: Applications of Geology, Geomorphology, and Geophysics to Criminal Investigations.” Earth-Science Reviews 69 (March, 2005): 235-247.