This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.
Soil is one of the most common forms of physical evidence found at crime scenes. For example, a vehicle suspected of having been used in an armed robbery may later be found to have soil from the crime scene adhering to its tires or wheel wells, thereby establishing that that vehicle was present at the scene. Similarly, a suspected rapist or mugger may have soil adhering to clothing or shoes. (Soil specimens are often particularly easy to obtain from shoes or boots with deep tread, as shown in Figure 5-1.) If soil found on a suspect’s shoes or clothing is consistent with soil present at the crime scene, it establishes that the suspect was probably present at that location.
Figure 5-1. Rubber-soled shoes or boots with deep treads are likely sources for soil specimens
Dennis Hilliard comments
Footwear and tire impressions in soil are an excellent example of examining different types of class evidence to increase certainty. The questioned impressions can be compared against impressions made by known footwear or tires to determine if they are consistent. In addition, the soil found adhering to the footwear or tires can be compared against soil from the scene to determine if they are consistent. If both types of class evidence are consistent, that increases the likelihood that the suspect person or vehicle was in fact present at the scene.
Obviously, impression evidence must be preserved before soil specimens are taken. Depending on the nature of the impressions and the underlying surface, impressions may be preserved by photographing them or by making casts.
Soil evidence by itself is seldom sufficient to secure a conviction, because it usually establishes only that a suspected person or object was present at the crime scene at some time, but not when that visit took place. Still, in combination with other evidence–and particularly if the suspect denies ever having visited the scene — soil evidence may provide an essential building block in the prosecution’s case. Equally, soil evidence can be exculpatory. For example, the police may suspect someone who has mud stains on his clothing that appear visually to be consistent with the soil at the crime scene. If subsequent forensic tests establish that the mud stains on the clothing are inconsistent with the soil at the scene, the police can redirect their efforts elsewhere.
But how can forensic scientists determine whether one soil specimen is consistent with another? After all, dirt is dirt, right? Not at all.
Soil is a complex mixture of mineral, vegetable, and animal material, and may also contain plastic, glass, metal, and other manufactured materials. Soil is anything but uniform. Specimens taken only a few centimeters apart may differ significantly in composition. Two specimens taken at a distance from each other, even if they are of the same general type, are almost certain to have sufficient differences in their composition, physical properties, and chemical properties to make it possible to discriminate between them. These differences mean that soil from any particular location has its own unique fingerprint.
Soil analysis is the process of determining what components make up a soil specimen, and in what proportions. Forensic scientists use microscopic examination and various physical and chemical tests to determine the characteristics of a questioned soil specimen (also known as an associated soil specimen, a suspect soil specimen, or an unknown soil specimen). By performing the same tests on a known soil specimen, a forensic scientist can, if the specimens are consistent in all respects, state with high certainty that the two specimens in fact originated from the same location.
A forensic geologist has primary responsibility for forensic soil analysis. Tests done by the forensic geologist are often sufficient to identify soil specimens as consistent. At times, however, a forensic geologist will call upon other specialists to complete more detailed analyses. For example, a forensic entomologist may be needed to identify insect eggs or larvae present in the soil specimen, a forensic botanist to identify pollen or other plant material present in the specimen, or a forensic chemist to identify trace amounts of organic or inorganic chemicals present in the specimen.
In this section, we’ll obtain and dry soil specimens and then use various tests to analyze and characterize those specimens, learning as much about them as possible within the constraints imposed by the equipment available in our home forensics lab.