Forensic Significance of Soil
Soil is classified as trace evidence, yet it is considered important evidence in crime scene investigation. The scientific discipline concerned with the study of soil is known as Pedology which is based on understanding the variety, distribution, and various properties of the soil.
Soil is a heterogeneous mixture of rock particles, minerals, plants, and animal debris. It is formed from rocks and minerals via weathering which later get pulverized by plants, animals, and man-made products.
The presence of various plants, animal debris, and human-made products such as glass, fertilizers, plastics, etc. makes the soil unique and specific to a particular place or location. The involvement of human activity such as mining, agriculture, industrial operations, etc. is also responsible for the variation in soil.
There are different types of soils present on earth and each type has specific characteristics such as color, texture, and structure.
The color of soil is the result of various minerals present in it. The presence of clay, silt, and sand in varied amounts is responsible for different structures and textures of soil. And all these properties can give information regarding the composition and origin of soil which plays an important role in determining the scene of occurrence as a primary crime scene or secondary crime scene.
Some scientists compare soil with human fingerprints in its uniqueness, although it is not fully correct. However, natural soils can be differentiated from man-made ones based on their compositions.
The value of soil as evidence relies on its prevalence at crime scenes and its transferability between the scene and the criminal. The physical, chemical and biological characteristics of the soil contribute to its forensic examination in a criminal investigation.
What Makes Soil an Important Forensic Evidence?
The properties which make soil significant evidence are individuality and transferability. The components of soil have varied physical, chemical, biological, and mineralogical properties which are individual to every location (Law of Individuality).
The individual characteristics of soil are also affected by geographical area, anthropogenic factors, and climatic conditions. The soil consists of very minute particles which are easily transferable from one surface to another surface( Locard’s Principle of Exchange). Therefore the soil is found on footwear, clothes, or the body of the victim or culprit.
As earlier discussed, the soil is considered to trace evidence, therefore there is a high probability that the culprit may miss noticing it on his/her clothes, footwear, or belongings, which can help the investigators to link the suspect with the scene of occurrence and victim.
How Soil Can be Analysed?
Soil is very easy to categorize by its color, texture, and structure. This can be done by the use of various analytical methods such as spectroscopy and X-ray diffraction methods. Spectroscopic examination analyses the color of the soil whereas the mineral components can be analyzed by the X-ray diffraction method.
Also, the soil is easy to locate, collect and separate. The methods involved in the collection and separation are time as well as cost-effective which makes soil analysis quick.
The analysis of soil is based on the principles of comparison by visual examination and microscopic comparison. The examination can be complex because of the heterogeneous nature of the soil which at the same time is highly useful in differentiating the samples from different locations.
However, the limitation of differentiating between the standard and non-standard methods of soil examination is a big drawback.
The physical properties of soil such as density and refractive index help to individualize the soil. Soil usually possesses the property of birefringence that is it shows different refractive indices when illuminated from different directions.
What Does the Soil Analysis Reveal?
Soil analysis helps to understand the localized area of the crime scene for investigations, the likelihood of a burial or clandestine grave, or the movement of scent by using human remains detection dogs. Soil analysis can determine the type of soil from a sample of unknown origin.
The examination of soil can also help to determine the age of bones which can be preserved for DNA typing, estimate the time of death, and the stage of the skeletonization process.
The soil examination methods such as microscopic methods help in understanding the structure of particles of the soil and certain unique components such as pollen grains, seeds, paint chips, plastic particles, minerals, etc. which determines the unique nature of the soil and its origin.
In certain cases, soil soaked in biological fluids is found, which is useful in forensic analysis of biological fluids that can determine the source of the biological evidence.
Soil is one of the potential trace evidence, found at almost every crime scene. Since it is present all over the landmass, therefore it is prevalent in every crime scene, either in small amounts or bulk quantities. The trace amount of soil at the crime scene can be sometimes missed or ignored by the investigators but it is very important evidence for forensic analysis.
In the absence of fingerprints or DNA or verbal testimonies in certain crime cases, the analysis of soil can be proved as a shred of important and reliable evidence for giving direction to the investigation. Soil is a piece of very good corroborative evidence that makes it more valuable.
Soil is something that is ignored by the culprits in almost every crime, therefore it is the duty of the investigator to not overrule the soil evidence and carefully conduct the forensic analysis on it so that it can provide valuable and reliable information.
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