Soil is a biogeochemical material found on the outermost layer of earth’s crust. It is composed of various layers where the distribution of constituents is not uniform. The heterogeneous nature of the soil is due to the variation in weathering, climatic and anthropogenic factors which vary with respect to distance. That’s why it is concluded that the soil of a specific area is individual to that area in respect of its chemical, physical and biological characteristics (Law of Individuality).
Trace evidence is one of the important piece of evidence present at a crime scene under the radar of a crime scene investigator since they are difficult to erase and easy to solve a case if analyzed properly. They are admissible in court as an evidence and soil is one of them.
Though soil is not connected directly to any human yet they still help in examination of crime scene and linking many unsolved mysteries too going in an investigation.
What is Soil?
The soil found in nature is composed of the components that are divided broadly as clay, silt and sand depending upon the size of the particles of soil.
Clay has the smallest sized particles, silt has moderately sized particles and sand has the largest particles.
The different types of soil found on earth vary according to the soil compositions in different percentages. Also the particle nature of soil is responsible for its transfer from object to object (Law of Exchange).
The transfer may take from the crime scene to items like shoes, garments or tires and subsequently from that item to another surface. The former is called primary transfer and the latter is secondary transfer.
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 contributes to their forensic examination in criminal investigation.
Soil is divided into 6 layers depending upon its composition:
- O-horizon – It is made up of dead and decaying organic matter called humus.
- A-horizon – It is the mixture of humus and minerals and is known as topsoil.
- E-horizon – It is composed of sand and silt.
- B-horizon – This layer is made up of clay and minerals.
- C-horizon – It consists of broken rock pieces and very little humus.
- R-horizon – Last layer of soil made up of solid rock.
Soil can be acidic or alkaline in nature depending upon the factors like- mineral composition, water salinity, fertilizers, pollution, rainfall, etc. The pH of the soil is considered as one of its identifying characteristics.
Also soil shows variation in its color due to presence of various minerals in different percentages which is unique to each location.
In 1856 Professor Christian Gottfried Ehrenberg (1795–1876) a natural scientist at the University of Berlin, acquired samples of sand from stations along railway lines and used a light microscope to compare the sand with the station from which the sand was most likely to have come from. He is considered the founder of both soil microbiology (a discipline of soil science) and microgeology (i.e. micropalaeontology, which is a discipline of geology).
Examination of Soil
The examination of soil requires the collection of two types of soil samples i.e., questioned and controlled.
Questioned sample is collected from the objects present on the crime scene like the clothes of the victim, footwear, tyres, digging tools etc. while the controlled sample is collected from the area under the vicinity of the crime scene for comparison.
The questioned soil samples present to the mobile objects like shoes, clothes, etc. should be collected as it is along with the objects in separate bags. If the soil sample is present on the immobile objects then it is scraped out using a scraper.
The controlled soil sample is collected from the ground using a spatula or spoon. It is seldom necessary to go deeper than ½ or ¾ of an inch of the surface for collection of controlled soil samples.
Wherever a footwear or a tire mark is found, tire control samples should be collected from as close an area to the impression as possible. It advised to collect the samples in separate plastic or glass containers which are properly labelled with the initials and the chain of custody should be maintained.
Methods of Soil Examination
The analysis of soil is based on the comparison of the questioned and controlled soil samples. The methods of soil examination are :
1. Side-by-Side Visual Comparison
It compares the soil according to their color and texture, which is a preliminary examination method.
2. Density Gradient Method
This method uses density of the soil as the comparing quantity. It involves the use of two organic liquids which differ in their densities and are filled in the glass test tubes(6-10mm in diameter and 25-40 cm in length). The liquids form two layers in the test tubes due to their difference in density (density gradient– heavier liquid being at the bottom and the lighter one on top). The soil sample is then added to the test tubes where the component particles of the soil get settled in the liquid having the same density as that of the particles. This gives rise to a density distribution pattern of soil which can be compared with various soil samples. Usually bromoform and bromobenzene are used to form the density gradient.
3. Microscopic methods
This method reveals the particle structures and types of minerals present in the soil samples. Binocular microscopes give information about the minerals present in the soil. Petrographic microscopes reveal the information of rock particles, minerals and biological matters. Further, more fine examination, like structures of the particles can be deciphered by the use of scanning electron microscope(SEM) and transmission electron microscope(TEM).
4. XRD method
X-Ray Diffraction method uses X-Rays to elucidate the unique structure of the mineral particles present in the soil samples.
5. DRIFT (Diffuse Reflectance infra-Red Fourier Transform) Method
Diffuse Reflectance Infra-red Fourier Transform method is a non destructive method which identifies organic matter, clay minerals and quartz in soil by virtue of the peaks of vibrational frequencies pertaining to the functional groups present in these naturally occurring substances.
Soil is a trace evidence which establishes a link between the crime scene, victim and suspect. The soil of one location is different from that of another location not only in its chemical constituents like ions, polymers and carbonaceous deposits, but also in physical parameters like color, texture and density. Such diverse characteristics serve as identification markers for different units of landscape and make soil a valuable evidence to link a suspect to the crime scene.