Soil is the uppermost layer of the earth’s crust, consisting of many organic and inorganic materials, including microorganisms. The soil composition differs from place to place. The soil of the same area at a distance of 100-200 meters differs in its composition. This feature of individuality makes soil a piece of critical evidence. In the upcoming sections, we will describe different chemical and instrumental methods of soil analysis.
Soil as evidence has high evidential value because of its high evidential value. Identifying extreme complexities in minerals, oxides, organic matter, microorganisms, etc., along with its physical features of size and density, can link a person to the crime scene. The techniques of soil analysis, such as wet sieving, dry sieving, screening, polarizing microscope, etc., help us to identify them.
Along with studying the methods of soil analysis, we will also look at the history of soil as evidence to know the advancement in the forensic significance of soil. Let’s start!
History: Soil as Evidence
- The soil was used 100 years back for investigation but fictionally in the series Sherlock Holmes.
- Hans Gross of Austria, in his handbook, mentioned dirt or soil in a shoe can help in the investigation, but there aren’t any such case studies regarding the soil as physical evidence.
- At present, according to many articles and case studies, soil contributes a lot to investigations.
- Forensic Geology monumental book by ‘Murrey [1 & 2]‘ was published along with the changes in the quality of examination.
Different Methods for Soil Analysis
Analysis of soil is complex due to soil complexity. However, this complexity and diversity help to discriminate soil samples with high discrimination power.
Mineral identification using a polarizing microscope requires skills and experience and to work calmly for many hours. To identify the complexity of soil, many methods of soil analysis have been proposed. Still, the international standardization of forensic soil examination is not yet fully worked out because of soil diversity.
Particle Size Discrimination
- The wet sieving method is used for fine particle separation at a fraction of less than 0.05 mm in diameter and a coarser particle fraction using a sieve with a 0.05 mm aperture size.
- The dry sieving method is an easy technique for the analysis of coarser particle fractions. In general, the particle size class is from 4 to 9 for better reproducibility. For high discrimination power, particle size classes are 3 in total, i.e., <0.05 mm, 0.05–0.2 mm, and 0.2 – 2 mm.
- Particle size analysis and multiple color measurement raise the discrimination power resulting in only 0.5 % of the sample pairs being indistinguishable. Refined particle analysis does not significantly improve discrimination power.
- Color and particle size discrimination of soil samples can be done easily and give valuable data for discriminating among similar samples.
- Air-dried samples color and particle size discrimination of soil samples can be done easily and give useful data for discriminating among similar samples.
- Color comparison alone is insufficient. This method is composed of color observations on:
- Air-dried and moisture soil color.
- Soil after organic matter decomposition by H2O2.
- Soil after removal of iron oxide using sodium.
- Soil ignited t 850o C in an electric furnace.
Namely, the surface of dried deposit by centrifugation after these methods mentioned above of soil analysis because it is more homogenous than bulk samples.