physical evidence

Soil is a common type of physical evidence found at the scene of crimes like hit-and-run accidents, automobile collisions and burglaries. Soil from the crime scene is also picked up by an automobile (tires), thus, providing a valuable link between the vehicle and therefore the crime. Similarly, soil or mud found adhering to clothing or shoes may provide the clue which will link a suspect to a specific crime site.

In general, soil is a naturally occurring material which usually contains clay, sand, rocks, coal,  plant material and also  its mineral constituents. Soil is made originally from rocks by the process involves erosion, oxidation, hydration. Soil is highly heterogeneous in nature and varies widely from place to place because of the results of wind, water, living organisms, mining and agriculture. Thus, soil from different places will have different characteristics. Therefore,forensic examination help to differentiate between the two different samples of the soil. Variety of physical and chemical methods have been used for comparison of two soils which are described below:

Methods of examination

1.Color: The visual examination of the soil based on the color, the most common preliminary method of comparison of two soil samples which is used by many scientists. If two soil samples differ in color then there chemical properties are  also diffferent. Basically the minerals directly provides the coloration to the soil. The following table 5 shows the substances that are responsible to impart the characteristic color to the soil particles.

Observe the colour of the soil us such and after drying in even at 105°C. Observe its nature particularly size and shape. Short out the foreign element (if it is there) as paper pieces, leafs, grass, seed, brick fragments, glasses, animal and wooden matters etc.

 2. Particle Size Distribution

Particle size distribution could be a very basic or elementary technique which forms a necessary base during the examination of a soil. There are several method used for the determination of particle size like sieving method, hydrometer method, counter coulter method and microscopic method.

Sample Preparation: sieve shaker.

Arrange the set of sieves in numerical order with smallest number (largest mesh size) at the highest and therefore the largest number (smallest mesh size) at the underside e.g. 5-10-18-25-35-60-80-90-100 120-200 and then on. Place the duvet on the shaker and pour the soil sample into the highest sieve. Place the cover on the upper sieve, fasten the binding straps Switch on the shaker and permit the shake to operate for few minutes. Then, switch the shaker and release the binding straps. Note-If power shaker isn’t available, shaking of the sieves can be done manually.

Remove the sieve cover and separate the sieves. Collect the soil retained in each sieve. The portion of sample to be utilised in the analysis is one taken from a sieve of middle range size.

 Observation of Particle Size Distributions

Apparatus: Set of sieves (ranging in size from ASTM No.5 to 200), Motor-driven shaker, chemical balance with an accuracy of 0.0002 gm, Standard Weights,


a. Take an approximately (50 g)  weighed  soil sample.

b. Arrange the set of sieve in numerical order and shake the soil in the same manner as mentioned in section 2.1.

c. Collect the soil detain in each sieve separately and reweigh accurately. Calculate its percentage as given below:

Percentage of soil retained on sieve No.(..)=(Weight of soil retained on sieve)x100 /(Total weight of soil taken)

3. Microscopic Examination – Microscopic examination method is used for the comparison of one soil (unknown) sample with another (known). After sieving, the different fractions of soil are then analysed under the microscope to compare the size and shape of the particles. Low power microscopes could reveal small soil components such as fibers, plant roots, and other debris etc. Whereas the high power microscope helps in identifying the small rocks and traces of minerals which can be used as a means of comparison and origin of the sample


 Apparatus: Stereo-microscope of high magnification range.

Method: Simple observations

Take some soil sample on a clean microscopic slider glass plate and make its thin layer. Place the slide with soil on viewing stage of stereo-microscope and take the microscopic observations of soil sample retained in each sieve  during particle size distribution.

a. Observe the colour of soil particles after drying at 105°C.

 b. Observe the nature of particles as- geometrical shape, extra particles, (brick dust, red ash, iron oxide or metal oxide, quarts grains, colourless mineral fragments), green minerals, etc.

 c. Find out the traces of foreign materials as -ding cloth fibres, hair, wooden particles etc.

d. Observe the results and compare with control soil sample.

4.  Microscopical Observation with Chemical Regents

Observe the soil sample on the microscope in the same manner. Moisten a small portion of soil with water and then add a small drop of concentrated hydrochloric acid (HCI) on it.

Observe the nature of reaction like bubbles and colour. Bubbles arising from solid particles indicate insoluble carbonates like chalk, or lime stone. Similarly yellow colour indicates the presence of iron.

5.Ignition Test

The loss on ignition (LOD) method was initially described by Davies in 1974. This method is used to determine the remaining portion of organic matter and the combined water in colloidal matter in the soil sample when subjecting it to the varied  temperatures. The cases in which the amount of soil evidence is not limited, can be used for this ignition loss method to determine the percentage loss of clay and humic material. The percentage of organic content is calculated by the following formula.

Apparatus: Analytical balance with an accuracy ± 0.0002 gm, Standard Weights, Alumina crucible (porcelain dish may be used), Muffle furnace of extreme temperature range (1000°C).


Take an exactly one gm of soil sample from sieve fractions dried at 105 “C in a alumina crucible and keep it during  a muffle furnace. Heat the sample at temperature between 750-800 “C for 1 hr. And then, cool it to room temperature. Reweigh accurately and note the loss in weight and difference in colour on ignition

Calculate the percentage of loss on ignition and compare it with a control soil sample.


Initial weight of soil sample – Wo g

Weight of soil sample after ignition- W₁) g

Weight loss on ignition- (Wo- W₁) g

Percent weight loss- (Wo-W₁)  /(Wo)    *100

6. Density Distributions of Soil Particles

 The density of the soil particle is that the weight of the particles per unit volume and it is usually expressed in grams per cubic centimetre. Density gradient method was first introduced into forensic science by Goin and Kark and now this technique is frequently used in many forensic laboratories to compare the two soil samples. The principle of this technique is to separate the particles of soil samples in a density gradient column according to their densities.

These methods are based on the observation that a solid particle will float in a liquid medium  of greater density, sink in a liquid of low density or remain suspended in a liquid of equal density. 

D1 = [(V1d1) + [(V2d2)] // (V1) + (V2)


d1  = density of liquid 1 (more dense; bromoform)

d2 = density of liquid 2 (less dense; xylene)

V1 = volume of liquid 1 (bromoform) in ml

V₂ = volume of density 2 (xylene) in ml

When the liquids of different densities are successively filled by equal volume in a tube, a density gradient within the tube are going to be formed. Based on this principle, density distributions of soil particles are observed:

Apparatus: Glass tubes of size 30cm x 5-10mm (closed at one end and fitted with corks), Bromoform ,Xylene ( or bromobenzene ,Graduated cylinder -10ml, Analytical balance capable of measuring ±0,01 g. Sample bottles.


Prepare the solution of mixture of bromoform and xylene in seven sample bottles (marked 1 to 7) separately within the following proportions (6 ml of every solution):

Bottle No. 1: Pure bromoform ( 2.89)

 Bottle No.2: 5 parts bromoform and part xylene.

Bottle No.3: 4 parts bromoform and pair of  xylene.

Bottle No.4: similar parts bromoform and xylene.

Bottle No.5: 2 parts bromoform and 2 pairs of xylene.

Bottle No.6:1 parts bromoform and 5 parts xylene.

 Bottle No.7: Pure xylene ( 0.88)

Shake vigorously each bottle and keep them for 4-6 hours so that both the two liquids mix together.

Mark off seven equal increments on the 30 cm long glass tube and stand it vertically in a rack or stand. Carefully pour each of solution in order into tube up to mark, beginning from bottle No. to 7 i.e. beginning with bromoform the most dense liquid.

Repeat the above for filling a second tube placed in the rack or stand. Keep both the tubes filled with liquids for 24 hours.

Pour carefully a small amount (50mg) of suspect soil sample into one tube and an equal amount of soil found at the scene of crime (control sample) into other tube. Be certain to label each tube. Most particles reach their depth within a few minutes, but allow the tubes to stand for 24 hours.

Compare the levels at which the soil particles have become suspended.

Remarks- If the levels at which the soil particles have become suspended are the same for the crime and control sample, both the soil samples have the same density distributions and thus, they may be of same place. If they are found at different levels, the soils are probably not the same.

7. pH Measurement of Soil Sample

-pH of soil is expressed as a figure or number between 0 to 14, which indicates the acidity of basicity of the soil based on the hydronium ions (HO’ or H’) measured in the soil solution. It is defined as the negative logarithm (Base 10) of the hydrogen ion (H) concentration in the soil, where hydrogen ion concentration is taken as moles per litre. To observe the  pH value of soil sample can be determined as follow:

Apparatus: a pH-meter and standard electrolytes buffer solution.

Method: Dissolve weighed quantity (one gm of soil sample in 100 ml water and stir thoroughly. Filter it. Take the filtrate and observe the pH value. Adding 10 ml, 20ml, 30ml, 40ml, and then on successively in solution, measure the pH values after each dilution and observe their variations, Similarly, measure the pH values for control soil sample and compare with suspect soil sample.

8.  Elemental Analysis

According to Dawson and Hillier in their paper “Measurement of soil characteristics for forensic Applications the United States determine the fundamental (chemical) compositions of soil particles by scanning microscopy using energy dispersive X-ray spectroscopy (SEM-EDX). Other methods are X-ray fluorescence (XRF), atomic absorption spectroscopy (AAS), inductively coupled plasma (ICP) spectrometry, neutron activation analysis (NAA) and energy and wavelength dispersive X-ray (EDX and WDX) microanalysis to measure the elemental composition of the soil Inductive coupled plasma has been used to measure the broad range of elements. ICP-OES and ICP-MS are the two instruments that may  provide the  data around 60 different elements

 Conclusions of examination of soil:

The following conclusions (reports) can be formatted according to information required. The above scientific examinations reveal that both the suspect soil sample marked (…) and control soil sample marked (…) are identical/similar/appeared to be similar/dissimilar/different with respect to their physical and chemical properties (as; colour, nature, particle size, ignition loss, density distributions etc.). Therefore, both the soil sample may be (or are appeared to be) of nearly same place’ different place.

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