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Soil Analysis
Chapter 13
What Exactly is soil?
• Definition varies depending on who you ask
• Farmers consider it to be the top 6-12 inches of the
earth’s crust where plants grow.
• Geologists-scientists who study the earth’s surfacedefine soil as the organic (carbon containing) and
mineral matter composing the earth.
• Engineers look at soil as material that can be removed
from an excavation site.
Soil and Crime
• When a forensic geologist is investigating a crime, all
natural and artificial objects that are on or near the
surface of the earth are considered soil.
• This includes rock, minerals, vegetation, glass, paint,
asphalt, etc.
• The presence of these objects in that area helps make
the area of soil unique from other areas.
Soils Vary
• Many scientists agree that no two places on Earth
have precisely the same soil.
• Soil from one area will be identifiably different from
the soil collected in another location.
• The properties of soil also vary depending on the
depth from which the sample is taken.
• In most forensic cases, only about one cup of the top
layer of the soil needs to be collected.
• The sample soil should be allowed to air dry to
prevent further decomposition of the material in the
soil
• Once dry, it is transferred to the crime lab to compare
with the soil sample that was found on the suspect or
on his or her belongings.
In the Lab
• Once in the crime lab, scientists use color as their main
identification technique
• Before observing the color, all samples are further dried at 100
C for one hour, because wet soil will appear as a different
color from dry soil.
• The presence of certain minerals in the soil can give it a
characteristic color
• For example, the presence of copper minerals appear green
while black minerals point to the presence of manganese and
iron.
• Odor and texture of soils are also examined during the initial
observation
TOP:
A soil collector (bottom
left) takes samples
from the scene of a hitand-run accident in
Missoula, MT., while a
detective and police
officer look on.
BOTTOM:
Mud that adheres
to car parts often
provides clues for
solving crimes.
From left to right: (1) A typical example of soil from a crime
scene found on the top edge of a shoe sole. (2) Soil is often
caught in the crevices on the bottom of a shoe or boot sole. (3)
This clod of soil has a footwear impression made by a shoe like
that in Fig. 1. Soil color and composition vary over relatively
short distances, both laterally and vertically. Therefore, it is
important to obtain samples from soils that most likely
originated from where the shoe picked it up.
• Samples are required from all
possible locations from where
the soil found on the
suspect's shoes may have
originated. Samples are also
needed from areas where the
suspect claimed the soil
originated (alibi
samples). Therefore, several
samples are carefully
compared with the questioned
soil on the shoes. Some
samples will be eliminated
based on color or
texture. Others will need to
be compared in more detail,
such as through microscope
studies of soil and grain
particles.