teacher background information
FSP13
Soil analysis
Soils
Why would soils be basic?
Soils are made up of many things. It is a
complex mixture of minerals, plant and animal
matter and tiny particles of man-made
substances such as glass, paint, concrete and
other materials. The contents of soil can vary
greatly between one site and another.
For the same reason that some soils have acids
added to them, soils are made basic by having
lime and other things added to them. This is
because some plants prefer an alkaline
environment.
Soils may be naturally basic too. Dry climates
generally have more basic soils. This is
because places that receive little rain are high
in carbonate (alkaline) because it has not been
leached (washed) out of the soil by water.
Soil is also made up of a variety of different
sized particles. If a soil is made up of mainly
small particles then it will be a fine soil. But if
a soil is composed of larger particles it would
be a coarser soil. Even clues like these can
help in an investigation.
How to test soils pH?
Forensic soil analysis usually begins with a
visual and microscopic examination. From
there various tests are done to ascertain the
colour, consistency, pH and mineral content.
Any foreign substances that may be present (ie
hair, plant matter) are also looked for as these
may provide important clues for the
investigation and may help explain the origin
of the soil.
There are two easy ways you can test soil pH.

Soil pH testing kits are available from
gardening and hardware shops. A small
sample of soil is collected, and then after
a chemical reaction with a ‘universal’ pH
indicator (one that has a different colour
for every point on the pH range) the
sample can be matched to a colour chart
The pH of soils may also vary. This is
important as nutrients in the soil are only
available to the plant within a certain pH
range. A soil of pH 4.0 is extremely acidic,
while one of pH 8.5 is very alkaline.

You can add water to the soil and if the
chemicals that are making it acidic or
basic are soluble in water, then you can
test the water to find out its pH.
Why would soils be acidic?
Some plants (azaleas) prefer an acidic
environment so adding materials such as peat,
manure and compost will make the soil more
acidic and make azaleas grow better.
Soils may also be naturally acidic. Moist
climates generally have more acid soils. This is
because high rainfall causes soils to weather
faster. Basic chemicals such as calcium and
magnesium, which are essential to living
organisms, are leached from the soil leaving
behind more stable materials rich in iron and
aluminium and more acid.
Action by humans such as mining, farming,
construction site development, and similar
events that remove topsoil can make the soil
more acidic.
Forensic investigations: Soil analysis (teacher information information)
FSP13 | revised May 2013 | © The University of Western Australia
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Licensed for NEALS
The plants involved in the classroom
crime scene
There are two plants involved: an azalea that
is in a pot on the teacher’s desk and a pink
hydrangea that is planted outside the broken
classroom window.
The common garden variety hydrangea
(Hydrangea macrophylla) is one of nature’s
very own pH indicators: pink flowers are
produced at pH values of 6.8 or higher, and
blue flowers at pH 6.0 or below.
Most flowers are coloured by their genetic
material however hydrangeas, unlike most
flowers, are coloured by the soil they grow in.
Azaleas (Rhododendron spp) are plants that
need to be grown in soil with a pH of 5.5.
In the crime scene, the soil on the desk, on
the carpet near the window and on the ledge
in front of the window should be a low pH (pH
5) indicating that the soil has come from the
azalea pot and NOT from outside.
If the soil on the carpet and ledge came from
outside it would have a higher pH as the
hydrangea is pink indicating that the soil pH
would be greater than pH 6.8. The perpetrator
of the crime has placed the soil on the carpet
and the window ledge to make it look like they
entered from the outside garden via the
broken window.
photos: A. Dook
References
http://www2.hawaii.edu/~nvhue/acid.html
http://www.ext.vt.edu/pubs/trees/430027/430-027.html
Forensic investigations: Soil analysis (teacher information information)
FSP13 | revised May 2013 | © The University of Western Australia
page 2