FSP13
soil analysis
Teacher Background Information
Soils
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.
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.
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 (i.e. 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.
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.
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.
Why would soils be basic?
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.
How to test soils pH?
There are two easy ways you can test soil pH.
1. 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
2. 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.
FSP13
soil analysis
Teacher Background Information
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.
Azalea’s (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.
A. Dook Photography
References
http://www2.hawaii.edu/~nvhue/acid.html
http://www.ext.vt.edu/pubs/trees/430-027/430-027.html