Soil 2

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                                                                   soil analysis
                                                          Teacher Background Information

Soils                                                      rainfall causes soils to weather faster. Basic chemicals
                                                           such as calcium and magnesium, which are essential
Soils are made up of many things. It is a complex
                                                           to living organisms, are leached from the soil leaving
mixture of minerals, plant and animal matter and
                                                           behind more stable materials rich in iron and
tiny particles of man-made substances such as glass,
                                                           aluminium and more acid.
paint, concrete and other materials. The contents of
soil can vary greatly between one site and another.        Action by humans such as mining, farming,
                                                           construction site development, and similar events
Soil is also made up of a variety of different sized
                                                           that remove topsoil can make the soil more acidic.
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              Why would soils be basic?
coarser soil. Even clues like these can help in an
                                                           For the same reason that some soils have acids
                                                           added to them, soils are made basic by having lime
Forensic soil analysis usually begins with a visual        and other things added to them. This is because
and microscopic examination. From there various            some plants prefer an alkaline environment.
tests are done to ascertain the colour, consistency,
                                                           Soils may be naturally basic too. Dry climates
pH and mineral content. Any foreign substances
                                                           generally have more basic soils. This is because
that may be present (i.e. hair, plant matter) are also
                                                           places that receive little rain are high in carbonate
looked for as these may provide important clues for
                                                           (alkaline) because it has not been leached (washed)
the investigation and may help explain the origin of
                                                           out of the soil by water.
the soil.

The pH of soils may also vary. This is important
as nutrients in the soil are only available to the
                                                           How to test soils pH?
plant within a certain pH range. A soil of pH 4.0 is       There are two easy ways you can test soil pH.
extremely acidic, while one of pH 8.5 is very alkaline.    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
Why would soils be acidic?                                    with a ‘universal’ pH indicator (one that has a
Some plants (azaleas) prefer an acidic environment            different colour for every point on the pH range)
so adding materials such as peat, manure and                  the sample can be matched to a colour chart
compost will make the soil more acidic and make
azaleas grow better.                                       2. You can add water to the soil and if the
                                                              chemicals that are making it acidic or
Soils may also be naturally acidic. Moist climates            basic are soluble in water, then you can
generally have more acid soils. This is because high          test the water to find out its pH.

                                                                    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

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