(FPGS) Properties of Soil Evidence Lesson

Properties of Soil Evidence

Forensic Characteristics of Soil

The legal application of Earth and soil science is known as Forensic Geology. Forensic Geologists are often called in to review soil evidence because of the complexity of soil content. Soil is the top layer of the earth's surface, consisting of rock and mineral particles mixed with organic matter. Minerals are a very important part of soil analysis because of the variety of minerals found in specific geographic regions. Soil samples that contain more rare minerals are often very useful in forensic investigations. A mineral is essentially any of a class of naturally occurring solid inorganic substances with a characteristic crystalline form and a homogeneous chemical composition.

When soil evidence is collected in a Forensic investigation, it includes other items normally found on the ground, such as asphalt, brick fragments, and cinders in addition to the dirt, rocks and minerals. Soil often varies in composition when digging down vertically into the various layers of soil as well as surface areas in lateral areas surrounding from the original site. The value of soil evidence is due to its transferability between crime scenes and suspect and victim and the uniqueness of soil samples in small areas. While soil often contains some of the same things in many samples, the smaller elements such as rare rocks or minerals, fossils, trace elements, or debris such as brick pieces can pinpoint an area much more specifically. Some examples of this include dried mud on a suspect's clothing that may link them to a particular crime scene, or a soil sample found with a body that has been moved. The more unique soil elements, such as rare minerals, in the soil from the body may help investigators trace the location the body was moved from. While it is considered class evidence, soil can be very useful in an investigation to generate leads or guide the focus of the investigation.

Types of soil typically encountered in a Forensic Investigation:

  1. Sand - A loose granular substance, typically pale yellowish brown, resulting from the erosion of siliceous and other rocks.
  2. Clay- A stiff, sticky fine-grained earth, typically yellow, red, or bluish-gray in color and often forming an impermeable layer in the soil.
  3. Silt- Fine sand, clay, or other material carried by running water and deposited as a sediment, especially in a channel or harbor.

Soil Horizons image with layers depictedSoil is formed by the act of weathering against rocks in the environment. Wind and water both contribute to the breakdown of the rock into increasingly smaller grains that eventually become "dirt" or soil. Water will deepen existing microscopic cracks in rock. As you learned in physical science, water expands when it freezes. When water freezes in the microscopic cracks in rock, it expands and breaks off tiny pieces of the rock. These tiny pieces eventually become the soil! Wind blowing against a rocky cliff face or water flowing against rocks, such as in a stream, also eventually wear away tiny pieces of rock which become soil. The soil formation process takes thousands of years. Soils often look very different from the rock they originally came from because of the content of organic matter, such as decayed plant or animal life, which becomes a source of nutrients and fertilizer for new plant growth and animal habitats. Soil is deposited over time in horizontal layers, known as horizons. The horizons vary over time and location for many reasons such as:

  1. Type of plant or animal life living there
  2. Inorganic materials present
  3. Climate variables
  4. Parent materials (type of rocks found in the area)
  5. Relief of the landscape (the slope and type of land form)
  6. Time

Examining Soil

Soil can be examined for both physical and chemical properties. The first step in examining physical properties of a soil sample is to observe it under low power with a microscope. This process will reveal any plant material, animal material, artificial materials, and specific rocks or minerals that may be present but not visible or clear to the naked eye.

Some specific physical characteristics that are observed in soil analysis include:

  1. Color - It is estimated that there are more than 1,100 different colors of soil!
  2. Type of soil - Sand, clay, and silt all have different properties!
  3. Presence of unique materials - Materials, such as magnetic particles, can give important clues!
  4. Fluorescence - Observing the soil sample for fluorescence using UV light can give clues to the type of soil or originating location.

Specific chemical characteristics observed in soil analysis include:

  1. pH - pH varies by soil type and composition
  2. Composition and presence of various compounds - Presence of Nitrates, Phosphates, Potassium, Carbonates, Iron, Chloride, Copper are generally examined.

While most soil can be differentiated by observation, it must be visualized when dry and when moist because it usually appears darker when wet. Minerals can also be observed in soil samples. Minerals are naturally occurring crystalline solids. There are more 2200 minerals known to exist, but most are rare. Only about 20 of those minerals are common and frequently encountered by forensic geologists. Thus, finding a mineral outside of those common twenty is quite rare and could provide information of great evidentiary value. The probability of finding two soils that are indistinguishable in both color and mineral properties in two locations separated by more than 1000 feet is about 1 in 50. In Georgia, some of the more prominent soil minerals are Kaolinite, Halloysite, Iron Oxides, Feldspar, and Kyanite. The Iron Oxides are responsible for giving Georgia the characteristic "red" colored soil and clay. Below are examples of kaolinite, halloysite, and iron oxide.

Testing Soil Samples

Besides microscopy, scientists can compare soil specimens using several other techniques to analyze the physical and chemical characteristics of the sample. 

  • The density of soil can be tested using a density-gradient column which is filled from the bottom to top with liquids of successfully lighter densities so when soil is added it "sorts" based on the different densities of its components.
  • The size of the particles of soil can be assessed using different sizes of sieves to "sift" the sample.
  • Soil samples are tested for the presence of magnetic materials using a magnet.
  • Depending on the location and soil type, chemical analysis can be done to compare soil samples.

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