Chapter 12 Soil Examination By the end of this chapter you will be able to: distinguish the different kinds of soil compositions discuss soil horizons explain the chemistry of soils distinguish the different kinds of sand describe the collection and examination of soil evidence All Rights Reserved South-Western / Cengage Learning © 2009 Forensic Science: Fundamentals & Investigations, Chapter 12

Introduction Factors such as temperature, rainfall, and the chemicals and minerals in the soil influence the production of soil. Soil from different locations can have different physical and chemical characteristics. Because of this, soil analysis has been helpful in such things as linking suspects to crime scenes and locating burial sites. Forensic Science: Fundamentals & Investigations, Chapter 12

History of Forensic Soil Examination Dr. Hans Gross is believed to be one of the first to recognize the importance of physical evidence. His Criminal Investigation, written in 1893, had ground breaking material in this science. Georg Popp is credited with being the first to use soil evidence to solve a crime. He linked soil samples found on a suspect with samples found at the crime scene. Forensic Science: Fundamentals & Investigations, Chapter 12

Soil Composition Soil is part of the top layer of Earth’s crust. It contains minerals, decaying organisms, water, and air in varying amounts. Soil texture describes the size of the mineral particles that make up soil. The 3 main grain sizes are sand, silt, and clay. The 3 subcategories of soil are loam, peat, and chalk. Forensic Science: Fundamentals & Investigations, Chapter 12

Soil Profiles Soils are formed in layers (horizons): Humus, the O horizon, is made of decaying organic matter. Topsoil, the A horizon, is a mixture of humus and minerals. Sand and silt makes up the E horizon. Subsoil, the B horizon, is made of clay and minerals. Broken rock, the C horizon, has very little humus present. Solid rock makes up the R horizon. Forensic Science: Fundamentals & Investigations, Chapter 12

Chemistry of the Soil The pH scale shows how acidic or basic something is. Forensic Science: Fundamentals & Investigations, Chapter 12

Chemistry of the Soil An important chemical property of soil is whether it is acidic or basic (alkaline). Materials that make up a soil are not the only factors that affect its pH level. Rainfall can change the pH value of a soil. Pollution and fertilizer also can change the pH value of soil. The pH value of a soil sample can help a forensic scientist match it to other samples. Forensic Science: Fundamentals & Investigations, Chapter 12

Sand The action of wind and water on rocks forms sand. This may take millions of years. Because water acts as a buffer, water produces sand more slowly than wind. Wind-blown sand becomes rounded more quickly because the grains strike each other directly without a buffer. Forensic Science: Fundamentals & Investigations, Chapter 12

Mineral Composition of Sand Sand from different locations contains different combinations of minerals. The most common mineral found in sand is quartz. Other minerals may be present in smaller quantities. Grains of sand can be rounded or angular depending on the amount of weathering and the mineral composition of the grains. Forensic Science: Fundamentals & Investigations, Chapter 12

Mineral Composition of Sand Other than quartz, mineral content of sand can include: feldspars micas iron compounds Sand also can be made of organic material such as coral and seashells. The following 2 slides show the 4 basic sources of sand. Forensic Science: Fundamentals & Investigations, Chapter 12

Mineral Composition of Sand —Continental and Volcanic Sand Note that the identifying feature of continental sand is quartz; whereas there is no quartz in volcanic sand. Forensic Science: Fundamentals & Investigations, Chapter 12

Mineral Composition of Sand —Skeletal and Precipitate Sand Skeletal sand gives off bubbles when mixed with an acid. Oolite formation is not a result of weathering but an example of depositions. Forensic Science: Fundamentals & Investigations, Chapter 12

Soil Collection In order to present credible evidence in court, a chain of custody log is essential. A person bags the evidence, marks it for identification, seals it, and signs it across the sealed edge (above, left). It is signed over to a technician in a lab for analysis who opens it, but not on the sealed edge. After analysis, the technician puts it back into the evidence bag, seals it in another bag, and signs the evidence log (above, right). Forensic Science: Fundamentals & Investigations, Chapter 12

Soil Examination The presence of soil unique to a certain area can show that a suspect or victim must have been in that area. Layers of soil or sand taken from shoes or the wheels of vehicles can show a suspect was present at a series of locations. Explain how each of the following is useful in the examination of soil samples: looking at samples macroscopically X-ray diffraction Forensic Science: Fundamentals & Investigations, Chapter 12

. . . . . . . . . . . . . . . . . Summary There are 3 grain sizes and 3 subcategories of soil. Soil forms in horizons. The pH scale measures how acidic or alkaline different soils are. Sand is formed by the action of wind and water. If investigators follow good collection procedures, they can analyze samples to see if a suspect was at a crime scene. Forensic Science: Fundamentals & Investigations, Chapter 12