Soil Analysis Chapter 13

What Exactly is soil? Definition varies depending on who you ask Farmers consider it to be the top 6-12 inches of the earth’s crust where plants grow. Geologists-scientists who study the earth’s surface- define soil as the organic (carbon containing) and mineral matter composing the earth. Engineers look at soil as material that can be removed from an excavation site.

Soil and Crime When a forensic geologist is investigating a crime, all natural and artificial objects that are on or near the surface of the earth are considered soil. This includes rock, minerals, vegetation, glass, paint, asphalt, etc. The presence of these objects in that area helps make the area of soil unique from other areas.

Soils Vary Many scientists agree that no two places on Earth have precisely the same soil. Soil from one area will be identifiably different from the soil collected in another location. The properties of soil also vary depending on the depth from which the sample is taken.

In most forensic cases, only about one cup of the top layer of the soil needs to be collected. The sample soil should be allowed to air dry to prevent further decomposition of the material in the soil Once dry, it is transferred to the crime lab to compare with the soil sample that was found on the suspect or on his or her belongings.

In the Lab Once in the crime lab, scientists use color as their main identification technique Before observing the color, all samples are further dried at 100 C for one hour, because wet soil will appear as a different color from dry soil. The presence of certain minerals in the soil can give it a characteristic color For example, the presence of copper minerals appear green while black minerals point to the presence of manganese and iron. Odor and texture of soils are also examined during the initial observation

TOP: A soil collector (bottom left) takes samples from the scene of a hit- and-run accident in Missoula, MT., while a detective and police officer look on. BOTTOM: Mud that adheres to car parts often provides clues for solving crimes.

From left to right: (1) A typical example of soil from a crime scene found on the top edge of a shoe sole. (2) Soil is often caught in the crevices on the bottom of a shoe or boot sole. (3) This clod of soil has a footwear impression made by a shoe like that in Fig. 1. Soil color and composition vary over relatively short distances, both laterally and vertically. Therefore, it is important to obtain samples from soils that most likely originated from where the shoe picked it up.

Samples are required from all possible locations from where the soil found on the suspect's shoes may have originated. Samples are also needed from areas where the suspect claimed the soil originated (alibi samples). Therefore, several samples are carefully compared with the questioned soil on the shoes. Some samples will be eliminated based on color or texture. Others will need to be compared in more detail, such as through microscope studies of soil and grain particles.