Soils

Soils

Soil on a suspect's shoe or splattered inside a car fender can provide forensic scientists with information about the travels of suspects and crime victims.

Soil is the product of biological, chemical, and physical alteration of materials at Earth's surface. Soils form in horizons, or layers, that are approximately parallel to the surface, have distinct properties, and are denoted by uppercase letters. The uppermost O horizon consists of decaying organic matter. It is underlain by the A horizon, or topsoil, which consists of a mixture of mineral and organic material. Beneath the A horizon is the B horizon, which consists of slightly altered mineral material, and the C horizon, which consists of the unaltered but loose parent material from which the soil developed (for example, sand). If intact rock is present, it can comprise an R horizon. Desert soils rich in calcium carbonate can also contain Bk or K horizons (the K is used to avoid confusion with the C horizon) that range from light accumulations of calcium carbonate to so-called petrocalcic horizons that are limestone formed in place. The term soil is also used loosely to refer to virtually any unlithified material at Earth's surface regardless of whether it has undergone the soil forming process known as pedogensis. Examples of materials that do not fall under the strict definition of soil include sand in dunes or along beaches and mud deposited by a recent flood. Because soils form by a complicated process that is influenced by factors such as temperature, precipitation, the mineralogical and chemical composition of the parent material, and even the nature of particles that may be washed out of the air during rainstorms, soil from different locations can have different physical and chemical characteristics that are useful to forensic scientists.

Soil recovered from shoes, clothes, and automobiles can be analyzed in order to determine if a suspect was or was not in a particular location. This is done by carefully comparing the color, particle size and shape, mineralogical composition, and biological components of a soil sample obtained from a suspect to those of soil from a known location. Particle sizes and shapes can be compared using reflected light microscopes . The chemical and mineralogical composition of the soil can be compared using techniques such as x-ray diffraction, in which a pulverized soil sample is subjected to x rays that produce patterns indicative of the crystal structure of minerals in the soil. Soils that are, or once were, adjacent to water may also contain distinctive shell fragments. The presence of soil unique to a particular area can show that a suspect must have traveled to that area, just as the absence of soil can be used to disprove an alibi. In some situations, layers of soil or mud can be used to establish presence at a sequence of locations.

The fictional British detective Sherlock Holmes is generally credited with the first use of soils as forensic evidence in the late nineteenth century, and soils have been employed as real life forensic evidence since the early years of the twentieth century. Holmes possessed the ability to distinguish different soil types and, using that information, make inferences about the travels of suspects. Real-life German chemist Georg Popp used goose droppings, sandstone fragments, and three different kinds of dust on a suspect's shoes to link to the same materials found at a murder victim's home, the place where the body was found, and the place where the murder weapon was found. Just as importantly, Popp used the absence of distinctive quartz crystals to disprove the suspected murderer's alibi he was walking in a specific field near his home when the crime occurred.

In more recent times, soil analysis was used in an attempt to track down the killers of Italian prime minister Aldo Moro in 1978. Investigators matched sand found on Moro's body to that found on an 6.8 mile (11-km) long beach north of Rome, which helped to focus their investigation. Another high profile case involved United States drug enforcement agent Enrique Camerena Salazar and his pilot Alfredo Zavala Avelar, who were killed by Mexican federal police in 1985. Their bodies were reported to have been found at the scene of a shootout between police and known drug dealers, implicating the drug dealers as murderers. Close examination of soil samples taken from the bodies, which contained an unusual combination of mineral and volcanic glass particles, revealed that the bodies had originally been buried in a remote mountainous area far from the shootout. This, combined with other forensic evidence, eventually showed that the federal police had been involved in the kidnapping, torture, and murder of the two.

Soil analysis is not restricted to cases involving politics and international intrigue. Soil found with a body inside a plastic garbage bag in New Jersey was identified as material that had been dredged from Newark Bay and used as fill to create new land along the shore. This clue led investigators to the victim's wife and daughter, who had killed him and temporarily buried the body beneath their home, which was built on the fill. California authorities were confounded when soil found in a murder suspect's car partially, but not completely, matched the soil around an oil well where the victim's body had been dumped. Further research showed that gravel from a different location had been spread around the well, explaining why the soil from the car was not an exact match with the natural soil in the area.

Small fragments of chert, a sedimentary rock made of silica, in cow manure collected from the back of a truck were used to prove that a herd of cattle had been rustled in Missouri and taken to Montana. Although the cattle rustlers had altered the brands on the cattle in an attempt to cover their tracks, they did not realize that the manure contained evidence that could have come only from Missouri. Another example of agricultural soil forensics is the comparison of soil samples to determine whether valuable plants were removed from protected government land and sold for landscaping.

see also Geology; GIS; Minerals.