WHAT IS SOIL?
The definition of soil depends upon who is working with it. Farmers are concerned with the topsoil in which they plant crops and they define soil in terms of the top few inches on the ground. Engineers, on the other hand, view soil as a component of construction material and are more concerned with its physical properties than its chemical make-up. Soil scientists study changes in soil chemistry and composition and are concerned with minute alterations and variations in soil. Forensic geologists look at soil a different way. Much of the soil that they study comes from areas that have been filled in such as garbage dumps, gardens, or soil patches around homes or businesses, gravesites, etc. They are concerned with the transfer of soil particles from such locations to objects such as cars or clothing, either accidentally or purposefully. The case described above is a good example. A forensic soil scientist might define soil as: earth material, either natural or manmade (concrete, gravel, other building materials), that is transferred from a crime scene to a person or object, or vice versa. This soil may then be recovered from that person or object or may have been shed and found at a different location. The objective of forensic soil analysis is thus to associate soil found at a crime scene to its source. This requires, of course, that a source has been identified and known soil samples have been collected.
Soil contains both organic and inorganic materials. The organic materials are essentially decayed and decaying vegetative and animal matter such as grass and other plants, and insects, animal droppings, animal parts, etc. Sometimes this collective organic fraction of soil is called humus. The inorganic part of soil is generally crushed rock and clay materials. These are made up of minerals. Minerals are generally combinations of metal and nonmetal ions. For example, iron and oxygen combine to form various minerals including ferric oxide (Fe2O3). Minerals are all crystalline solids with regular arrangements of atoms. Many have distinct colors that can be imparted to a soil sample. There are more than 2000 minerals but only about 50 occur commonly in US soils. Rocks are generally made up of several mineral types with definite percent compositions and the soil near the rocks will reflect this diversity. The emphasis on the analysis of soil from a forensic science standpoint is to compare soil from the original location to the person or object to which it was transferred, in order to show that the person or object was, at one time, in the location where the soil originated. In this comparative analysis, the forensic geologist seeks to measure those physical and chemical properties that are most effective in deter mining if two soil samples could have come from the same location, given the time, material, and instrumental constraints that are always a part of forensic science work.
