In this lesson, students practice at being forensic geologists: scientists who study soil from a crime scene and determine its origin. They examine three different types of soil—sandy soil, clay soil and loam—and observe their properties, noting color, texture, and odor. In addition, they add water to each sample and make more observations about texture. Using this information, their newly learned vocabulary, and a chart from Mr. Mugg, they identify the samples and match the crime scene soil evidence to the suspect samples.
Because soil can get easily onto clothes, shoes, or in treads, it is often an example of trace evidence. Evidence from a crime scene can be transported to other places and vice versa, a phenomenon known as Locard’s Exchange Principle. This principle states that a criminal must leave trace evidence as well as take something away from the scene. In this case, the trace evidence is soil from the bicycle tires of the suspects.
Soil is composed by half of air and water, both elements necessary to sustain life. Rocks, humus (dead plant or animal material), molds, and worms typically make up the remaining half. The size of these particles helps to determine the type of soil. Sand contains the largest particles and clay the smallest. Although sandy soil contains mostly sand, it also reveals a small amount of clay, but very little, if any, humus. Sandy soil does not absorb water well. Instead, water runs through it. Because there are few nutrients in sand, plants do not grow well in it.
Clay is mostly just that—clay. It has a little humus and sand, as well. This type of soil holds water and dries slowly. When wet, it is very sticky. However, it is hard, like pottery, after drying.
Loam contains the most humus, which makes it excellent for growing plants. It contains gravel, sand, and clay, and its color ranges from dark brown to black.
Depending on where the soil is located, it could contain non-organic items. In cities, soil often yields paper, glass, metal, cement, or other items. A forensic geologist considers whatever is in the soil to be a part of it, whether it is natural or not. These items help the forensic geologist to match the soil to its original location.
The activities in this lesson address Next Generation Science Standards practices of Asking Questions and Defining Problems, Planning and Carrying Out Investigations, and Engaging in Argument from Evidence as well as disciplinary core idea PS1.A. In addition, they address Common Core State Standards CCSS.ELA-Literacy.CCRA.SL.1 and CCSS.ELA-Literacy.CCRA.SL.2. See the Standards Matrix included in the appendix for more detailed information.