1. Soil, Unit 7 Chapter 12
Pawson, 2014
PVMHS

2. Learning objectives: By the end of this unit you should be able to:
Recognize various soil types
Discuss soil horizons
Explain the chemistry of soils
Distinguish the different kinds of sand
Describe the collection and examination of soil evidence

3. When soil is not just “dirt”
Police in California found a dead body on the platform of an oil well. Investigators noticed the rocks used to construct the platform seemed unique. Test showed that the rocks were a special type that came from a site 300 miles from the well’s location. They knew finding matching samples of the rocks on a suspect would be key to solving the case. Police arrested an acquaintance of the victim. The suspect kept repeating, “I wasn’t anywhere near the crime scene!” The police suspected otherwise. Physical evidence linking the suspect to the crime scene was needed. After they obtained a search warrant, the police took possession of work boots that had been worn by the suspect. They took the boots to a laboratory. Once in the lab, forensic scientists attempted to match soil samples from the suspect’s boots to soil from the area where the body was found. The suspect was not worried. To him, dirt was dirt. He was unaware of the testing that could be done to compare the soil from his boots to the soil found on the body. Forensic geologists performed chemical and microscopic tests on the soil on the suspect’s boots, as well as samples taken from his car. The test proved that the samples from the suspect came from the site where the body was found. Faced with convincing evidence, the suspect admitted his guilt and was sentenced for the crime.

4. Vocab, pg. 339 in text Clay Geology Humus (hyoo-muh s) Leaching
Mineral
Rock
Sand
Silt
Soil
Soil profile
Weathering

5. History of soil in forensics
Dr. Hans Gross, Austria, was one of the first scientists to apply soil science to crime investigation.
Wrote the book Criminal Investigation in 1893
In 1904 Georg Popp, Germany, first forensic investigator to use soil to solve a crime.

6. Soil Properties pH, Texture, color, and structure
Texture affects porosity and permeability
Soil Porosity: the amount of open space between each soil particle
Soil Permeability: the ability of a liquid to flow through soil

7. Sand Silt Clay 0.05–2 mm diameter 0.002–0.05 mm diameter
less than mm
Diameter
Water
Water
Figure 3.25
Natural capital: the size, shape, and degree of clumping of soil particles determine the number and volume of spaces for air and water within a soil. Soils with more pore spaces (left) contain more air and are more permeable to water than soils with fewer pores (right).
High permeability
Low permeability

8. Soil Soil is the top most layer of the Earth’s crust
Soil contains minerals, decaying organisms, bacteria, water, and air.
Soils form in horizons , characteristics and properties differ from horizon to horizon.
Soil texture:
Describes the % of sand, silt, and clay in a soil.
This is the most important physical property of soil and will provide information about:
Water-holding capacity
Water-flow potential
Fertility potential

9. Soil Factors affecting soil
Temperature
Rainfall
Chemicals and minerals in the soil
Soil physical and chemical characteristics vary by location
Soil pH – soil can be acidic or alkaline
Soil analyses help link:
Suspects to crime scenes
Locating burial sites

10. Particle diameter size
There are 3 main soil grain sizes:
Sand
Silt
Clay
Soil particle diameters range over 6 orders of magnitude
Boulder >2 m
Cobble < 2.5 m to > 64 mm
Pebbles < 64 mm to > 2 mm
Sand < 2 mm to > 0.05 mm
Silt < 0.05 mm to > mm
Clay < mm

11. Sand
<2mm to >0.05mm
Visible without a microscope
Sand is formed by weathering, the action of wind and water on rocks.
If a sand grain contains only one mineral, it is considered a crystal.
If a sand grain contains two or more minerals, it is called a fragment.

12. Sand
Considered non-cohesive – does not stick together in a mass unless it is very wet.
Sand has less nutrients for plants
Space between sand particles promote drainage and entry of air
Holds little water and prone to drought

13. Types of sand Mineral Composition of Sand
Sand from different locations contains different combinations of minerals.
The most common mineral found in sand is quartz.
There are four basic sources of sand:
continental
volcanic
skeletal
precipitate
Continental Sand
Continental sand is composed mostly of quartz, micas, feldspars, and dark-colored minerals like hornblende or magnetite.
Volcanic Sand
Volcanic sand is usually dark in color due to the presence of black basalt or green olivine and its primary source is from mid-ocean volcanoes.
Skeletal (Biogenic) Sand
This type of sand is made from the remains of marine organisms, such as microorganisms, shells, and coral.
Precipitate Sand
Water contains dissolved minerals and when the water evaporates, these minerals come out of the water solution (precipitate), and form crystals.
Calcium carbonate precipitate forms small, round structures called oolites.

14. Skeletal
Volcanic
Continental
Precipitate

15. Silt
<0.05mm to >0.002mm
Particles not visible without a microscope
Quartz & feldspar are the dominant minerals
Feels like flour (not gritty like sand)
Small size retains more water for plants
Holds more nutrients than sand
Highly erosive and easily washed away

16. Clay <0.002mm Wet clay is sticky and plastic (it can be molded)
Pore spaces are very small
inhibits movement of air and water
Large capacity to hold water
Shrink-swell capacity – clays soils swell when wet and dry up and crack when dry

17. Effects of shrink-swell

18. Recipe for good soil
What do you think would be an ideal proportion of sand, silt, and clay for soil?
45% minerals
With an even mixture of sand, silt, and clay
25% air
25% water
5% organic matter
Remains of dead plants and animals

19. “Types” of soil Loam Peat Chalk Mixture of sand, silt, and clay
Best soil for agriculture
Peat
Primarily composed of decaying organic matter.
Found in bogs
Acidic
Often mixed in with other soils due to its ability to retain water
Chalk
Made up of various sized pieces of a rock called chalk
Alkaline
Poor choice for agriculture

20. Soil Horizons Soil looks different at different depths in the earth
Each layer has unique characteristics

21. Soil Horizons O Horizon A Horizon E Horizon B Horizon C Horizon
Surface litter
Leaves are partially decomposed
Thick in forests
Thin in deserts
A Horizon
Topsoil
Humus, living organisms, inorganic minerals
Contains most nutrients for plant growth!
E Horizon
Zone of Leaching!
Dissolved and suspended materials move downward
B Horizon
Subsoil
Yellowish in color due to presents of
Iron, Aluminum, humic compounds, and clay
Leached from A and E horizons
Rich in nutrients
C Horizon
Weathered parent material
Partially broken down inorganic minerals
Bedrock

22. Chemistry of the soil
The materials that make up the soil determine it’s pH.
Rainfall can also alter pH.
Soil can be alkaline (above 7) or acidic (below 7).

23. Soil collection
1. Collect all samples as soon as possible before the environment of the crime scene can be changed by humans or animals walking through the scene or by weather conditions, like rain.
5. Be careful not to remove soil stuck to shoes, clothing, or tools found at the crime scene and package these items separately in appropriate containers.
6. Carefully remove soil from vehicles and package these soil samples separately.
2. Collect baseline samples and samples that appear different or out of place from the rest of the environment of the crime scene.
7. Document all samples by taking photographs, drawing sketches, and labeling the evidence collection containers.
3. Collect at least four tablespoons of material from several locations at the scene.
8. Collect additional soil samples from the four compass points (north, south, east, and west) within a few feet of the crime scene. Collect another set of samples 20 to 25 feet from the crime scene.
4. Sketch the crime scene and note on the map where the soil samples were obtained.

24. Forensic Archaeology
Applied to locate potential gravesites or buried artifacts relevant to a criminal matter.
Detection methods:
Cadaver dogs
Aerial photography – allows large areas of ground to be examined for disturbed or unusual areas. Cameras are used to document the area and examined later – shadows may indicate height differences in soil, color variation may occur from mixing topsoil with subsoil, & changes in vegetation growth could be visible.
Thermal imaging – a decomposing body buried at ~1m is a few degrees warmer than surrounding soil.
Magnetometry – like an MRI for the dirt, can show areas that have been disturbed. It its moved around an area in a grid pattern.
Metal detectors – detect objects up to 40 cm deep.