Part 2 of the Earth Materials and Processes Module focuses on soils--how they are classified, their texture and structure, how liquids move through them, and their engineering properties. The module concludes with the effects of land use activities on soil which results in erosion, pollution, and desertification.
Environmental Geology requires a working knowledge of introductory geology. If you need a review, go to our Planetary Geology Internet Course for information on planetary processes, rock-forming minerals, the rock cycle and earth geological processes. You might want to visit the Planetary Geology Home Page for additional information.
Soil is one of the most important earth materials we encounter each day, but finding an exact definition of soil is difficult. The definition depends on the context of the question. Soil scientists and most ordinary people think of soil as a fine-grained, well-weathered earth material that is able to support plant growth. Soil scientists focus on the physical and chemical properties of soil. Engineers think of soil as any earth material that can be removed from a site without blasting. Engineers focus on particle size and the amount of organic material in soil when they consider the properties of soil for engineering applications.
Environmental geologists must understand soil from many perspectives. Soil characteristics affect agriculture, engineering, hydrology, natural hazards and other aspects of land use. Understanding soil development and soil character is crucial to good land use planning.
Soil is an important part of the geologic cycle and soil charactistics are influenced by parent material, climate, topography, weathering, and the amount of time a particular soil has had to develop. Unsurprisingly, variations in climate, parent material, type of weathering and amount of time produce distinct soils that express these variations.
As soil develops, weathering creates distinct layers in soil. We call these layers soil horizons, and each soil horizon has distinctive characteristics. Every soil has a soil profile, a list of the horizons that describe a particular soil. This soil has several horizons. The description of these horizons classifies the soil into a certain group. (Photo by Dr. Ray Weil, University of Maryland, photo courtesy of NASA.)
The Twelve Soil Orders, Soil Taxonomy by Univerisity of Idaho.
Soil Science Education Page by NASA: A site for younger people, but full of useful information and excellent photographs.
Land Degradation and Desertification NRCS -- Natural Resources Conservation Service: When bad things happen to good soil.
Soil Taxonomy by NC State University
1: Investigate the six most common soil horizons. Know the materials they contain, and how they form. Remember that color is an important indicator of soil chemistry. Go outside and find a road cut or trench. See if you can identify some soil horizons. Tell us what criteria you used to make your classification, and give us a geographical location for your investigation For instance, Idaho Falls, ID, roadcut near Interstate 15. (If this is impossible, use the web to find a picture like the one above, and make your analysis from the picture.)
2: Open this pdf document which shows a map of soil regions for Wisconsin, a precipitation map for Wisconsin, and a description of how soil maps are made. Write a brief paragraph about the relationships between precipitation and each of the soil types in Wisconsin. What other factors enter into the soil type of an area? (hint: the last page of the document has other types of maps for Wisconsin.)
A soil's profile depends on its age, its parent material and the climate in which it developed. Soils can be compared in terms of their relative profile development.
Below is a picture of an ultisol from Florida. Compare it to the soil picture above. Note the differences in color and number of horizons. (An ultisol from Florida. Photo by Dr. Ray Weil, University of Maryland. Photo courtesy of NASA.)
A soil’s profile depends on its age and its conditions of formation. Soil profile is the primary criteria for soil classification.
Soils can be compared in terms of their relative development. Weakly developed soil profiles are generally younger and may have fewer horizons; well-developed soils are generally older and have more horizons.
Relative development of a series of soils allows their arrangement in a soil chronosequence. A soil chronosequence gives information about the history of the landscape. The relative development of the soils in a chronosequence tells the investigator about the climate and depositional history of the area.
Click the picture to learn more about the Thistle Landslide
in Utah (USGS photo)
A soil may be saturated or unsaturated with water or other fluid, depending on whether the pore spaces between the grains of the soil constituents are filled with gas or liquid.
Fluid moves vertically and/or horizontally depending on pressure gradients, chemical gradients, and gravity.
Water molecules are attracted to each other, a property we call cohesion. Water molecules also tend to stick to soil surfaces, a property called adhesion. The amount of water a soil can hold depends on the relative amounts of clay, silt, sand and organic material present in the soil.
In this diagram, adhesion holds the water molecules to the surface of the clay layers, and cohesion holds water molecules next to each other.
Change in the amount of fluid in a soil affects its properties. View the USGS Geohazards website to learn more about how landslides work..
Calculating the moisture content of soil is as follows:
Moisture content affects the engineering properties and stability of soils. A soil that is stable in dry conditions may become unable to support the structures built on it when saturated with water.
Groundwater is water beneath the surface. Precipitated water, either direct or indirect (runoff), infiltrates into the soil through the unsaturated zone. The horizon below which all void spaces are filled with water is called the water table.
Water moves through soil (and rock) in convoluted patterns at the scale of individual soil grains, although the overall direction of groundwater movement is fairly straight on a larger scale.
Imagine you want to buy some land and build your dream house. You review all the building sites available to you, and find two sites that fit your criteria. You visit both sites two days after a hard rain.
The first site is fairly flat-lying and you notice that the soil is still quite wet (water is still standing in puddles), and the reddish soil sticks to your shoes.
The second site is on a gentle slope and the owner tells you he ploughed the site just last week. The soil is light brown in color, and quite dry even though it rained only two days ago. Deep rills cross the plough rows.
Compare the two sites. Would you buy one of these lots? Why or why not? If you decided to buy one of them, what considerations in building will you have to take into account?
Write a short (1-2 paragraph) report about your imaginary land purchase, and e-mail it along with assignments 1 and 2 to hughscot@isu.edu.
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