How to identify a soil EarthCache

Background:
Soil identification (don’t call it ‘dirt’ around a soil scientist!) is an important method for pedologists and gardeners alike. Pedology is the study of soils, particularly pedogenesis (how it forms), soil morphology (composition, structure, organization), and classification.

It is important to study soil because it serves as a zone of interactions between climate, soil life (plants and animals), mineral material of the original weathered rock, and its relation to the landscape. It also:

• Provides a medium to grow produce
• Produces and stores gases like CO2
• Stores heat and water
• Provides a home for plants, animals, and microorganisms
• Filters water and wastes
• Provides a material for construction, medicine, art, etc.
• Decomposes waste
• Provides record of geologic, climatic, biological, and human history

Because it covers only 10-11% of the earth’s surface, soil should be studied to learn how it should be used and conserved properly. A massive soil survey was conducted in Denali from 1997 to 2002 during the months of July, August, and September. Researchers dug over 2,000 soil pits, each one meter deep. Their goal was to describe and map all the soils of Denali by covering one sixth of the Park each year – about 405,000 hectares, or 4050 square kilometers. For future soil studies, researchers now have a baseline dataset of soil and environment types for the entire Park. Because this study required researchers to visit nearly every corner of the park, they created a comprehensive ecological database that includes plant species. Creating this baseline dataset is vital for monitoring ecological changes.

With the right technique, you can almost identify soils in the field as accurately as in a laboratory. Remember that sand is gritty, silt is smooth, and clay is plastic and sticky. Sand grains are large enough to be seen with the unaided eye and can be felt individually between fingers. Silt cannot be seen with unaided eye or felt between fingers. Clay can only be seen with an electron microscope.

Try the following technique suggested by McDaniel et al. (1997) in Maley (2009):

1. Fill the palm of your hand with dry soil.
2. Moisten the soil enough that it sticks together and can be worked with your fingers. Do not saturate it. If it sticks to your fingers, it’s too wet.
3. Knead the soil between your thumb and fingers. Take out the pebbles and crush the soil clumps.
4. Estimate the sand content by the amount of textural grittiness you feel.
5. Estimate the clay content by pushing the sample between your thumb and index finger to form a ribbon.
a. Less than 27 percent: the ribbon is less than 1 inch (2.5 cm) long.
b. 27-50 percent: the ribbon is 1-2.5 inches (2.5-6 cm) long.
c. More than 40 percent: the ribbon is 2.5 inches (6 cm) long.
6. Combine sand and clay percentages and subtract from 100% to get silt content.

How to use the ternary diagram:
Another way to think about this diagram is placing silt at the right-hand corner, placing sand in the left-hand corner, and placing clay in the top corner. Notice that for whatever particle size is in the corner, the percentage is 100%. Percentages decrease in each line moving away from that corner.

Figure 3: Blank ternary diagram of sand, silt, and clay. Image from Long Island University.

1. Estimate the percentage of clay, silt, and sand in the soil. The three percentage values must add to 100%.

Figure 4: Percentage lines top to bottom: Clay, silt, and sand. 100% for each is circled.

2. On the right side of the diagram, find the percent of silt you found (for example, 40%, orange line). Highlight the line that radiates downward from that number to the left.

3. On the bottom side of the diagram, find the percent of sand you found (for example, 30%, green line). Highlight the line that radiates up and to the left.

4. On the left side of the diagram, find the percent of clay you found (for example, 30%, pink line). Highlight the line that radiates horizontally to the right.

5. Where all three lines meet is the category your soil fits into. The example soil is clay loam (orange rhomb).

Figure 4: The example ternary diagram showing a 'clay loam' classification

How to access this EarthCache:

This cache is located along the Roadside Trail which can be accessed by the Denali Visitor Center. Please only find this cache from the trail instead of cutting in from the road (and return the same way).

What you need for this EarthCache:
Water for estimating sand and clay content. This is probably best done on a non-rainy day.

Please remember to practice Leave No Trace ethics to preserve this experience and environment for future visitors:

• Hiking off-trail is not required at this EarthCache.
• Collecting samples is not allowed in National Parks.

To log this EarthCache: Email the answers to the following questions to me (click my username up top, send message).

1. What percentage of sand, silt, and clay did you determine is in the soil?

2. Based on the sand-silt-clay triangle, what type of soil is this? Ignore all other particle sizes in the outcrop for now.

3. Describe some properties of the soil you have identified (research out of field. The OSHA site ‘Field Method for Identification of Soil Texture’ is a good place to start).

Sources:

Clark, M.H., and Duffy, M.S., 2006. Soil survey of Denali National Park and Preserve. National Cooperative Soil Survey.

Lambert, D. The Field Guide to Geology. 2007. New York: Checkmark Books.

Maley, T.S., 2009. Field Geology Illustrated. Ann Arbor: Sheridan Books.
Soil Survey Division Staff, 1993, Soil Survey Manual, United States Department of Agriculture Handbook No. 13, p. 437.

Soil Survey Staff. 2004. Soil Survey of Denali National Park Area, Alaska. U.S. Department of Agriculture, Natural Resources Conservation Service and U.S. Department of the Interior, National Park Service.

Why Do We Study Soil? . 2 June 2010.

This EarthCache was created by the Geological Society of America summer 2010 GeoCorps intern at Denali National Park and Preserve.