InTeGrate Modules and Courses >Critical Zone Science > Module 1: CZ Background > Unit 1.2 - Role of Soil
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Unit 1.2 - Role of Soil

Timothy White (Pennsylvania State University)
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The focus on soil in this unit is accomplished by browsing and reading or browsing (in some detail) information from nine websites as well as a book chapter. This effort will help students to understand issues relating to soil erosion, the state factors of soil formation, methods of soil description and classification in the field, soil orders, soil surveys and threats to soil. Questions are posed that require written responses and the in-class activity involves a web-based soil survey using the Natural Resources Conservation Service Web Soil Survey. This activity can be accomplished individually or by groups and should involve a short report of findings.

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Learning Goals

By the end of this lesson students should be able to:

  • Recognize soil as a socially relevant mineral and biotic entity.
  • Identify the five state factors of soil formation and basic information regarding their importance, and apply that knowledge to a site.
  • Discuss how soils are described and classified into 12 soil orders and how those soil orders are geographically distributed.
  • Explain five of the major threats to soils.
  • Apply information from the Web Soil Survey to land-use planning and decision-making.

Context for Use

This unit is designed to cover two ~75 minute long classroom sessions. The first of these sessions will focus on how soil forms and the various threats to soil. Students will need access to computers and the internet to complete in-class web-based assignments. Soil consists of mineral and organic matter derived from a variety of sources and it is vitally important to sustaining life on Earth including human society. Soils are not randomly distributed about our planet, but instead occupy space determined by the overlapping domains of the state factors of soil formation. Ultimately, soil records the overlap of atmospheric, lithospheric, hydrologic, and biologic processes, the innermost workings of the CZ. A major activity of this unit is to use online resources to consider the distribution of soils at a site chosen by the student and cleared with you (instructor), and the implications of the distribution to understanding CZ processes and land use at the site.

Critical Zone function is represented by coupled physical, biological, and chemical processes that involve study by experts in geology, soil science, biology, ecology, geochemistry, geomorphology, and hydrology, to name a few of the relevant sciences.

While these various disciplines are equally important for understanding the Critical Zone, they are linked by the presence of soil, considered by many to be the central component of the Critical Zone. This concept is perhaps best illustrated by the SoilCritZone logo (used with permission from Vala Ragnarsdottir, and see; not licensed for use beyond this site) shown here: a four leaf clover-like emblem of yellow (at top, representing atmosphere), green (right, biosphere), blue (left, hydrosphere), and brown (bottom, lithosphere) arcs surrounding the acronym SoilCritZone; the design of the logo symbolizes how soil and the Critical Zone exist within the overlapping region between the four arcs or "spheres." The spheres closely overlap with the state factors of soil formation you will be introduced to later in this lesson: parent material, climate, topography, biota, and age.

Various aspects of soil will be considered in this lesson. To begin, consider the following four definitions of soil:

  1. The upper layer of Earth that may be dug or plowed and in which plants grow; and, the superficial unconsolidated and usually weathered part of the mantle of a planet and especially of the Earth (Merriam-Webster).
  2. Earth material which has been so modified and acted upon by physical, chemical, and biological agents that it will support upright rooted plants. The term as used by engineers includes, in addition to the above, all regolith (American Geological Institute Dictionary of Geologic terms).
  3. (i) The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics (Soil Science Society of America Glossary).
  4. The naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter (humus) on the surface of the Earth, capable of supporting life. In simple terms, soil has three components: solid, liquid, and gas. The solid phase is a mixture of mineral and organic matter. Soil particles pack loosely, forming a soil structure filled with voids. The solid phase occupies about half of the soil volume. The remaining void space contains water (liquid) and air (gas). Soil is also known as earth: it is the substance from which our planet takes its name (Wikipedia).

Though the four definitions share common attributes, they do differ. For example, agronomists and most soil scientists focus their studies of soil on the rooted zone, or rhizosphere (approximately to depths of one meter), while the geologist's perspective is much broader and deeper, encompassing the full thickness of material down to the original parent material (shallow, to depths up to one hundred meters or more). Nonetheless, most scientists agree that soil is a complex biomaterial that promotes the growth of terrestrial organisms, that it is crucial to life on Earth, and that it is the product of material derived from weathering of parent material, decomposing plant matter, and atmospheric deposition. Furthermore, as soil resources are finite, humanity should view them as non-sustainable and learn to care for and sustain these important resources. For these reasons and more, this unit focuses on soil, the "heart" of the Critical Zone.

Description and Teaching Materials

Lesson Plan: Unit 1.2 - Day 1 (70 minutes total)

Before Class

  • Pre-class reading: McCauley, Jones and Jacobsen (2005) (reference below, 12 pgs). Supplemental reading: Brady and Weil (2002) (reference below, ~25 pgs).
  • Pre-class browsing: Learn about soil characterization protocols, soil horizon, soil texture, soil color, and soil structure at:

During Class

  • Lecture (15 min): "State" factors of soil formation, (review of the material covered in the Brady and Weil reading assignment). This activity should reinforce the notion that the state factors of soil formation very much guide CZ science, the primary difference being that CZ science delves deeper into the subsurface and considers broader swaths of geological time.
  • In-class Videos (~25 min): Class will watch the 4 short videos that describe field activities involving soil study and soil properties; soil horizons, boundaries and transitions; soil color and texture; and soil moisture:
  • In-class browsing (~15 min): Soil erosion and threats to soils (websites listed below, requires laptop use in class).
  • Class discussion (~15 min): Think-pair-share format.
    • What aspects of the Critical Zone other than soil are impacted by soil erosion and other threats to soils, and are those impacts beneficial or deleterious? For example, while erosion of weathered material may deplete a soil, the transport and deposition of those particles leads to the sustained build-up of flood plains and deltas elsewhere. Soil erosion, in moderate amounts, introduces nutrients into the water cycle that various organisms rely on to survive and thrive.


Activity - "12 Soil Orders" Report Browse "12 Soil Orders" website to learn about how we classify soils in the U.S., as well as the Compendium website to learn how other nations approach the description and classification of their soil resources. The students should also study the global map of the distribution of soils.

Address the following 3 questions (typed, ~2 pgs dbl spaced) based on the two websites above:

  1. Do you observe any generalized pattern to the distribution of any of the soil orders?
  2. If so, can you attribute the distribution to any understanding you may have of the state factors of soil formation and the concepts of the CZ?
  3. Can you draw any conclusions regarding the relative importance of any of the state factors of soil formation from your observations? How might this conclusion relate to understanding CZ processes and function at a site?

Lesson Plan: Unit 1.2 - Day 2 (75 min total)

During Class

  • Class discussion (~15 min): Think-pair-share review of the three questions from above.
  • Class demonstration of WebSoilSurvey website (~15 min) (focus on how the tool works and how to use it to address land-use planning and decision making)
  • Activity - "WebSoilSurvey" Report (in-class)
  • In-class activity (~45 min): Soil surveys, soil maps and land-use planning.
  • Based on your reading, review the information derived from the NRCS SoilSurvey website to assess or infer the following factors. Be sure to cite each separate report that you reference when answering these questions. A final report from this exercise should include a soil map of the chosen study site with either a description of the site location or a larger scale map in which the site can be located, and the following information:
    1. The dominant soil type at the study site.
    2. Any characteristics of the dominant soil that effect land use and whether or not the effect is positive or negative.
    3. Whether or not the state soil exists at the study site. If not, where is the closest location where the state soil is mapped?
    4. A brief description of the importance of the state factors in the formation of the dominant soil and the CZ at the study site. Compare/contrast the dominant soil to the state soil. (See NRCS State Soil info:
    5. What are the primary threats to soil at the site?
    6. Optional: Are there any soil exposures located at the study site? If so, what soil type is exposed? If not, where is the closest soil exposure to your study site? Include a photograph of any soil exposures you visit.
    7. The following activity sheet may help to guide development of student reports: WebSoilSurvey Activity Sheet (Microsoft Word 2007 (.docx) 32kB Dec23 16)

Teaching Notes and Tips

Each day, a "think-pair-share" approach to metacognition is recommended to organize the class discussions: pose the question(s), provide time for the students to think and discuss with a subset of nearby classmates, then finish the discussion with the full class. It is best to collect the students' written response before the discussion lest they try to write or redo their responses in class. (Learn more about Think-pair-share and about Metacognition.)

Most of the second class will focus on soil surveys, soil maps and land-use planning. The activity is based on the NRCS WebSoilSurvey tool (link below). The instructor should take several hours before class to become familiar with this relatively simple tool. The students can choose sites to study or the instructor can assign sites. If a nearby site is available for field study then the instructor should consider having the students collect information on that site(s). This assignment can be finished up as homework but it is important that the students leave class knowing how to use the information available from this site to answer the questions. Note that some parts of the country (the West) have relatively limited amounts of information available. If this is your situation, identify a suitable target (one with actual data links) that you can direct students to before class. Students should be encouraged to view this information as useful/usable in their semester project---as baseline information on site characteristics of a potential study site or proposed CZO.


  • Students will write short essays answering the listed questions about soil orders. Student responses should demonstrate the ability to distinguish and analyze the geographic distribution of different soil orders and should formulate a relationship between the distribution and environmental factors (the state factors of soil formation). The rubric for this assignment can be viewed under course level assessment and is titled "Short Essay Rubric."
  • Students will also assess and describe the distribution of soils at a study site. The written report should illustrate the students' ability to comprehend, apply, analyze, synthesize and evaluate the data to guide interpretations of the dominant environmental soil-forming factors and threats to soil at the site as well as how their observations and interpretations may relate to land use at the site.

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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »