Unit 2: Soil Characteristics and Their Relationship to Land Use Practices

Unit 2 supports the following overarching goals of the Growing Concern Module:

1. Use soils and land use history data to develop a plan for sustainable soil management in one or more agricultural settings.
   
2. Predict agricultural challenges that might result from climate change using systems thinking.
   

By the end of the unit, students will be able to:

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Lettuce crop in central California

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• Describe the soil properties of porosity and permeability.
  
• Characterize the porosity and permeability of a soil sample.
  
• Interpret and assess the effects of land use practices on the porosity, permeability, and erosivity of the soil.
  
• Make recommendations for sustainable agricultural practices in a hypothetical scenario.
  

This unit directly supports multiple InTeGrate guiding principles. Students engage in the methods of geoscience by testing through prediction. They consider the interdisciplinary nature of soil sustainability by relating soil properties to increased anthropogenic soil erosion rates due to agriculture and then explaining the effects of various agricultural methods on these soil properties. Students then engage in systems thinking by offering solutions to prevent soil erosion, and relating the importance of prevention efforts to the potential for increased erosion due to climate change.

This unit is designed to be used in any 50-minute introductory soils/agriculture, geology, or environmental science class as an introduction to geoscientific thinking. It is designed to work in a wide range of class sizes and it can be readily adapted to fit any classroom environment. A small amount of readily available equipment is required, and if possible, a locally representative soil sample should be collected. Directions for assembling simple soil models are also provided.

Humanity's growing population and demand for food has led us to rely heavily on industrialized agriculture, whose methods impact soil significantly. How we till the soil and what we plant in it causes changes in its composition, porosity, and permeability, all of which affect the erodibility of the soil. Therefore, it's crucial to understand how to measure these soil properties and to learn how to solve erodibility problems caused by losses in porosity and permeability.

In this unit students will learn the nature of soil and its properties. This will be followed by an in-class "lab" in which they will describe and quantify soil porosity and permeability and then relate the loss of these two properties to unsustainable agricultural practices. In the post-activity homework, students will offer solutions to soil loss in the role of an agricultural expert, and their work will feed into the agricultural fact sheet in Unit 6 that acts as the summative assessment for the module. Instructors can customize the module to focus on soil in their local area but are also provided with instructions for assembling simple models of a generic soil. This unit takes approximately 50 minutes and is intended for introductory classes in soils/agriculture, environmental science, or geology. A short pre-work homework assignment is provided. Students will read basic information about soil and answers questions in writing to begin the unit with a basic understanding of the nature of soil and its properties.

Instructions

Pre-work homework

Prior to the classroom portion of the unit, students should read all of "What is Soil?," including the three subsections "How Do Soils Form?," "Soil Types," and "What Makes Soil, Soil?" This should take approximately 10 minutes. To increase comprehension and help them relate the reading to this unit, have students answer the questions on the attached Pre-Work Questions for Students.

• What is Soil? (Acrobat (PDF) 385kB Jul31 14)
• Pre-Work Questions for Students (pdf version) (Acrobat (PDF) 35kB Sep21 14)
  Pre-Work Questions for Students (MS Word version) (Microsoft Word 2007 (.docx) 16kB Sep21 14)
• Answers to Pre-Work Questions -- private instructor-only file
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Have your students hand in their answers at the beginning or the end of class.

Soil property identification

This in-class activity consists of an opening "think/pair/share" activity, followed by a hands-on soil porosity and permeability activity, and wrapped up by a class discussion and a homework assignment.

1. Project the Opening and Wrap Up Slides (PowerPoint 2007 (.pptx) 1MB Oct15 14) to review the "student version" of the learning objectives for the unit.
2. Think/Pair/Share activity: (10 minutes) Start with a pre-activity "think/pair/share" exercise to make the students think about what they know about soil (learn more about think-pair-share). Instruct students to think about what they learned about soil formation in the pre-activity homework, then ask them to divide into pairs, and have each pair write a description of soil in fewer than 30 words. Have your own description ready that includes reference to soil components, factors that affect its formation, and why it is important. You can write your own description or use the following, which is also included in the Opening and Wrap Up Slides (after "What is Soil?"):
   
   

   Soil is a mixture of minerals, water, air, organic matter, and organisms. It plays many roles, including providing a medium in which plants (including our food) and a large number of the microorganisms on earth grow; filtering our water; interacting with the atmosphere to cycle important substances in and out of the air; and serving as engineering media for construction of foundations, roadbeds, dams, and buildings. Soil is important to us simply because life as we know it would not exist without it!
   
   Every soil originally formed from parent material: a deposit at the Earth's surface. The material could have been bedrock that weathered in place or smaller materials carried by flooding rivers, moving glaciers, or blowing winds. Different soils form in different places around the world because of CLORPT: climate, organisms, relief (landscape), parent material, and time. Over time, sun, water, wind, ice, and living creatures help transform, or change, the parent material into soil. As a soil ages, it changes because its components—minerals, water, air, organic matter, and organisms—constantly change. Important characteristics of soil that tell us how it will behave are: texture (is it sand, silt, clay, or a mixture of these?); structure (peds); and color.
   
   

3. How Full is Full? (30 minutes):
   
   This activity can be conducted as a whole-group activity for small classes or in small-to-medium groups for large classes. Give all students instruction sheets (see links below) and soil samples that will be used to simulate both a natural and an agricultural soil. Instructions are provided in the link below for collecting and preparing soil for the activity. If possible, a local soil sample should be collected; otherwise, instructions are provided for assembling a soil model. If you are teaching a large class for which you cannot provide enough soil and/or equipment, this part of the activity can also be demonstrated by the instructor.
   
   
   • Soil Model Instructions (Acrobat (PDF) 184kB Oct16 14)
   • How Full is Full? (pdf version) (Acrobat (PDF) 638kB Oct16 14)
     How Full is Full? (MS Word version) (Microsoft Word 2007 (.docx) 992kB Oct16 14)
4. Group discussion (10 minutes): The instructor should spend time at the end discussing with the whole class their answers to question #5 (Do you think this compacted soil is more vulnerable or less vulnerable to erosion by runoff?). Start by asking them their answers and encourage them to debate until they agree on the correct answers. Some students may believe that compacted soil is less vulnerable to erosion by runoff; however, the opposite is true. If rainfall cannot infiltrate (percolate down into) the soil, there will be more water flowing on the surface wearing away the soil. After students have shared their answers and this discussion has occurred, Slide 4 in the Opening and Wrap-Up Slides can be shown to illustrate this point. Ask the students to look carefully and interpret what they see. Then point out how the compacted field equipment track (traffic lane) is wetter than the adjoining planted areas. The planted areas themselves are likely more compacted due to tillage than are soils from natural areas surrounding this field. A brief discussion of soil compaction should follow. Ask the students to summarize what they have learned about soil compaction in the context of this lesson (compacted soil has lower porosity and is more erosive than uncompacted soil). Next, if possible, show students at least the first few minutes of this YouTube video that demonstrates the benefits to soil of one of the sustainable soil management practices your students will be addressing in their post-activity homework. (If time allows, show the entire video.) If you are unable to show the video, show Slide 5 in the Opening and Wrap-Up Slides and read the description of what they are looking at. The description is contained on the notes page for the slide (under the View tab).
   
   
   • Opening and Wrap Up Slides (PowerPoint 2007 (.pptx) 1MB Oct15 14)
   • Video: http://www.youtube.com/watch?v=q1aR5OLgcc0&feature=player_embedded#at=274
5. Homework:
   
   Finally, assign the following homework:
   
   
   • Post-Activity Homework (pdf version) (Acrobat (PDF) 54kB Sep21 14)
     Post-Activity Homework (MS Word version) (Microsoft Word 2007 (.docx) 18kB Sep17 14)
   • Reading for Post-Activity Homework (Acrobat (PDF) 801kB Aug6 13)
   • Rubric for Post-Activity Homework (pdf version) (Acrobat (PDF) 48kB Sep21 14)
     Rubric for Post-Activity Homework (MS Word version) (Microsoft Word 2007 (.docx) 16kB Sep17 14)

Notes for the porosity and permeability activity:

1. Clear, plastic 10-ounce cups work well for this. Do not use the larger colored plastic beverage cups as they are difficult to see through. To save time, punch 12 to 15 holes approximately 2 mm in diameter in the bottom of the cups ahead of time. Try to make all of the holes approximately the same size and make the same number of holes in each cup. If your soil is very loose, you may want to cut some filter paper or coffee filters to the size of the bottom of the cups and place them inside the cups on top of the holes so that you don't lose your soil.
2. Remind students that when they are reading the measurements on their graduated cylinder or measuring cup it should be sitting on a desk or a counter rather than held in their hands. Remind them also that even though their water volume need not be perfectly precise, they should at least be consistent in their reading, either reading from the meniscus (the dip in the middle) or from the edge each time.
3. If your natural soil is a heavy clay soil, your students may not find any measurable permeability after they have compacted it. Let them know that is fine. (Try the activity yourself first, and if you find little to no permeability to start with, you may want to wet your clay soil, mix in some peat moss or mulch, and then let it dry again without compacting it after mixing. That will give you a "natural," uncompacted soil that should work for this activity even though it is not quite representative of the location where it was collected.)
4. If you are teaching a large class and use the demonstration method to test the porosity and permeability of the soil, have some students assist you with the activity. You should still have the class break into small groups or pairs, if possible, to answer the questions.

Prior to beginning the module, the instructor should collect the Pre-Work responses from students to assess their level of comprehension of what soil is.

The instructor should also collect the "How Full is Full?" porosity and permeability worksheets (formative assessment) from each small group to review how well students have learned to describe and quantify soil porosity and permeability and to determine how effectively students related these features to various land use practices.

Finally, the instructor should review the final homework assignment (summative assessment) which can be used to evaluate students' comprehension of the impact of soil management practices on agricultural sustainability. A portion of this assignment can be used in the Unit 6 agricultural fact sheet that is the summative assessment for the module. A rubric for this assignment is included above.

• How Full is Full? Modified with permission from teachengineering.org; retrieved 6/25/13.
• Nearing, M.A., F.F. Pruski, and M.R. O'Neal, 2004. Expected Climate Change Impacts on Soil Erosion Rates: A Review, Journal of Soil and Water Conservation, 59(1):43-50.
• Soil Science Society of America. 2001. Glossary of soil science terms; accessed 8/14/13.
• Soil Basics; retrieved 7/31/14, Soil Science Society of America, reprinted with permission
• Wortmann, Charles S., and Paul J. Jasa, Rev. October 2009, NebGuide: Management to Minimize and Reduce Soil Compaction, University of Nebraska – Lincoln Extension
• Learn more about Think-pair-share from Pedagogy in Action