Lesson 3: Soil Properties

Learner Outcome

Students will understand how soil properties influence the rate and control of erosion. They will grow their knowledge of how soil properties relate to the management of soil erosion and sedimentation.

Success Indicator

Describe the different properties of soil and their relationship to erosion.

Skill Level

Advanced; ages 14-18

Time Needed

One 90-minute class or two 45-minute classes 

Life Skills

Keeping Records, Learning to Learn, Problem Solving

Materials

  • Lesson Three Slides
  • Different soil textures, including samples from the soil that will be used in the rainbox experiment and from the school grounds 
  • 5 Spray water bottles
  • 5 Shovels
  • 5 Nutrient test kits for Nitrogen, Phosphorus, Potassium, and pH
  • 6 Tennis ball containers with 5 holes drilled in the bottom
  • 6 mesh screen circles to fit inside tennis ball container
  • 3 funnels that fit on top of tennis ball container
  • 12, 6 oz cocktail cups
  • 20 oz of each sand, loam, and clay – dried and clumps sieved or broken apart
  • A package of index cards

Optional

  • 6-inch cylinder (coffee can, steel ring)
  • Stopwatch
  • 500 ml beaker

Lesson Slides

Lesson Slides

Click on “File” and then select “Make a Copy ” of the lesson slide deck to use it with your youth.

Introduction

Background Information

Furthering students’ knowledge of basic soil properties enables them to understand the relationship between the susceptibility of soil to erode and how to best manage erosion and sedimentation.

Soil Properties

Soil Texture

image of coarse sand
Sandy soil allows water to infiltrate quickly.

Soil texture refers to the proportion of different-sized particles present in a soil.  Sand has the largest soil particles, followed by silt, with clay having the smallest particles. Most soils are combinations of these different-sized soil particles.  The various sized soil particles have different physical characteristics that influence the rate of soil erosion.  Sandy soils have low runoff rates because water easily infiltrates and permeates the soil profile.  Sandy soils, however, usually have low organic matter, low native fertility, and low water-holding capacity, making it a challenge to establish vegetation.  Silty soil is better able to retain moisture, but doesn’t aggregate well, making it the most susceptible to erosion by water or wind.  Clayey soils have the finest particle size and when detached and suspended in runoff, create turbidity challenges due to their extremely small size, lightweight, and inability to settle out of water.  When wetted, they become very sticky.  These small clay particles are very cohesive making them less likely to be detached and erode compared to silt particles.

Soil Structure

image of blocky soil structure
Blocky soil structure is an indication that water can move into and through the soil, decreasing the risk of erosion.

Soil structure refers to the naturally occurring aggregation of sand, silt, and clay particles into clusters. Individual aggregates are called peds. Soil structure affects infiltration, how the water moves through the soil profile and surface runoff. Unlike soil texture, the structure can easily be altered or destroyed by mismanagement. For example, soil can be compacted through heavy construction or agricultural machinery, destroying soil structure. There are different types of structure (which are illustrated more completely further in this lesson) called prismatic, columnar, platy, blocky, granular, and structureless (massive and single-grained).

Infiltration

The infiltration of water into the soil is the result of the relationship between soil texture, soil structure, and bulk density.  Soils that allow rapid infiltration are less likely to experience erosion since there is limited water runoff.  Soils with slow infiltration experience greater susceptibility to erosion.  The rate of infiltration is affected by soil characteristics including how easy it is for water to enter (is the soil compacted, reducing infiltration?), how much water the soil can hold, and how fast the water moves through the soil.  The soil texture and structure, vegetation types and amount of cover, water already present in the soil, soil temperature, and rainfall intensity all play a role in controlling infiltration rate and capacity.  For example, coarse-grained sandy soils have large spaces between each grain and allow water to infiltrate quickly.  Vegetation creates more porous soils by both protecting the soil from raindrop impact, which can cause soil particles to stick together sealing off the pores and loosening soil through root action.  This is why forested areas have the highest infiltration rates.

Soil Fertility

Soil is a complex physical, chemical, and biological system providing a number of ecosystem services, one being a substrate for plant growth.  When implementing strategies to manage soil erosion, the primary goal is to establish vegetation.  Soil develops into different layers called horizons. The surface horizon of a soil, called the topsoil, tends to be richer in organic matter, abundant in nutrients, and have a pH (the measure of the acidity or alkalinity of an aqueous solution) more conducive for nutrient uptake by plants.  Too often, however, the topsoil erodes or it is removed by construction activities, leaving the subsoil at the surface.  Subsoil tends to have a less desirable pH, low nutrient content, and a soil texture that can limit plant growth.  Soil fertility plays a large role in determining the success of establishing vegetation on exposed soil and reducing soil erosion.

Daily Blog Breakdown

Take a few minutes for students to describe their blog posts. Questions to facilitate processing might include:

  • What was the most fun about doing the blog?
  • What are some decisions you made to carry out your posting?
  • Why is it important to share your ideas?
  • How does blogging help you contribute to your own understanding?
  • Was this the first blogging experience for you? 
  • Do you think blogging about your lessons helps you to better understand what you are doing/ learning each day?

Soil Properties: Texture

Exploring Soil Properties

Determine Soil Texture

Students will be exploring the different properties of soil, beginning with soil texture. Explain to students that soil has four basic components: water, air, minerals, and organic matter. Collectively these four components make up soil. Water and air fill the pore spaces or holes created by soil particles and aggregates. Organic matter is derived from decomposed and decaying living things like plant material, bacteria, and animals. The mineral component of soil originates from solid rock broken down over time. The mineral component develops into three soil particle size ranges from largest to smallest: sand (2mm to 0.05mm), silt (0.05mm to 0.002mm), and clay (<.002mm).  Soil texture refers to the percentage of soil particles in a given sample. The soil texture triangle (below) displays the 12 soil textural classes. Soil texture relates to erosion because depending on the texture, a soil might be more highly erodible (silt vs. clay). It could have limited infiltration, leading to greater runoff (clay).

Pass out the student handout, “Exploring Soil Properties.” Students can follow along to the following teaching steps. Soil scientists use a field test called hand texturing to determine what soil texture they have. 

  1. Have the students watch the video titled, “Hand Texturing by Feel”.
  2. Encourage students to share their observations with the class.

Note: The soil texture lab can be extremely messy, and teachers may want to cover the floor or tables with brown craft paper or newspapers.

Demonstrate hand texturing for the students

  1. Begin by holding a golf ball size sample and spraying it lightly with water from a mist bottle.  Work the water into the soil until evenly moist.  
  2. Squeeze the soil into a ball and open the hand.  If the soil ball falls apart, this indicates sandy textured soil.  
  3. If the soil remains in a ball when squeezed, likely there is silt and clay in the sample.  Take the ball and show the students the “smush and push” method.  
  4. With your thumb, smush the soil sample gently and push it across your index finger.  This should form a ribbon and depending on the length gives an indication of what textural class the soil falls into.  
  5. Demonstrate the differences between a sand, loam, and clay soil for students.  
  6. Allow the students to try hand texturing themselves by having samples of different soil textures. Pass out the Hand Texturing by Feel handout at the end of this lesson for specific step-by-step instructions. 
  7. Use the samples collected from the school campus grounds during the site assessment as well as the soil that will be used in the Rainbox Experiment (if it is different from what was collected from the school grounds).  
  8. Students should record the soil texture from the rainbox on the Rainbox Throwdown Lab handout (see Lesson Two).

Additionally, try to obtain samples from each of the soil textural classes. Work with the local Cooperative Extension office or the local Soil and Water Conservation District to identify areas in your county with different soils. One might also use Web Soil Survey to find locations with different soil textures.  Play sand could be used as a sample for sand. A garden soil usually can be considered loamy, and throughout much of the country, clay can be fairly ubiquitous, especially in areas of new development or road construction.

Exploring Soil Properties Handout

Use this student handout to guide explorations of soil property activities, including:

  • Soil texture
  • Soil structure
  • Infiltration
  • Soil nutrients
Soil Texture by Feel

Use this student handout to determine what soil type you have.

Soil Ribbons

Watch this video to learn how to make a soil ribbon to determine how much sand, silt, or clay is in your soil.

Soil Properties: Structure

Soil Structure

Briefly introduce the concept of soil structure to the students.  Explain that soil structure is the aggregation of soil particles into groups or clumps called peds based on physical and chemical properties. These soil clumps or peds form shapes that affect water infiltration and runoff from the soil surface and the ability of water to move through the soil.  Soil structure is arranged into five primary shapes: granular, blocky, columnar or prismatic, platy, and two structureless conditions: massive, and single grained.

  1. Watch the video, “Soil Structure (coming soon).  Encourage students to look closely at what the soil scientist demonstrates.  They should see that the scientist takes a clump of soil and naturally breaks off a ped to examine more closely to determine structure.  
  2. To explore soil structure, gather the students outside and demonstrate how to collect a soil sample with a shovel by keeping as much of the soil sample intact.  
  3. Then show them how to determine structure by gently breaking off pieces or “peds” of the soil. 
  4. Supply each group with its own shovel to take a large scoop of soil.  Students should break off a clump of soil and hold it in their hand to observe it closely.  Using the illustrations in the student handout “Exploring Soil Properties” and from what was shown in the video, students should determine the type of soil structure they have.
  5. Students should determine the structure they will use in the rainbox throwdown experiment and record it on the “Rainbox Throwdown Lab Sheet” (found in Lesson Two). Teachers might work again with their local Cooperative Extension agents or Soil and Water Conservation District to obtain other soil structure samples to enhance student understanding.
Soil Structure Handout

Use this student handout to determine what soil structure you have.

Soil Properties: Infiltration

Infiltration

Explain to students that infiltration is the process of water entering the soil. When the soil has a stable structure and numerous surface pores, rainfall easily enters the profile.  If the soil structure is lost through compaction or crusting, the surface pores are sealed resulting in limited infiltration and high rates of runoff.  The runoff can carry soil particles, surface applied chemical fertilizers, or pesticides down the landscape. These materials can end up in streams and lakes or in other undesirable locations. 

Infiltration Demonstration

  1. Set up a demonstration using six different columns of soil. Prior to class:
  2. Using empty tennis ball containers with holes drilled in the bottom and a mesh screen in the bottom, fill the first container with a sandy soil, the second with equal volume loamy soil, and the third with a clay soil.  You may have to dry the soil prior to this demonstration and break up any clods or large aggregates of soil.  You may even put the dry soil in a non-food blender and break all the soil peds apart.
  3. In the other three columns, fill one with sand, the second with loam, and the last with clay.  Using the handle of a mallet or hammer, tamp down the soil to compact it.
  4. Place each of the soil columns in a 6 oz clear cocktail cup to capture any runoff.
  5. Place a funnel on top of each soil column to minimize soil splash when you pour water.

During class ask students to explain how soil texture may influence infiltration. Follow up by asking, “Which texture will water infiltrate the fastest?” “How do you think this relates to erosion?” 

  1. Using student volunteers, pour equal volumes of water (about 8 oz or more depending on the volume of soil) into the funnel at the same time and observe the rate at which the water infiltrates.
  2. Have students share their observations. Ask them to consider, “What would happen to the water if the soils were compacted?” “Does soil texture influence compaction?” “Can you think of a time when you have observed this outside?”
  3. Pour equal volumes of water on the compacted soils and observe the rate of infiltration.
  4. Encourage students to share their observations and consider what other variables might influence erosion.
  5. This experiment can be varied in multiple ways; pour water on saturated soil to observe the influence of soil moisture on infiltration; plant grass seeds in the soil columns and see if vegetation influences infiltration; or add compost or other soil conditioners as a variable. Have students determine additional experiments to try.
Soil Pour-Through

What happens when water is poured over different soils? Watch this simple experiment to understand the relationship between soil texture and structure with water movement in and through the soil.

Benefits of no-till farming

Watch NRCS agronomist, Ray Archuleta, share the reasons why undisturbed soils are productive for agriculture and minimizes erosion.

Soil Properties: Fertility

Soil Fertility

Construction sites typically have no topsoil for engineering reasons, leaving behind a subsoil horizon.  In agricultural systems, the topsoil can be lost from poor management practices.  The removed topsoil layer contains the most fertile soil for the establishment and growth of vegetation.  The remaining subsoil layer has low native fertility and sometimes a low pH, a problematic condition for establishing vegetation.  A common practice is to apply fertilizers and liming agents to the subsoil layer to provide soil nutrients.  For example, North Carolina is famous for its acidic soils and requires large amounts of added lime to amend the pH and improve the availability of essential nutrients.  When measuring pH, a value less than 6 would be an indication of acidic soil.  Lime then would be needed to raise the pH.

Nitrogen (N), phosphorus (P), and potassium (K) are essential plant nutrients found in the soil.  There are many more nutrients that plants need for growth and development, but N, P, and K are required in large amounts and will be the focus of this lesson.  These nutrients are often limited in quantity due to the movement of the nutrients through the soil profile.  This means that when rain falls and water enters the soil profile, it picks up these nutrients and washes them away if they are not being used to nourish plants.  The common practice of applying fertilizer helps to supply these plant nutrients, vital for establishing vegetation.

Testing Soil Nutrients and pH

  1. Provide each group with a soil test kit (see Resources Appendix) to measure the amounts of nitrogen, phosphorus, and potassium present in the soil and also measure the pH.   
  2. Obtain a soil sample from the rainbox used for the demonstration or that will be used in the rainbox experiment.  Either demonstrate to the students how to use the soil test kits or have them follow the protocols given by the kit instructions.  Students should record their findings on the Rainbox Throw down Lab handout.

Testing Soil Fertility (optional)

Most states offer soil sample testing services through the state department of agriculture or other non-profit organizations.  The cost for testing can range from no expense to citizens to a moderate fee.  If your resources allow it, obtain a soil sample testing box and have each group of students take a sample of soil from the area on the school campus where the soil was collected for the Rainbox Throwdown.  The samples should all be all from one area.  Have students use a trowel to collect a sample that is 4-6 inches deep.  Take each subsample collected by the groups and mix it into a small plastic bucket.  From the bucket, put a sample into the soil testing box and return it to the soil testing lab.  Results typically take 2 to 6 weeks (depending on the time of year) and the local Extension agent can help interpret the recommendations. Applying the proper amount of lime and fertilizer is essential not only for the vegetation but also for the protection of groundwater. NC State University’s Cooperative Extension published a “Gardener’s Guide to Soil Testing” that is a good reference.  

Nutrient Testing

Learn how to use a simple soil nutrient test and determine your levels of nitrogen, phosphorus, and potassium.

Testing Soil pH

Learn how to use a simple soil pH test and determine your soil’s pH levels.

Soil Sampling and Fertility

Join Anthony Growe, Richmond County Cooperative Extension’s Agriculture Agent, as he shows us how to take a soil sample and what we can find out.

Assessment

Assessments

Concept Card Mapping

Concept Card mapping takes the idea of concept mapping (Novak, 1998) and gives students the opportunity to build relationships between concepts they have learned. Concepts are written on index cards and students arrange them to show linkages.  Students activate their prior knowledge and build a visual map showing the connectedness of their knowledge.  It gives teachers an opportunity to understand the sophistication of student thinking, their level of knowledge and can be used to reinforce certain concepts that seem to be weak.

Using cards (PDF in this section), cut out the given cards or write these concepts on index cards and have students work in pairs or groups to make a map.  Be sure to provide blank cards for students to write in their own concepts as well. Give students a minute or two individually to first develop their own connections.  Once pairs or groups have made their linkages, they can form sentences between the concepts.  For example,  soil “structure” increases “infiltration.”

The Daily Blog

Blog Assignment: Students should write a short summary on how soil properties influence erosion.  Students can draw from additional expert resources including the Natural Resources Conservation Service (NRCS), the Cooperative Extension Service, and state soil and water agencies, citing the research these organizations have conducted on erosion and soil properties. Students may investigate global occurrences of erosion and the different kinds of soils in these areas.

Documenting Soil Properties

Teachers may have students document the properties of the different soils that they are exploring.  The Data Collection section of the Rainbox Throwdown handout can be used to assess student understanding.

Concept Mapping Cards

Use these concept mapping cards to assess your student’s understanding of soil erosion and soil property concepts.

Concept Mapping Cards Jamboard

Try this digital version of concept mapping using Jamboard.

image of lesson 3 assessment jamboard

Learn More

Learn More
Soil Science Careers

Many opportunities exist for students interested in soil science and applying their interest in ways that steward the environment. Give students the bookmarks below and have them research possible careers related to soil science.

Soil Careers: Agriculture Extension Agent

Meet Anthony Growe, an agriculture agent with North Carolina Cooperative Extension. Learn about what Mr. Growe does as part of his every day work!

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