Water Quality - Nutrient Management and Cropping Systems - Lesson 8 Soil Structure

Author

Chrissy Rhodes

Published

5/24/2014

Water Quality - Nutrient Management and Cropping Systems - Lesson 8 Soil Structure

Topics:

Lesson Plans - 9th - 12th

Target Grade Level / Age Range:

Grades 9-12

Time:

50 minutes

Purpose:

To teach students the significance and properties of soil structure through class discussion and hands-on activity.

Materials:

3+ soil samples
- Dry samples derived from various gardens or fields. Variation of samples would be best.
- Some examples could be sand, loam from a garden, silt or clay rich soil from a streambed, etc.
- Label each sample with a letter or number
Rubber tubs for soil samples
- Pie tins, paper plates, or cookie sheets (with rimmed edges) would also work
Water in spray bottles
Magnifying glasses
Rulers
Newspapers and paper towels for easy clean up

Suggested Companion Resources (books and websites)

Lab-Aids Soil Organism Study kit
Lab-Aids Biology and Chemistry of Soil Experiment Kit

Vocabulary (with definitions)

Soil texture: the relative proportion by weight percentage of sand, silt, and clay of the mineral soil separates
Sand: the largest soil particle size, consisting of particles from 0.05 mm to 2 mm
Silt: the medium sized soil particle size, consisting of particles from 0.002 – 0.05 mm
Clay: the smallest soil particle size, consisting of particles smaller than 0.002 mm
Structure: the arrangement of soil separates into units (peds or aggregates)
Cation: a positively charged ion; most nutrients necessary for plant growth are found in cation form in the soil
Anion: a negatively charged ion
Organic matter: material from an organic (carbon-based) substance, including plant and animal material
Ped: a naturally occurring soil structure that persists through cycles of wetting and drying
Aggregate: a synonym of “ped,” an aggregate is a soil structure that persists through cycles of wetting and drying
Tilth: the condition of tilled soil, especially in respect to suitability for sowing seeds
Granular structure: a small soil structure noted by its spherical peds, resembling cookie crumbs
Blocky structure: a moderately sized soil structure that is identifiable by its nearly square or angular blocks
Prismatic structure: a soil structure noted by its vertical cracks
Columnar structure: a soil structure similar to prismatic, but with an additional salt cap
Platy structure: a soil structure identifiable by its flat, layered structure
Non-structure: soils that lack a true soil structure
Single grained: a classification of non-structure, wherein each soil particle is separate and does not form aggregates
Massive: a classification of non-structure, wherein the soil does not form separate aggregates and instead forms large clods.
Soil horizon: a layer generally parallel to the soil crust whose physical characteristics differ from the layers above and beneath it
Pedology: another term for soil science; from the Greek words “pedon” meaning soil, and “logos” meaning study
Micropore: pores small enough that water inside is considered immobile, though partially plant-available; tends to mean pores inside peds
Macropore: large pores responsible for downward flow of water through profile; tends to mean pores between peds

Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)

A large part of understanding soil structure is understanding soil texture. The two are interrelated in many ways.
- Soil texture is defined by three main particle sizes: sand, silt, and clay
  - Sand is the largest particle size, silt is the moderate particle size, and clay is the smallest particle size.
  - As a general rule, the larger the particle size, the better the soil will drain. This is because with large grains, large pockets are created between particles. This is made less possible with clays, because the particles are small and the pockets between particles, if present, are much less interconnected.
  - As another general rule, the smaller the particle size, the higher the capability of retaining nutrients. Many things can affect this, including surface area, parent material, age or weathering of the soil, and overall charge of the soil particles.
    - Soil particles are made of weathered minerals, and therefore have chemical structures. While sand and silt are larger, this can be seen best with clays. Clay particles are not round; they are flat and form in sheets. Clays can be 1:1, 2:1, or 2:1:1. This denotes how many layers of different types of minerals are present in the clay particle. For instance, Kaolinite clay is a 1:1 clay with one Si sheet and one Al sheet. This is important, because the charge of the soil particle is what will retain the minerals that plants need. The negative charge of the soil particle is exhibited as Cation Exchange Capacity (CEC). Though the chemical structures of soil won’t be explained in-depth in this lesson, a basic understanding that soil has a charge and that it relates to its productivity is important.
- Soil texture is important to producers because it influences the way they can treat their field to maximize sustainability.
  - There are 12 texture groups in soil. They range from sand, silt, and clay, to loam, silty loam, sandy loam, sandy clay, and silty clay. Here is a soil textural triangle, used by soil scientists and agronomists to help identify soil types:
  - To use the triangle, simply look at the edges to find the percent of each soil type, and the intersection of the three lines is the texture group of the soil. For instance, a 20% clay, 30% silt, 50% sand soil would be categorized as a loam.
  - In the lesson, students will use the textural triangle to help decipher the texture group of their soil sample. To do this, students will need to make a soil ribbon. The ribbon test is fairly simple, but it can get messy. Students will take a ped (naturally occurring soil structure) in the palm of their hand, and wet it until it gets to about Play Doh consistency. Then, they will push the soil in between their thumb and base knuckle of their index finger. They will try to make as long of a “ribbon” as possible, and will let the soil break under its own weight. There are many videos online illustrating this clearly, and three are listed in the procedures below.
  - Once students have a couple ribbons from their ped, they will measure the ribbons. Hopefully the ribbons are comparable length. If the ribbon is greater than 2”, the sample would be in the top tier of the textural triangle (sandy clay, clay, or silty clay). If the ribbon is 1” – 2”, the sample would be in the middle tier (sandy clay loam, clay loam, or silty clay loam). If the ribbon is less than 1” but still ribbons a bit, it would be sandy loam, loam, silty loam, or silt. Pure sand should be easy to identify, and loamy sand will stick some, but will not be able to ribbon at all.
  - Once the tier is identified, then students will need to observe their sample more carefully. The ribbon test identifies how far up on the triangle it is, but further observation and texturing will show if it is farther to the right or left of the triangle. The attached PowerPoint goes over characteristics of the soil textures. In summary, sand feels gritty and looks glittery or sparkly. Silt feels floury, soft, or silky, and looks dull. Clay feels sticky and looks shiny.
    - In order to feel these textures differently, students can rub out a sub-sample. This means that they take a small pinch of the sample, place it in the palm of their hand, and add much more water. Then using the index finger on the opposite hand, rub the sample and feel for textures. While things like color and brightness can be seen in the ribbon sample, the subsample method can help exaggerate texture or feel.
    - If you are really interested in students getting the most out of this activity, the below videos can really help illustrate these concepts. While it is good to learn by doing, there is a lot that can be learned about soil with just some samples and water if students know what to observe and how to apply that knowledge.
- As mentioned previously, soil texture and structure are interrelated. As you may have noticed with the ribbon test explanation, a soil’s ability to stick together has a lot to do with the clay content of the soil. However, there is a happy medium in a soil’s ability to stick together; soil needs to have structure to accommodate roots and hold nutrients, but it also needs to have pores for water and air to be able to move freely.
  - A soil with poor structure, whether it be from weathering, compaction, or another cause, will have trouble supporting life. A well-structured soil will be high in organic matter (carbon based plant or animal material), stable or strong, not compacted, and well-drained.
  - The drainage of the soil is seen in the micropores and macropores of the soil. Micropores are the pores inside of a ped. The water inside a micropore is considered immobile as it cannot travel through the soil, but it is still partially available to the plant. Macropores are the pores between the peds. Water is mobile through these pores, and they are responsible for water movement through the soil.
    - A note on water movement in soil: though we usually think of water as moving down through soil, it primarily moves to dry areas. Though a portion of water in soil will be acted upon by gravity, other portions of water in soil will be acted upon by the soil particles themselves, and can pull water up or sideways to a drier area.
      - When water moves through a soil, it can cause leaching. Leaching means that nutrients or particles initially at the top of the soil can be moved downward through the soil. This creates horizons in the soil, where you can visibly see different textures, colors, and characteristics of soil at various depths. The collection of these horizons is known as the soil profile. In most Iowa soils, an A horizon, which is topsoil, will be clearly visible and can reach greater than a foot in depth. Below that is a B horizon, which is called subsoil. The B horizon will have a greater clay content, because of the leaching of the small clay particles. The B horizon will also likely be a lighter color than the A horizon, because it will have less organic material, since plant and animal material stay primarily at the surface.
    - This lesson talks about 5 soil structures and two non-structures. They are granular, blocky, prismatic, columnar, platy, single grained, and massive.
      - Each of these structures is described in the attached PowerPoint
      - The lesson is written such that soil samples would be provided for the students so they could observe and deduce what kinds of soil structures they see. If this is not possible, there is an additional PowerPoint available with pictures of soil structures that can be used instead.
        If soil samples cannot be obtained, the soil texturing activity can be omitted, but this will decrease the length and depth of the lesson. Students could still fill out the worksheet in class by describing the pictures of the structures and predicting which is which.
      - To quickly summarize the soil structures, there are two that are ideal in terms of soil tilth. They are granular and blocky. Though blocky peds can get too large and impede water movement, they still allow movement between peds. Prismatic, columnar, and platy structures all inhibit water movement. Columnar soils are more common in arid areas. Platy soils are more common in compacted or forest soils. Prismatic soils are more common in B horizons or higher clay content areas.
This lesson also talks briefly about careers. The agriculture industry is growing faster than people are joining it. Teaching students about agriculture can help prepare them for a relevant career.
- Agriculture has 40% of the world’s jobs. Between 2015 and 2020, it is expected to see 57,900 jobs opening annually in the agriculture industry for graduates with a bachelor’s degree or higher ( https://www.purdue.edu/usda/employment/).
- Some jobs relating to soils are:
  - Soil scientist
  - Agronomist
  - Agriculture engineer
  - Soil conservationist
  - Crop consultant
  - Research technician
  - Conservation planner
  - Crop production specialist
  - Research scientist
  - Watershed technician/specialist
  - Landscaping business owner
  - And many more!
Before the class period:
- Review vocabulary, PowerPoints, videos, and activity instructions
- Gather soil samples
- Gather water spray bottles, rulers, tubs for soil samples, magnifying glasses, etc.
- Print worksheets and assessments

Interest Approach or Motivator

Ask students what they know about the structure of soils. Ask students if they’ve seen a sandy beach or a garden. These are different soil structures, and they impact how the soil can be used!

Procedures

Walk through PowerPoint presentation Slides 2-17 (Soil Structure.pptx) with students. Ask questions throughout and monitor students for understanding of material. Be sure to highlight the particle sizes and how to identify each. After that, use the presentation to walk through the types of structure. Allow time for questions between structure types, and encourage students to take notes. Then, the presentation will prompt students as they observe their soils and walk through the texturing process.
1. What is Soil? “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.”--Soil Science Society of America
2. Soil has three particle sizes:
  1. Sand - .05 mm – 2 mm
  2. Silt - .002 mm – .05 mm
  3. Clay - < .002 mm
3. Other solids in soil:
  1. Course fragments (includes stones) - > 2 mm
  2. Organic matter (includes plant and animal material)
4. Soils aren’t just sand or silt or clay; usually they are a mix of these.
5. The textural triangle helps soil scientists understand the properties of a soil based on the percentages present of each particle size
6. Scientists judge soils by feel and appearance
7. Properties of sand
  1. Sand is very well drained
  2. Feels gritty to the touch
  3. Can look sparkly or glittery in soil
8. Properties of Silt
  1. Silt is moderately well drained
  2. Feels soft, floury, or silky
  3. Looks dull in color
9. Properties of Clay
  1. Has poor drainage
    1. Because of small particle sizes, the space between particles is very small
  2. Feels sticky
  3. Looks more shiny than silt
  4. Plate-like in chemical structure
10. Soil Texture Basics
  1. Smaller particles tend to have a higher capability of holding minerals
  2. Clay particles help soils stick together
  3. The larger the particle size, the better the drainage
  4. The naturally occurring soil structure unit is called a ped or aggregate
  5. Structure is important because it influences water and air movement through the soil
  6. Size, shape, and strength of peds all influence the overall structure
  7. Micropore: pores small enough that water inside is considered immobile, though partially plant-available
    1. Pores inside peds
  8. Macropore: large pores responsible for downward flow of water through profile
    1. Pores between peds
11. Granular
  1. Peds in this structure are small and nearly spherical
  2. Water moves well through granular soils
  3. Most common in topsoil
12. Blocky
  1. More common in subsoils where there is more clay accumulation
  2. Peds can be nearly square, or more angular
  3. If peds are larger, it can slightly impede movement of water
13. Prismatic
  1. Soil forms pillars and displays prominent vertical cracks
  2. Drainage is poor
  3. More common in subsoils
  4. Columnar structures are similar, but have an additional salt cap at the top, and are more common in arid climates
14. Platy
  1. Common side effect of compaction in soils
  2. Also found more in forest soils
  3. Impairs water movement
15. Single Grained
  1. Sand at the beach is single grained in structure.
  2. Peds are essentially nonexistent
16. Massive
  1. The opposite of single grained
  2. the soil has formed one or few large aggregates
17. Good Structure
  1. Large pores
  2. Ideal amount of water movement
  3. Strong peds
  4. Helps plant roots obtain the right amount of water, air, and nutrients
18. Bad Structure
  1. Small pores
  2. Inhibited water movement
  3. Weak or nonexistent peds
  4. Inhibits water, air, and/or nutrient uptake in plants
19. Is structure changeable?
20. Soil compaction crushes peds and deteriorates soil structure
21. Too much tillage can degrade ped formation
22. Some tillage, and the incorporation of organic material and root systems can help repair soil damage
Bring students into groups at slide 18 (Observe your samples!)
1. Note: If soil samples could not be obtained for class, use the PowerPoint titled Identify the Structure.pptx and walk through it as a class. Discuss each structure and have students guess each structure. Give students time to take notes for each picture on their worksheet.
2. Create as many groups as you have soil samples.
3. Hand each student a copy of the Soil Structure and Texture Worksheet (attached).
4. Give each group their dry soil sample. Do not give them the water yet.
Ask each group to observe their soil sample quietly, without touching it.
1. Have students take notes on their worksheet. Have them hypothesize what kind of soil structure they see and describe their soil’s characteristics.
After about a minute, ask students to share some of what they see. Encourage them to use vocabulary words, like ped, aggregate, platy, and granular. Allow them to use any notes they took earlier.
1. Go through each sample as a class, displaying an aggregate from the sample as you analyze it. Point out characteristics that are pertinent. Have students record the structures of each sample at this time.
Ask students what these structures can mean, or why they are important. Bring up ideas like building basements and roads. Also talk about planting crops and what plants need from soil (nutrients, air, and water).
1. When discussing water, talk about micropores and macropores. Does this soil structure allow water movement between peds? Why or why not? How would this impact a plant’s growth?
Next, bring each group a spray water bottle. Before you continue with this activity, have students roll up their sleeves, pull back their hair, etc. The soil will most likely not ruin their clothes, but it can get messy.
Hand out rulers, and tell students that they will be performing a ribbon test. Tell students that this test will help them discover the proportions of soil particles in their soil samples. Take a couple guesses from students on what they think will be a prevalent soil particle size based on the structure or look of their sample.
Now, walk through the ribbon test with students (slide 19 of presentation). Have each student take one ped in the palm of their hand. Then, have students use the spray bottle to wet the sample (over the tub or designated area) and work the soil until it becomes about play doh consistency.
1. Students with pure sand will find this very difficult, because it will not stick together. Students with pure clay will also find this very difficult, because it will take a lot of water to make the hard clay workable. Talk about these challenges and why they happen as they arise. (i.e.: clay is dense and pores are not interconnected so water doesn’t move through it easily, and sand and silt need some clay in order to stick together)
When the samples become workable, have students place their sample at the base of their index finger. Have students carefully work their sample into a ribbon with their thumb. Students should “push” the soil from the sample away from their body while trying to maintain a ribbon. Students should not simply press the sample flat. They should try to make the ribbon as long as possible before it breaks under its own weight.
1. Here are some example videos of ribboning:
  1. Soil texture by feel, by UCDavisIPO: https://www.youtube.com/watch?v=GWZwbVJCNec
  2. How to test your soil – texture (sand, silt, clay composition), by Central West Local Land Services: https://www.youtube.com/watch?v=fufeaLBLGlk
  3. Eye on Agriculture Today: Soil Texture By Feel, by KSREVideos: https://www.youtube.com/watch?v=IOyaBxj767s
Once students have made a few ribbons from their soil sample (if their sample is capable of ribboning), have them measure the length of their ribbons.
1. According to the chart (in the PowerPoint, slide 20), what are the possibilities of soil textures it could be?
Next, have students look and feel their samples more closely (slide 21). If there are multiple possibilities for textures given the length of their ribbon (for example, sandy clay loam, clay loam, or silty clay loam), have students look for clues to help them decipher the correct category. Is the sample gritty? Does it feel soft or floury?
1. Describe to students how to texture a subsample by rubbing it in the palm of their hand. This will help exaggerate texture differences between sandy and silty samples of the same clay content.
Based on these analyses, have students write their soil texture analysis on their worksheet.
If time allows, let students go to other soil samples and try to texture these as well. Have students look for similarities and differences between soil samples.
1. Have students record soil textures of these samples on their worksheet as well.
To end class, have a “so what?” moment. Soil has structure and texture. So what?
1. Talk about the importance of soil structure in sustainability. A healthy soil must have good soil structure, or it will not be productive. While some practices like planting cover crops and conservation or no-till can help promote soil structure, other things like overworking the soil or driving on overly wet soils can degrade soil structure.
2. Introduce the careers of agronomy and soil science. Soil scientists and agronomists are real people that study things like pedology, structure, and soil health. Agronomists can help food producers by testing their soil for nutrients, and helping producers put those nutrients back into the soil. Both soil scientists and agronomists can help producers make positive decisions on their farm that will not only help them benefit today, but will help the land be productive for years to come.
At the end of this class or at the beginning of the next, give students the Soil Structure Summary Assessment (attached). Once they have been turned in, go over each question with students. Help them recall the information before moving on to the next subject.

Essential Files (maps, charts, pictures, or documents)

Did you know? (Ag facts)

Iowa has some of the most productive farm land in the world.
Iowa’s primary soil parent material is glacial till.
Iowa is the No. 1 producer of corn, soybeans, pigs, and eggs. These are all possible because of soil!

Extension Activities (how students can carry this beyond the classroom)

Have students bring in a soil sample from home to test for texture.
Keep the soil samples used in this lesson. Test each sample for pH and research importance of soil pH, and how it is corrected.
Give students the opportunity to research various aspects of soil science, including soil chemistry and soil biology. Assign a two-page paper with three valid sources in which they can describe the field, the importance, and jobs available within it.

Sources/Credits

Lesson plan development was funded by the Resource Enhancement and Protection Conservation Education Program (REAP CEP). Resource Enhancement and Protection Program (REAP): Invest in Iowa our outdoors, our heritage, our people. REAP is supported by the state of Iowa, providing funding to public and private partners for natural and cultural resources projects, including water quality, wildlife habitat, soil conservation, parks, trails, historic preservation and more.
Soils: Student Oriented Individualized Learning of Soils, Thirteenth Edition, by Andrew Manu and John W. Schafer
Soil Fertility and Fertilizers; An Introduction to Nutrient Management, Seventh Edition, by John L. Havlin, James D. Beaton, Samuel L. Tisdale, and Werner L. Nelson
Careers in Soil Science: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=nrcs142p2_054277
Employment Opportunities for College Graduates in food, Agriculture, Renewable Natural Resources, and the Environment; USDA: https://www.purdue.edu/usda/employment/
Good Soil Structure; eXtension: http://articles.extension.org/pages/62385/good-soil-structure
Soil texture by feel: https://www.youtube.com/watch?v=GWZwbVJCNec
How to test your soil – texture (sand, silt, clay composition): https://www.youtube.com/watch?v=fufeaLBLGlk
Eye on Agriculture Today: Soil Texture By Feel: https://www.youtube.com/watch?v=IOyaBxj767s
Soil Texture and Soil Structure; University of Hawaii and Manoa: http://www.ctahr.hawaii.edu/mauisoil/a_factor_ts.aspx
Soil Texture; FAO: ftp://ftp.fao.org/fi/cdrom/fao_training/FAO_Training/General/x6706e/x6706e06.htm
Soil Structure; FAO: ftp://ftp.fao.org/fi/cdrom/fao_training/FAO_Training/General/x6706e/x6706e07.htm
Soil Judging in Iowa, PM 1106; Iowa State University Extension and Outreach

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