Biology of Soil - Lesson 3 - How Soil Behaves

Biology of Soil - Lesson 3 - How Soil Behaves

Target Grade Level / Age Range:


Estimated Time:

45 min.


Students will understand the characteristics of a healthy soil, including how energy, water, and nutrients cycle through it; healthy soil structures; and the impact of soil textures.


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

Vocabulary (with definitions)

  • Sand: The largest particle size of soil, with each particle measuring between .05 and 2 mm in diameter
  • Silt: The second-largest particle size of soil, with each particle measuring between .002 and .05 mm in diameter
  • Clay: The smallest particle size of soil, with each particle being less than .002 mm in diameter
  • Aggregate: a material or structure formed from a loosely compacted mass of fragments or particles
  • Pedon: the smallest 3D unit of land that has a representative sample of all soil horizons
  • Ped: an aggregate of soil
  • Slaking: “Slaking is the breakdown of large, air-dry soil aggregates (>2-5 mm) into smaller sized microaggregates (<0.25 mm) when they are suddenly immersed in water.”
  • Microbe: Microscopic organisms like bacteria and fungi that live in the soil, consume organic material, and contribute to soil biomes and soil structure.

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

A healthy soil behaves differently from an unhealthy soil. A healthy soil is robust, has a rich network of organisms, is strong, fertile, and cycles elements well. An unhealthy soil may be degraded, have poor structure, not have microbial activity, and succumbs easily to stress.

Soil can have many characteristics, from color to texture, and depth to structure. Each of these characteristics tells about the soil and how well plants can flourish in it. Soil texture is often one of the first things referred to because of how impactful it is in holding water and nutrients. There are three particle sizes in soil: sand (the largest), silt, and clay (the smallest). Sand is so large, we can see each individual particle. In soil it looks sparkly and feels gritty. Sand does not hold water well (because of the large pores between particles) and does not hold nutrients as well (because it has less surface area than silt and clay). Silt looks dull and feels floury or silky in the soil. Clay looks shiny and feels sticky in the soil. Clay does not drain well (because of a lack of pores between soil particles), and has a high nutrient holding capacity (because of lots of surface area). Silt is in between sand and clay in size, water holding capacity, and nutrient holding capacity.

Soil particles are not the only things in soil, however. A healthy soil should be about 45% minerals (soil particles), 5% organic matter (plant residue, insects, microbial life, etc.), 25% water, and 25% air. Water and air are essential to a healthy soil and growing healthy plants. This means that soil structure is essential to healthy soil and growing healthy plants.

There are many different soil structure types. Soil structure is dictated largely by the size and shape of the soil. Picture digging in your garden. When you pull your spade up, you have multiple clumps of soil. Each of these clumps or aggregates is called a ped. The study of soil is called pedology.

Some soil structure types, like single-grained or massive, are really a lack of structure. Singular-grained structure is when each particle has no association with any other. Think of sand on the beach – each grain of sand is loose and unassociated. Massive structure is like the opposite; this is when a large area is one large ped.

Healthy soils have peds that stick together. Water should be able to move within these peds (through micropores) and around these peds (through macropores). When healthy soils are subjected to stress, like wind, rain, or compaction, they should be resilient and not flake, degrade, or succumb to these stresses.

One good way to test a healthy soil is with the slake test. The slake test subjects peds of soil to water so that you can observe the way the structure holds up to the stress. To do a slake test, you will need two large clear jars, vases, or pitchers; two pieces of ¼” mesh (to create a basket to fit at the top of the jars); and two contrasting peds of soil. One ped should be taken from an area that is rarely or never tilled (think a yard or no-till field). One ped should be taken from an area that is consistently tilled or worked (think a garden or a conventionally tilled field). Both samples should come from the top two to three inches of soil. It is best to take the samples from areas that are in close proximity to each other to try and ensure a similar soil type for each.

You will fill both jars/vases/pitchers with water and place your mesh screen at the top like a basket. The screen will need to be submerged in the water. Then, place the peds into the jars at the same time. The jar with the tilled soil ped will begin to flake quickly. Large pieces of soil will come off the ped. It does not have great structure so it will not be as resilient to the stress of the water. The jar with the untilled soil ped will hold its structure better, and for longer. It will eventually begin to flake, but smaller pieces will fall from it.

One reason for this structural integrity is microbial activity. Soil microbes thrive when undisturbed. They build networks, eat organic material, and make plenty of their own (through excretions and eventually dying in the soil). This activity provides “glues” that help build soil structure. These glues help keep soils together and keep them resilient.

In this test you can also compare visual qualities such as color. Dark brown and black soils are indicative of high organic matter content (meaning lots of plant and animal material). Organic matter is food for microbes and is high in nutrients, so this is good for soil. Light brown soils are likely more eroded and don’t have as much nutrients. Gray or blue soils are indicative of standing water, meaning they historically haven’t drained well (think high in clay). These colors are more common in subsoils (far beneath the soil surface in Iowa). Red soils are indicative of old soils that have been highly weathered and are now oxidized. These soils are more prevalent in the south and in tropical or desert regions.

Interest Approach – Engagement (what will you do to engage students at the beginning of the lesson)

Ask students what they think soil should look like. What should its color be? What should its texture be? Is there a correct answer?


Activity 1: Soil ribboning and discovery

  1. Ask students what they know so far about soil. What is soil health? Why does it matter?
    1. Write students’ answers on the board. Include all relevant answers.
  2. Ask students to describe a healthy soil. Based on what they know, what would a healthy soil be like?
    1. Introduce characteristics discussed in the background section.
      1. A healthy soil is approximately 45% mineral matter (sand, silt, and clay), 5% organic matter, 25% air, and 25% water.
      2. A healthy soil has good structure.
      3. It is resilient to stresses like erosion and compaction.
      4. It drains well, holds moisture, and holds nutrients.
      5. It has a thriving microbial life that supports soil structure, provides nutrients, and breaks down organic material.
  3. Tell students they will be observing soil samples and taking notes on them.
  4. Instruct students to take out a notebook and turn to a new page. Have them label the page for soil observations.
  5. Prior to class, have shallow tubs or trays with various soil samples set up. Try to keep the aggregates as unbroken as possible. Label each tray with a letter or number. Provide a spray bottle with water, a ruler, and a towel with each tub. Consider also providing an informational sheet like the one linked above in Essential Files about soil texture and how to do a ribbon test.
  6. Hand out the soil ribboning kits (soil in tubs, water bottles, rulers, towels, info sheet) to each group of approximately four students. Tell students they will first be observing without touching.
    1. Tell students to use their science journal/class notebook to write down any and all observations. Consider making sketches of the structure. Prompt them with soil characteristics, like color, shiny or dull, sparkly or slick in appearance, etc.
  7. After a minute or two, tell students they will be handling the soil and making soil ribbons. Tell students that this is a common test for soil scientists, farmers, and agronomists to do in a field to determine soil texture. Soil texture is important to determine because it can impact nutrient application and irrigation rates.
  8. First, have each student take one ped of soil. Tell them to place the ped in the palm of their hand and use the provided spray bottle to get the sample wet enough for it to be workable like play-doh or modeling clay.
    1. Have them work and knead the sample into a small ball. This is called a bolus.
      1. Depending on the soil samples, some students may not be able to work their sample at all. Tell them to record all observations about their experience trying to work an unworkable sample. Why do they think that has happened?
    2. Once their bolus is acceptable, tell students to knead the sample using the side of their forefinger and their thumb into a ribbon. The ribbon should be about a centimeter wide. Their goal is to make a ribbon with the soil as long as possible before the soil breaks from its own weight.
      1. Soils high in sand will not ribbon well. Soils high in clay will ribbon extremely well.
    3. When all students have successfully ribboned their samples multiple times (to verify results), have them measure their ribbon length in centimeters and inches and record in their notebooks.
    4. Point students to the linked soil texture guide from the Carry Institute and have them deduce what texture their soil is.
      1. In order to determine texture, students will need to measure the ribbon and make observations about the look and feel of the samples.
      2. As another visual aid, consider projecting the following graphic on the whiteboard. The soil textural triangle helps break down soil types by the percentage of each sediment type. This triangle is also broken down by ribbon length to help students visualize how clay content directly relates to the ribbon test.
  9. Once students have completed their observations, allow some time for cleanup and recording those observations in the students’ notebooks.
  10. Talk with students about why soil texture matters. Ask students if anyone had a high clay content soil. What would happen if this soil got too dry? It would be hard as a rock! Did anyone have high sand content? Would this soil hold water well? Not very!
    1. Have students consider the purpose of soils. They are to be a medium for crops to grow and obtain nutrients and water through their roots. Soils also need to hold air so as to not drown crops.
    2. The smaller the particle size of soil, the better it holds water and nutrients. However, if the soil is too high content in clay (the smallest particle size), it can hold water too well and create ponds. A balance is best in soil particle size.
  11. Spend some time sharing out with the class about the different soil samples each group had. Have groups share what they observed about their samples, and what some of the consequences of that soil type might be.

Activity 2: Slake test and biological indicators

  1. In the previous activity, students learned that 5% of soil is made up of organic material. Have students brainstorm what might make up that organic material. What is it? Where did it come from?
    1. Some answers may be manure, residual plant matter, insects, animals, and microbes.
  2. Introduce students to the idea that microbes are plentiful in the soil. Decomposing plant and animal matter feed these microbes and allow them to build networks, live, die, and then become part of that organic matter themselves.
  3. If done the same day as activity 1, have students look at the soil samples in their trays. If not done the same day as activity 1, have students turn back to their notes on those samples. What did the structure of their soil samples look like? Did some groups have well structured soil? Did other groups not? What is the shape of the structure? How large are the aggregates? Can students visibly see organisms in it?
  4. Help students remember what characterizes a healthy soil.
    1. A healthy soil is approximately 45% mineral matter (sand, silt, and clay), 5% organic matter, 25% air, and 25% water.
    2. A healthy soil has good structure.
    3. It is resilient to stresses like erosion and compaction.
    4. It drains well, holds moisture, and holds nutrients.
    5. It has a thriving microbial life that supports soil structure, provides nutrients, and breaks down organic material.
  5. Tell students that good structure helps keep soil resilient, and microbial life helps build structure.
  6. Begin setting up the Slake Test. Take your two contrasting soil peds and explain to the classroom how their sources differ. One should have a strong structure from a largely undisturbed soil, and one should have poorer structure from a more disturbed soil.
    1. In agriculture, soils from fields that use cover crops and no-till systems have stronger structure because without disturbing the soil, natural processes (like microbial activity) can help build and repair soils.
    2. Have two large, clear containers filled with water at the front of the room. Each should have large mesh baskets at the top to hold the soil peds in the water.
    3. Instruct students to place themselves in an area where they can see the jars clearly. Tell them to get their science notebooks out to note their observations.
    4. Place the two contrasting peds into the jars at the same time. Consider setting a digital timer or stopwatch so students can record times of their observations.
    5. Review with the class which sample is which. How are they behaving? Are they both holding their structure? Are pieces of the sample flaking off? How big are those flakes?
      1. Point out water quality differences, as well. The water from the soil with poor structure will likely look much dirtier. Individual particles are flaking from the sample instead of smaller clumps. This is also indicative of poor structure.
      2. Tell students to continue observing the samples for a few minutes. Tell them to sketch their observations, as well. Prompt them with questions to promote their note taking.
  7. Ask students what this test tells them. Why would a scientist do this test? Why would they care?
    1. This test measures how well a soil can hold up to being immersed in water. In the real world, rains can come very quickly and saturate or even flood a field. In these extreme weather instances that are becoming more and more common, how well can that soil be expected to recover? How resilient can this soil be in the face of stressors?
    2. Maintaining soil health is imperative in sustainability. We will not discover new arable land to provide more food. We must care for the land we have to continue using it for generations.
  8. Ask students which sample would withstand erosion better. Which sample held its structure better? How can you tell?
    1. The sample from the less disturbed soil would resist erosion better. It was able to take on water better without flaking apart. It had the structure, pore space and pore connectivity to hold water more like a sponge rather than collapsing on itself and falling apart.
  9. Talk with students about how soil structure is created and repaired.
    1. In soils, life begets life. Plant growth creates organic material that can feed insects and microbes, which die and create organic material to feed more insects, microbes, and even vertebrates. This cycle of organic material provides nutrients to the soil and helps plants to grow. It also helps microbes create communities and the glues that help hold the soils together. One of these glues is called glomalin, and is a protein produced by fungi. This and various other organic compounds help stabilize soil and make it strong.
    2. Iowa’s native vegetation is largely prairie. Prairie soils are deep and rich in organic matter, nutrients, and microbial activity. Native prairie grasses have extremely deep root systems that also help hold the soil structures together. The modern practice of using cover crops is an echo of this; by keeping the soil covered and having roots grow in the soil throughout the year, more organic matter can be introduced and the roots can help keep the soil in place.
  10. Iowa is lucky to have the soils we do. Iowa’s least productive soils are still higher quality than soils in many parts of the world. Wrap up class by having students journal about how soil health is important in all parts of the world, and brainstorm ideas to build and/or maintain soil health in a variety of areas (like Iowa, deserts, sandy soils, clay soils, etc.). Allow students to research soils of the world if time and resources are available.

Did You Know? (Ag facts)

  • Iowa’s state soil series is the Tama Soil Series. This series is notable for its windblown sediment (loess), and was first named in 1917 in Iowa.
  • All of Iowa was once covered by glaciers. Glacial till is the parent material of much of Iowa’s soils.
  • Iowa has one of the largest loess deposits in the world. Loess is windblown sediment. The Loess Hills in western Iowa are deposited silt from the bed of the Missouri River.

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

  • Have students bring in various soil samples from home and perform more slake tests. Whose soil has the best structure? Why do they think so?

Suggested Companion Resources (books and websites)



Chrissy Rhodes

Organization Affiliation

Iowa Agriculture Literacy Foundation

Agriculture Literacy Outcomes

  • T4.9-12.d: Evaluate the benefits and concerns related to the application of technology to agricultural systems (e.g., biotechnology)
  • T4.9-12.e: Identify current and emerging scientific discoveries and technologies and their possible use in agriculture (e.g., biotechnology, bio-chemical, mechanical, etc.)

Iowa Core Science Standards:

  • HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
  • HS-LS2-7: Design, evaluate, and refine a solution for reducing the impacts of human activities on the environment and biodiversity.
  • HS-ESS2-2: Analyze geoscience data to make the claim that one change to Earth’s surface can create feedbacks that cause changes to other Earth systems.
  • HS-ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.