Target Grade Level / Age Range:

Grades 9-12

Estimated Time:

Teaching time: 50 minutes

Observation time: 8 weeks


Students will identify characteristics of a healthy soil.


  • Two samples of soil – one from a cultivated field and one from a natural scape
  • White, cotton underwear
  • Shovel or spade
  • Permanent, waterproof marker
  • Stake
  • Hammer
  • Multiple test site with different soils, different vegetations, and different access to water

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

Vocabulary (with definitions)

  • Soil health the continued capacity of a soil to function as a vital, living ecosystem that sustains plants, animals, and humans.
  • Decomposition – decay of plant material from more complicated structures into more simple ones.
  • Soil quality the ability of the soil to function, e.g., to be able to decompose organic matter and release nutrients from it.
  • Carbon cycle the route that carbon molecules make from being incorporated into plant tissue, to dead plant material on/in the soil to being released in the air after decomposition and so on.
  • Climate conditions general term for weather conditions over a longer period.
  • Environmental conditions any condition outside the plant that determines its growth (including weather, soil nutrient level, soil compaction).
  • TBI parameters the TBI (Tea Bag Index) parameters are a characteristic for how much and how fast organic material is broken down.

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

Soil health is the foundation of productive farming practices. Fertile soil provides essential nutrients to plants. Important physical characteristics of soil-like structures and aggregation allow water and air to infiltrate, roots to explore, and biota to thrive. Diverse and active biological communities help soil resist physical degradation and cycle nutrients at rates to meet plant needs. Soil health and soil quality are terms used interchangeably to describe soils that are not only fertile but also possess adequate physical and biological properties to "sustain productivity, maintain environmental quality and promote plant and animal health" (Doron 1994).

According to the (USDA) Natural Resource Conservation Service, "Soil quality is how well soil does what we want it to do." In order to grow our crops, we want the soil to hold water and nutrients like a sponge where they are readily available for plant roots to take them up, suppress pests and weeds that may attack our plants, sequester carbon from the atmosphere, and clean the water that flows through it into rivers, lakes, and aquifers.

Healthy, high-quality soil has:

  • Good soil tilth that allows plant roots to easily penetrate large volumes of soil
  • Sufficient depth
  • Sufficient, but not excessive, nutrient supply
  • Small population of plant pathogens and insect pests
  • Good soil drainage including porous surface with many pores connected to subsoil
  • Active, diverse, and large population of beneficial organisms
  • Contains high levels of relatively fresh residues that provide beneficial organisms with food
  • Includes high levels of decomposed organic matter
    • helps retain both water and readily leachable nutrients
    • net carbon sequestration
  • Low weed pressure
  • No chemicals or toxins that may harm the crop
  • Low to moderate concentrations of salt
  • Resilience to degradation and unfavorable conditions

Soil fertility is only one component of soil quality. Fertile soils are able to provide the nutrients required for plant growth. These are the chemical components of soil. Some plants need certain nutrients in large amounts, like nitrogen, phosphorus, and potassium, which are called macronutrients. Other nutrients, like boron and manganese, plants only need in very small amounts. In high-quality soil, nutrients are available at rates high enough to supply plant needs, but low enough that excess nutrients are not leached into groundwater or present at high levels toxic to plants and microbes.

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

Provide students a sample of soil from the top two to three inches of the soil profile recently collected from a cultivated field and one from a natural scape. Ask students to observe each sample to and decide if the soils are healthy. Have them brainstorm characteristics of what makes a soil healthy or what qualifies a soil for us to consider it healthy. What information would they need to collect from the soil samples (aside from visual observation) to know if the soils are healthy?


  1. On a large writing surface write “What makes a healthy soil?” Explain to students that today they are going to ‘Soil their Undies.’ Select one or more of the following videos to play for the students.
    1. What Underwear Can or Cannot Tell You About Soil Health
    2. Soil Your Undies Challenge
    3. Soil My Undies Challenge: Measuring Soil Biology with a Pair of Briefs
  2. Process the videos by asking questions. Why did the underwear deteriorate? Revisit the question “What makes a healthy soil?” Present the content in the PowerPoint What Makes a Healthy Soil.pptx. Ask processing questions – italicized and in parenthesis below – to facilitate student conversation and deepen understanding.
    1. The Level of Soil Organic Matter Is Maintained (what is organic matter?)
      1. Functionality maintained (substrate, role in soil structure, ground cover) (How can farmers increase organic matter in their fields?)
      2. Net carbon sequestration (How do plants use carbon? How could carbon sequestration help combat climate change?)
      3. Soil organic matter level matching land use and soil texture (Do clay soils need more or less organic matter? Do grasslands need more or less organic matter than fields?)
      4. Soil Fertility Is Optimized
        1. Nutrient additions at least match removals and losses (What kinds of nutrients (NPK) might farmers add? How do farmers know how much nutrient to add? (soil tests))
        2. Fertility is adequate for land use (Should low fertility soil be kept in crop production? Usually the answer is no.)
        3. Nutrient storage capacity is maintained (How much nitrogen or phosphorus can soils hold?)
        4. Minimize nutrient loss off-site (What ways do farmers minimize nutrient loss? Cover crops, buffer zones, bioreactors, etc.)
      5. Optimized Water Entry, Storage and Supply
        1. Water infiltration, permeability and storage maximized to meet land-use needs (What things can help improve water infiltration? Plant roots, worm holes, etc. What things can help improve water storage? Organic matter.)
        2. Drainage is minimized where there is risk of dryland salinity or acidification (How can farmers minimize drainage? Buffer zones)
        3. No constraints to water use (supply function)
      6. Enhanced Soil Biological Function
        1. Resilient and diverse biological community (self-regulating) (What types of living organisms are found in soil? Worms, bacteria, fungi, plants, etc.)
        2. Biota undertake key functions, such as nutrient recycling (What function does bacteria and other microorganisms play? Energy and nutrient recycling.)
        3. Optimal biological functioning (efficient, beneficial, dynamic)
        4. Absence of disease expression (What disease could be present? Fungi, etc.)
      7. Supports Productive Land Uses
        1. Supports plant growth/land use requirements (Should all land be cultivated? Grazed? Built on?)
        2. Maintains resource condition and ecosystem services
      8. Enhances Environmental and Community Health and Well-Being
        1. Healthy soils = healthy food = healthy people
        2. Profitable while conserving soil resource and reducing environmental impact
        3. Fit for purpose (What types of soil support different human or animal activities? Firm ground for building. Flat loamy fields for cultivation. Hilly grassland for grazing. Etc.)
  3. Now that students have an understanding of what makes healthy soil, explain the procedure for Soiling the Undies. Have students pick two patches of ground on school property or elsewhere with permission that can be easily monitored. The patches of ground should have different soil types and different plant communities, if possible.
    1. Students should dig a hole large enough and deep enough to fully bury the underwear leaving only the waistband above ground.
    2. The underwear should be free from any dyes or other chemicals that could inhibit the experiment. White underwear that have been washed in cold water (no detergent) work best.
    3. Once soil is filled back in around the underwear put a stake into the ground to mark the spot.
    4. Write the date on the stake with a waterproof marker.
    5. After eight weeks, dig the underwear up and compare the decomposition of the underwear at the two sites. Note: the temperature should be above 50 degrees for the time frame. Temperatures below 50 degrees will significantly inhibit bacterial growth and other microorganism activity.
  4. Have students write a one-page reflective paper as an assignment to discuss how one change to Earth’s surface (specifically soil health increase or decrease) can create feedbacks that cause changes to other Earth systems (like climate, water cycle, etc.). What geoscience data can they find that supports their position?

Did You Know? (Ag facts)

  • Tama soils occur in 28 Iowa counties in eastern Iowa as well southeast Minnesota, southern Wisconsin, northwest Illinois and western Indiana. The soil series was first identified and named in Black Hawk County, Iowa, in 1917. Today, in Iowa, the Tama soil is identified as the soil series on more than 933,000 acres.
  • Tama soils are formed in wind-blown, dominantly silt-sized particles, known as loess, and are more than 60 inches in depth to an underlying earth material. The topsoil and subsoil have textures of silty clay loam. The soil profile is dominated by silt-sized particles and generally less than five percent sand-sized particles. Clay-sized particles range from 27 to 35 percent in the topsoil and subsoil. The soil developed under native prairie vegetation. The deep penetrating fibrous roots of the grasses produced a thick, dark colored topsoil that ranges in thickness of 12 to 14 inches on slopes that experience minimal accelerated erosion. The topsoil layer has a granular structure that provides for water infiltration into the soil. The soil has excellent internal drainage and is termed well-drained meaning that it does not hold excess water for more than 30 consecutive days during the year. The soil occurs on slope gradients ranging from 0 to 18 percent, however, most Tama soils occupy slopes of 2 to 9 percent. In fact, in Iowa, the map unit Tama silty clay loam, two to five percent slope gradients with a topsoil thickness of more than seven inches occupies more than 478,000 acres.
  • The Tama soil has potential to provide 11 to 12 inches of plant available water in the upper 60 inches of the profile. The soil is highly productive. Currently, under high level management estimated corn yield potential ranges from 230 to 240 bushels per acre. On sloping phases of the Tama soil, two to five, five to nine, and nine to 14 percent gradients water erosion is a major concern where row-crop agriculture is practiced. The silt-sized particles are easily eroded by water where not protected by plant residues or sod-based plants. Therefore, Tama soils are both highly productive for plant growth and easily erodible when not protected by conservation practices.

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

  • Challenge students to create a song and/or a music video to highlight what they’ve learned. See this example for inspiration.
  • Soil is a mineral resource. Have students conduct research to evaluate competing design solutions for developing, managing, and utilizing soil resources based on cost-benefit ratios.

Suggested Companion Resources (books and websites)


  • This material is based upon work supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under number NR196114XXXXC003. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
  • The grant by which this project is funded is administered by the Conservation Districts of Iowa.
  • The Tea Bag Index: a novel approach to collect uniform decomposition data across ecosystems:
  • Iowa State Soil: Tama Soil Series:


Will Fett

Organization Affiliation

Iowa Agriculture Literacy Foundation

Agriculture Literacy Outcomes

  • T1.9-12.b. Describe resource and conservation management practices used in agricultural systems (e.g., riparian management, rotational grazing, no till farming, crop and variety selection, wildlife management, timber harvesting techniques)

Iowa Core Science Standards:

  • 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-ESS3-2. Evaluate competing design solutions for developing, managing, and utilizing energy and mineral resources based on cost-benefit ratios.

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.