Students examine the components of different soils and recognize how sand, silt, and clay particles affect air space and water absorption. Grades 6-8

infiltration: to pass through a substance by filtering or permeating

loam: ideal garden soil that has a well-balanced mixture of sand, silt, and clay

percolation: the movement of water within and through the soil

permeability: the ability of soil to allow the passage of water

porosity: the percentage of soil volume that is not occupied by solids

As with agriculture, we generally don’t think much about the soil. In fact, “soil” has a negative connotation. We call it dirt and wash it off our clothes and our bodies. In reality, the soil is essential to our survival and that of nearly every organism on Earth. Our planet is mostly made of rock with an iron-nickel core. Plants and animals, including us, occupy a thin veneer on its surface. Our existence is possible because of a thin layer of soil that comes between the planet’s rocky interior and us.

Soils are slowly produced by the weathering of rock. Constant exposure to wind and rain cause the rocky crust to slowly break down into smaller particles. It can take centuries to produce fertile topsoil. As rainwater seeps into cracks, temperature extremes cause the water to freeze. The rock expands, contracts, and fractures. These weathering actions are helped along by organisms that live on and in the soil. Soils are composed of inorganic material derived from rock and organic material derived from living and dead organisms. Both are important to support plant growth. Some scientists believe that without life, soils are just dirt.

As weathering breaks down inorganic material, particles of various sizes are produced. Soil texture refers to the relative proportions of different-sized particles found in the soil. Scientists classify soil particles into three categories. The smallest particles, which measure less than 0.002 millimeters, are called clay. Clay is important in holding nutrients. Clay particles form plate like structures that act like magnets, holding nutrients until they are displaced by another element, absorbed by a plant root, eaten by a soil microbe, or chemically absorbed into the soil. The next largest particles are called silt. Silt particles range in size from 0.002 millimeters to 0.06 millimeters. Sand refers to the largest particles. Sand grains range in size from 0.06 millimeters to 2 millimeters. Soils vary in their proportions of clay, silt, and sand. Soil scientists classify different soil types using the soil triangle. Each side of the soil triangle represents the amount of a soil component, clay, silt, or sand. The relative amounts of these three particle sizes intersect within the triangle and determine to what type of soil those proportions correspond.

The ability of a soil to accept and retain water is largely determined by the relative amounts of clay, silt, and sand present. Porosity refers to spaces in the soil that can hold either air or water. Permeability is defined as the rate at which water can travel through soil. Table 8 lists properties of particle size that relate to soils’ interactions with water. Soils with desirable properties for farming are called loams. Loamy soils typically contain about 50 percent air space, which allows root systems to “breathe” (i.e. obtain O2 for respiration). The solid half of soils is about 90 percent minerals and10 percent organic material. Usually, loamy soils have names that more accurately reflect their composition, such as clay loam or silt loam. Although the organic fraction of most soils is small in volume compared to the mineral fraction, it plays an important role in supporting plant growth. The organic material is composed of living organisms, plant roots, and plant and animal residue. A single gram of healthy topsoil may contain 100 nematodes (small roundworms),1million fungi, and 1 billion bacteria. Present in smaller numbers may be earthworms and a wide variety of insects. Organic material contains a significant amount of nutrients, and it, together with plant roots, help

• decrease erosion;
• increase water infiltration and storage;
• act as a pH buffer (to maintain an acid-base balance);
• decompose organic material, releasing nutrients;
• recycle carbon, nitrogen, and other nutrients; and
• retain available nutrients such as metal ions. 

The soil is a “bank” for nutrients that are taken up by plants, and these nutrients must be replenished for continued plant growth. Before the advent of modern agriculture, farmers relied solely upon tillage to break down existing organic material and release existing soil nutrients. This practice is still used in many less developed countries.

1. Ask students questions to assess their prior knowledge. Questions could include:
   • What is in soil? Are there different types of soil?
   • What is the purpose of soil?
   • Does soil contain organic or inorganic material? Does it contain air?
   • What affects a soil's ability to hold water?
2. After completing this lesson, students will be able to:
   • list aspects of soil composition,
   • appreciate that soils are living and dynamic,
   • recognize that soils vary in composition,
   • describe where nutrients in soils come from,
   • recognize that plants take up water and nutrients from the soil, and
   • recognize that growing crops can deplete agricultural soils of nutrients.

Preparation:

Soil separation. In Step 6, students are asked to observe three different soil types (potting soil, local soil, and sand) that have been mixed with water and allowed to settle. For this demonstration, clear plastic,12-oz bottles work well. Fill each bottle about 2/3 full of soil. Place potting soil, local soil, and sand in separate bottles. Add water to near the top of each bottle. Place caps on the bottles, shake the contents well, and place the bottles in a location where they will not be disturbed. Prepare at least one day before making observations.

Group 1: Dry soil investigation. Make available potting soil and local soil, enough to be contained in the center circle of Master 2.1, Dry Soil Investigation (about 1 tsp for each group of 4 students). Also have hand lenses available.

Group 2: Soil and air space. Make available three clear test tubes that can hold 50 mL. If these are not available, you can use graduated cylinders. Make available at least 1 oz (30 mL) each of potting soil, local soil, and sand. Also have available a ruler and a container that holds at least 120 mL of water.

Group 3: Soil and water. Make available three 100-mL graduated cylinders that are clear or translucent. Make available at least 120 mL each of potting soil, local soil, and sand. Also have available a container that holds at least 120 mL of water.

Activity 1: Properties of Soils

1. Ask students, “Aside from essential elements, what else do you find in soils?” Write students’ responses on the board or on an overhead transparency.
   • At this time, accept all answers. Student responses may include rocks, sand, clay, insects, worms, bacteria, bits of wood, and water. If necessary, point out that these materials contain many of the essential elements.
2. Ask students, “How would you categorize the components of soils?"
   • Student responses will vary. Guide the discussion to bring out the fact that soil consists of nonliving inorganic material such as clay, silt, and sand as well as living and nonliving organic material such as dead plant material, bacteria, insects, and worms.
3. Ask students, “How do soils help plants grow?” Write student responses on the board.
   • Student responses will vary. Guide the discussion to bring out the following:
     • Soils provide support for plants’ root systems.
     • Soils provide essential nutrients.
     • Soils hold water and make it accessible to plants.
4. Ask students, “Can healthy soil support the growth of crop plants forever, or does it ever go ‘bad’?”
   • Student answers will vary. If not mentioned by a student, guide the discussion to bring out the fact that soils are like a “nutrient bank” and that crop plants growing in them make constant “withdrawals.” Over time, the nutrients in a soil become depleted because the nutrients are removed from the ecosystem in the harvest. These nutrients must somehow be replenished (such as through the application of fertilizers) if the soil is to regain its ability to support healthy crop growth. Explain that plants growing in forests, wetlands and other non-agriculture ecosystems return their nutrients to the soil where they are recycled by soil organisms and reused by plants. In agricultural systems, soil nutrient retention may be promoted by planting cover crops or utilizing no-till systems that return plant matter to the soil.
5. Explain that a healthy soil can take hundreds of years to form and it is a precious natural resource. Ask students, “What happens to the environment when an agricultural soil loses its ability to support crops grown by farmers?”
   • Student responses will vary. Students may respond that when a soil stops supporting crop plant growth, farmers will have to clear additional land to grow their crops. You may point out that this is happening in some agricultural soils that formerly supported rain forests. Here, farmers grow food on land until they deplete it of nutrients. They must then clear additional land for their crops. Lack of plant growth increases the rate at which erosion takes place. Over time, such erosion can produce desert areas. Students may have heard of the dust bowl that was created in the North American plains during the 1930s. Drought conditions killed the crops and, without either the crop plants or the natural prairie vegetation that farming replaced to hold the soil together, the topsoil was blown away by high winds.
6. Show the class the bottles of potting soil, local soil, and sand that were previously mixed with water and allowed to settle. Explain how they were prepared. Ask students to gather around the bottles and make observations about the different soils.
   • Students will observe that the different soils separate differently. At this point, students will not know what is found in each layer. They should record their observations and refer back to them later in the lesson.
     • The potting soil will show a thick layer of dark material on the bottom, a thick layer of cloudy water, and a thinner layer of organic material on the top.
     • Local soils may differ, but a typical soil will show layering similar to potting soil, though there may be less organic material floating on the surface.
     • Most of the sand will form a very thick layer on the bottom of the container. There will be a thick layer of clear water and a very thin layer of material on the surface.
7. Remind students that soils contain both organic and inorganic material. Ask, “Can you identify the organic material in each container?” Responses will vary. If necessary, explain that the organic material is less dense than the inorganic material and floats on the surface of the water.
8. Explain to students that the cloudiness in the water comes from inorganic particles called clay that are so small that they can remain suspended in the water. Point out that most of the nutrients in the soil are found in the organic material and the clay.
9. Ask students, “Do all soils support the growth of plants equally well?”
   • Most students will recognize that since soils differ in their amounts of organic material and clay, they will vary in their ability to support plant growth.
10. Explain that they are now going to investigate some other properties of soils that affect plant growth. Divide the class into groups of 4 students and direct them to their work areas.
    • Student groups will explore three different aspects of soil. Depending on the size of your class, there will be two or three student groups assigned to each of the three different activities. Therefore, you will need to set up multiple lab stations.
      • Group 1: Dry soil investigation.
      • Group 2: Soil and air space.
      • Group 3: Soil and water.
11. Pass out the appropriate masters to the groups as follows:
    • Group 1: Master 2.1, Dry Soil Investigation (1 copy per student in group) Master 2.2, Graphic Organizer (2 copies per group)
    • Group 2: Master 2.3, Soil and Air Space (1 copy per student in group)
    • Group 3: Master 2.4, Soil and Water (1 copy per student in group)
12. Instruct students to follow the directions on their handouts, record their observations, and answer any questions.
    • Give students approximately 15 minutes to complete their investigations.
    • Collect students’ answers to the questions posed on Masters 2.1, Dry Soil Investigation, 2.3, Soil and Air Space, and 2.4, Soil and Water.
    • See attached answer key for discussion questions.
13. After the groups complete their investigations, reconvene the class and ask each group to take turns reporting their results.
    • Student reports will vary. For each type of investigation, summarize the results on the board or an overhead transparency. As necessary, ask guided questions to bring out the following:
      • Group 1: Dry Soil Investigation:
        • Soils differ in their composition.
        • Soils contain organic and inorganic particles of varying size.
        • Soils contain microorganisms that cannot be seen but are critical to plant growth.
        • Visual inspection cannot fully evaluate the nutrient content of soils.
      • Group 2: Soil and Air Space: As water was slowly added to the soil samples, students should have noted that both the potting soil and the local soil produced air bubbles that rose to the surface. Fewer air bubbles would be seen when water was added to sand. After the water was allowed to percolate into the potting soil, students should have observed that the final water level was approximately halfway between the surface of the soil and the line drawn on the test tube. This means that the potting soil contained about 50 percent air space. The local soil also would contain a significant amount of air space, though it may be less than the potting soil. The sand would display only a small amount of air space, depending on the grain size. Make sure to bring out the following points:
        • Soils differ in the amounts of air space that they contain.
        • Average soils that support crops consist of nearly 50 percent air space. An astute student may recognize that wetland rice is an important exception to this general rule.
        • The air space in soil can be occupied by either air or water.
        • Soils need both air space and water to support a plant’s root system.
        • Plant roots absorb nutrients from the soil water.
      • Group 3: Soil and Water: As water was slowly added to the potting soil, students should have noted that water was immediately taken up by the soil and that some water reached the bottom of the graduated cylinder in less than 1 minute. The results with local soil would vary, depending on its composition. Most soils would accept the water less quickly than the potting soil, and the rate at which the water percolates through the soil would be somewhat slower. The sand will accept the water almost as quickly as potting soil. Make sure that students recognize that differences in soil texture mean that soils differ in their ability to accept water (infiltration) and transmit it (percolation).

After conducting these activities, review and summarize the following key concepts:

• Properties of soils that are important to support plant growth:
  • The soil is firm enough to support the plant’s root system.
  • The soil contains the essential plant nutrients.
  • The soil contains adequate amounts of organic material and clay.
  • The soil contains about 50 percent air space.
  • The soil allows water to infiltrate and percolate through it.

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