Plant-Soil Interactions (Grades 9-12)
Students will explain the roles of diffusion and active transport in moving nutrients from the soil to the plant, describe the formation of soil and soil horizons; and describe the events in the Great Dust Bowl, how they relate to soil horizons, and how those events affected agricultural practices.
- Master 3.1, What Do You Know about Roots? 1 per student and (1 to project)
- Master 3.2, Moving Water and Nutrients into Roots (1 per student)
- Master 3.3, Experiments with Roots (1 to project)
- Seedling Preparation: (For each team of 4 students)
- 1 drinking glass
- 1 hand lens
- 6 pinto bean (or other type) seeds
- 1 cup of water
- 1 paper towel
- Diffusion Demonstration: (For each team of 4 students)
- 1 paper or Styrofoam cup
- 1 large container
- 1 bottle of food coloring
- Water (enough to fill the large container)
- 1 sharp pencil
- Master 3.4, The Plant Vascular System (1 to project)
- 1 paper or styrofoam cup
- 2 pieces of celery stalk
- 1 bottle of food coloring (blue works well)
- Master 3.5, Soil Horizons (1 to project)
- Master 3.6, Where Does Soil Come From? (1 per student)
- Master 3.7, Soil Formation (1 per student)
- Pens or pencils in different colors
- Master 3.8, Disrupting the Soil Horizons: The Dust Bowl (1 per student)
- Master 3.9, Farming Practices (1 per team of 2-3 students)
- Master 3.10, Dust Study (1 to project)
Essential File (map, chart, picture, or document)
dust bowl: a well-known drought that was a natural disaster that severely affected much of the United States during the 1930s. The soil, depleted of moisture, was lifted by the wind into great clouds of dust and sand which were so thick they concealed the Sun for several days at a time. The “dust bowl” effect was caused by sustained drought conditions compounded by years of poor land management practices that left topsoil susceptible to the forces of the wind.
Background Agricultural Connections
Plants use their root systems for structural support, stability, and nourishment. If you have ever seen a tree toppled by high winds, you have some idea of why trees are so stable. The primary function of the root system is to absorb water and nutrients from the soil. To do this, the root system is ever changing over the course of the plant’s life, capable of growing year-round, if conditions for growth are met and there is not competition from the plant’s top system. Roots also may serve as storage organs for starch or sugars. Carrots, beets, radishes, turnips, and potatoes are examples of storage roots.
The growth of roots is similar to the growth of shoots. However, there are important differences. In general, the more extensive a root system is, the more water and nutrients it can absorb. If you examine a root using a magnifying glass, you will see a large number of delicate root hairs growing out from the surface of the root (see Figure 4). This system of root hairs greatly increases the surface area of the root available to contact and absorb water. A single rye plant 60 centimeters tall is estimated to have a root system with a total length of 480 kilometers. Its surface area is more than 600 square meters—twice that of a tennis court!6
The tip of an actively growing root is called the root cap (see Figure 5). The root cap produces a slimy secretion called mucilage that helps lubricate the root as it pushes its way through the soil. Just behind the root cap is the zone of active cell division, and behind it is a zone of cell elongation. The cells of the elongation zone grow by taking in water and swelling. The root cells contain salt and sugars. Because the root cells contain more solutes than the water in the soil, water flows into the cells by diffusion. This causes the cells to elongate, forcing the root deeper into the soil. Behind the elongation zone is the zone of cell differentiation. The cells in this area give rise to the cells of the vascular system, which transport water up the stem and sugars down from the leaves.
Roots may stop growing during the winter not because they have become dormant like the buds at the top of the plant, but rather because the temperature is too cool to support growth. In order for roots to grow, they must have adequate moisture and temperature. Many people are under the misconception that roots grow in search of water. This is not the case. Roots can only grow where the conditions are suitable for growth. This means that roots grow where water is already present.
The transport of water and nutrients into, within, and out of a plant depends on three important processes:
- DIFFUSION refers to the movement of a liquid or gas from a region of higher concentration to one of lower concentration. This movement is a natural consequence of random molecular movement and does not require added energy to accomplish. During photosynthesis, carbon dioxide moves down its concentration gradient to enter a leaf cell. At the same time, oxygen moves down its concentration gradient to exit the leaf cell.
- OSMOSIS is a process similar to diffusion but refers to the movement of water. When water enters plant roots, it moves down its concentration gradient since the concentration of water is higher in the soil than in the root tip.
- ACTIVE TRANSPORT refers to the movement of a liquid or gas from a region of lower concentration to an area of higher concentration. This movement against a concentration gradient can only be accomplished by using energy to help molecules move opposite the direction that diffusion would take them.
- WATER is absorbed by the root hairs and brings along with it any chemicals, including nutrients that are dissolved in it. Most nutrients are present in higher concentration in the root hairs as compared with the soil water. Active transport is used to move the nutrients deeper into the root system until they reach cells of the vascular system. The importance of active transport can be demonstrated by exposing plants to a chemical that interferes with cellular respiration. Without a supply of energy-containing ATP molecules produced through respiration, the rate of nutrient movement slows greatly.
The Plant Vascular System
Although plants do not have a circulatory system like humans, they still must transport material from one part of the organism to another. The plant stem contains a vascular system that connects the leaves to the roots. The plant’s vascular system is composed of xylem tissue that transports water from the roots to the rest of the plant and phloem tissue that transports sugars produced in the leaves to the non photosynthetic parts of the plant (see Figure 6). The xylem is composed of dead cells that form long, empty tubes. Some tubes are wide, and others are narrow. The cell walls within the tubes are either missing or contain a series of holes that permits the passage of water. The cells that gave rise to the xylem lay down thick cell walls that contain a polymer called lignin. Lignin lends strength to the xylem and prevents it from collapsing under pressure.
The capability of xylem tissue is truly amazing. In the case of the tallest trees, water must be transported from the roots up, over 100 meters and against gravity, to the leaves. Water is thought to move through the xylem by a process known as cohesion-tension. According to this view, water can be pulled upward if the diameter of the tube is sufficiently small and that the column of water is continuous, that is, without air bubbles. A further requirement is that the tube be made of a material to which water molecules will adhere. Within each xylem tube, the water molecules are attracted to adjacent water molecules, forming an unbroken chain. The plant loses water through evaporation from its leaves by a process called transpiration. As water is lost, a negative pressure or tension is created that pulls water up from the xylem. Transpiration is the process that drives the transport of water from the roots up through the stems to the leaves.
While water is moving up the plant, sugars and amino acids must move from the leaves downward to the non photosynthetic parts of the plant. Phloem tissue is composed of tubes made from living cells called sieve cells. Holes at the ends of their cell walls form sieve plates. The cytoplasm of one sieve cell connects with the cytoplasm of adjacent sieve cells through these holes, forming a continuous cell-to-cell sieve tube. As the sieve cells mature, they lose their nuclei and other organelles.
Beside each sieve cell is a smaller companion cell that has a nucleus. The companion cells are thought to regulate the activity of the sieve cells.
Experiments have demonstrated that this movement occurs at a rate that is thousands of times faster than could be achieved by diffusion. Sugars are thought to move through the phloem by a process called pressure-flow. According to this view, water and dissolved sugars flow through sieve tubes from areas of higher pressure to ones of lower pressure. Sugars made in the leaves are transported into the phloem by active transport. The high concentration of sugar causes water to flow into the phloem cells, increasing what is called the turgor pressure within the cell. This high turgor pressure forces the sugar-water solution into the adjacent phloem cell, increasing its turgor pressure. This process repeats, moving from cell to cell until the solution reaches a cell where it will be used. Once at its destination, the sugar is removed from the phloem by active transport. Water, too, is removed from the phloem cell, regenerating the lower turgor pressure needed to keep the flow moving.
The Dust Bowl
The importance of maintaining healthy soil was made clear by the events of the so-called Dust Bowl that affected a large area of the Great Plains in the 1930s. During the late 1800s, an unusual amount of rain fell on the Great Plains. This led farmers and agricultural experts to overestimate how much rainfall the region could expect. This unusually wet period caused more people to settle in the area and begin farming. In 1930, an extended drought began, which caused crops to fail. High winds carried massive amounts of topsoil eastward. Throughout the 1930s, the area, including the Texas and Oklahoma panhandles as well as parts of New Mexico, Colorado, and Kansas, experienced a series of huge dust storms. Some of these storms blew dust all the way to Chicago and eventually to Cleveland, Buffalo, Boston, and New York City. During the winter of 1934–1935, red snow fell in New England.19
A number of factors worked together to create the Dust Bowl. Certainly, an extended period of high temperatures, wind, and drought were important. However, people too played a part. Early settlers used the land for grazing livestock. Later, as mechanized farming began to spread, many farmers used deep plowing techniques that eliminated the native grasses that held the soil together. High grain prices during World War I caused farmers to plant even more crops, which made the problem worse. Therefore, when the drought hit, the topsoil simply blew away.
To help prevent erosion, the federal government supported the planting of millions of trees from Canada to Texas. These trees helped to anchor and protect the soil. Farming practices too were modified. Farmers began to use a technique called contour plowing that helped the soil retain water. They also began to allow portions of their fields to lie fallow each year to help the soil regenerate.
Interest Approach - Engagement
- Ask students questions to draw on prior knowledge by asking the following questions:
- What is diffusion and active transport?
- How are soil nutrients moved from the soil to the plant?
- What was the Great Dust Bowl? When did it occur? Why did it happen?
- After your class discussion inform your students that in this lesson they will:
- explain the roles of diffusion and active transport in moving nutrients from the soil to the plant;
- describe the formation of soil and soil horizons; and
- describe the events in the great Dust Bowl, how they relate to soil horizons, and how those events affected agricultural practices.
Activity 1: From Soil to Roots
In Step 5, students are asked to observe the root systems of young seedlings. For this activity, any type of seeds may be used so long as the roots have grown about 1 or 2 cm. Pinto bean seeds are easy to obtain and work well. To germinate the seeds, place several seeds in a row along one side of a paper towel as shown in Figure 3.1a. Carefully roll up the paper towel from bottom to top. Place the rolled paper towel into a glass of water so that the seeds are at the top and out of the water glass (Figure 3.1b). Water will wick up through the paper towel and keep the seeds moist. Prepare enough seedlings so that each team of 4 students will have a seed to observe. Assume that just 1/2 of the seeds you prepare will germinate. Set the glasses of seeds in a location where they will not be disturbed. The seeds will need approximately 5 to 6 days for the roots to grow enough for observation. During the germination period, be careful to replace any water that is lost through evaporation. n
Students learn about plant roots and their role in obtaining water and nutrients from the soil for plants to use.
- Remind students that there are air spaces in soil. These air spaces can be filled with water containing dissolved nutrients. Ask, “How does the plant obtain nutrients from the water that is in the soil?”
- Students’ responses will vary. If necessary, guide the discussion to mention the plant’s root system.
- Display a copy of Master 3.1, What Do You Know about Roots? Use a piece of paper to cover all but the first statement. Read the first statement and ask the students to indicate by a show of hands whether they agree or disagree with the statement.
- This discussion is designed to help you assess the students’ prior knowledge of the topic. If necessary, review for the class the essential features of diffusion and active transport.
- Molecules move randomly due to their kinetic energy.
- This movement causes molecules to intermingle.
- The net movement of molecules is from an area of higher concentration to one of lower concentration.
- The net movement of molecules stops when the concentration of the molecules is the same everywhere.
- The movement of the molecules comes from their kinetic energy and does not need additional energy (unlike active transport).
- Active Transport
- Active transport is a process used by cells to move molecules from an area of lower concentration to one of higher concentration.
- It requires energy.
- If your students already have been introduced to the energy molecule ATP, you may mention it as the source of energy for active transport.
- This discussion is designed to help you assess the students’ prior knowledge of the topic. If necessary, review for the class the essential features of diffusion and active transport.
- Continue revealing the rest of the statements, one at a time, and asking students whether they agree or disagree with the statements.
- After students vote on each statement, ask for 1 or 2 volunteers to explain why they voted as they did. At this time, do not correct wrong answers. The students will come back to these statements later in the lesson. Answers are found and revealed to students in Step 15.
- Explain that they will now investigate the mechanism by which roots obtain nutrients from the soil. Divide the students into teams of 4. Pass out to each team a young seedling (taken from the paper towel germination) and a hand lens.
- This activity refers to the way that most plants obtain their nutrients through the root system. Legumes that carry out nitrogen fixation in their roots are a special case and are not dealt with here.
- Instruct the students to take a minute to observe the seedling’s root system with the hand lens and write down their observations on a piece of paper.
- The root hairs are white and very fine. Provide a dark background against which the root hairs are more easily visible.
- After the students have recorded their observations, ask volunteers to describe what they saw.
- Students will report seeing one large root emerging from the seed. They also will describe fine white hairs growing out from the root.
- Remind students of the first statement from Master 3.1, What Do You Know about Roots? “Plant roots have tiny hairs that absorb water.” Ask, “Why do you think that plants have so many root hairs?”
- Student responses will vary. Guide the discussion to bring out that more root hairs mean more surface area with which to contact water and nutrients in the soil. In Lesson 2, students learned about particle size and surface area. In this case, the small projections on the root are another example of the importance of increasing surface area.
- Ask students, “How do nutrients in the soil water get into the root hairs?”
- Students’ responses will vary. At this time, accept all answers.
- Explain that students will now investigate the process by which water enters the root hairs. Keep the class in their teams. Pass out to each team 1 copy of Master 3.2, Moving Water and Nutrients into Roots.
- Ask students to read over the procedure on the handout. Explain that the cup represents the root hair, the larger container represents the water in the soil, and the food coloring represents the nutrients dissolved in the water.
- After students have completed their investigations, reconvene the class and ask volunteers to explain what happened when the holes were poked through the cup.
- Students will report that the colored water slowly entered the cup.
- Ask students :
- “Why did the colored water enter the cup?”
- Students’ responses will vary. Guide the discussion to bring out the fact that although the concentration of water was the same on both sides of the cup, the concentration of the food coloring was higher outside the cup compared with inside the cup.
- “What is the process called where a substance moves from an area of higher concentration to an area of lower concentration?”
- The process of diffusion was summarized in Step 2. Students should recall that diffusion involves a net movement of a substance from an area of higher concentration to one of lower concentration.
- “Where does the energy co me from to drive this process?”
- Students should recall from the discussion in Step 2 that the kinetic energy of the molecules in solution drives the process.
- “Why did the colored water enter the cup?”
- Display a copy of Master 3.3, Experiments with Roots. Cover the bottom section with a piece of paper. Reveal the first experiment and read it aloud. Ask the students what this data tells them about how nutrients move from the soil into the roots.
- Since the concentrations of some essential elements move from an area of low concentration to one of higher concentration, this suggests that energy was required for the movement and the process involved was active transport.
- Reveal the second experiment, read it aloud, and discuss its meaning.
- Students should recognize that since the chemical halts ATP synthesis there would not be energy available to support active transport. Without active transport, those essential elements that depend on active transport to reach high concentrations will exhibit much lower concentrations in the root hairs as compared with the first experiment. Other essential elements that are transported by diffusion will be expected to have their concentrations unchanged.
- Conclude the activity by displaying Master 3.1, What Do You Know about Roots? once again. As before, ask students to indicate by a show of hands whether they agree or disagree with each statement. Ask volunteers to explain why they changed their minds about their answers.
- Students should be able to respond to the statements about roots as follows:
- Before holding a class discussion, give each student 1 copy of Master 3.1, What Do You Know about Roots? Instruct students to write on their copies of Master 3.1 why each statement is true or false. Students should include specific evidence from the lesson that supports their conclusions. Students can use their answers during the class discussion. You may also wish to collect students’ papers to assess their understanding.
Activity 2: From Roots to the Plant
Use a sharp knife to cut celery stalks into pieces approximately 5 cm (2 inches) long. Make sure that the cut surfaces are flat and will allow the celery to rest upright when placed into the paper cups. Approximately 2–3 hours before class begins, put one piece of celery into a cup containing the food coloring. Wrap the other piece of celery in plastic wrap until needed.
This activity helps students think about how plants have specialized tissues for moving water and nutrients from the roots to all other parts of the plant.
- Explain that getting nutrients into the plant roots is an important first step . Ask students, “How does water, and the nutrients it contains, get from the roots to the rest of the plant?”
- Accept all reasonable answers at this time.
- Hold up a piece of celery and a cup containing food coloring . Ask students to predict what the celery will look like after it has been in the food coloring solution for a while.
- Do not correct misconceptions at this time. Students will come back to this in Step 4. If students are unfamiliar with the existence of a vascular system in plants, they may predict that the entire stalk of celery will be blue inside. If they know that plants have a vascular system, they would predict that there are specific places within the stalk that are blue (blue dots seen on the surface of the cut end).
- Display a copy of Master 3.4, The Plant Vascular System. Briefly review the information on the master so that students understand that plants have specialized mechanisms for moving water and nutrients.
- Reconvene the class and hold up the piece of celery that has been in the food coloring. Ask volunteers to use what they have learned about the plant vascular system to explain why their earlier predictions were or were not accurate.
- Students should see that the food coloring was transported up the celery stalk and was visible as a series of colored dots along the top of the stalk. Some students may have seen this demonstration before and remember what the result is, but have not thought about what it means about the existence of a specialized mechanism for transporting water and nutrients.
- Reinforce that the movement of water took place through the plant’s xylem system, which explains why the food coloring was present in discrete places in the celery.
- Conclude the activity by reminding students that photosynthesis produces sugars in the leaves. Ask them how the sugars, needed for energy, reach the lower parts of the plant.
- Students should recall from Master 3.4, The Plant Vascular System that phloem tissue is used to transport sugars downward from the leaves. You can point out that in the case of the celery stalk, the xylem and phloem tissues lie next to each other in structures called vascular bundles.
Activity 3: Soil Formation and Horizons
This activity introduces students to soil formation. The way in which soil is formed results in layers, called soil horizons.
- Display a copy of Master 3.5, Soil Horizons. Ask students to make observations about the soil they see in the pictures.
- The main observation that students should make is that there appear to be layers in the soil that differ by color and thickness. Explain that soil includes layers and that the layers are called soil horizons. Give each student a copy of Master 3.6, Where Does Soil Come From? Ask students to work in teams of 2-3 to make and record observations.
- Give teams 2–3 minutes to write brief observations. In the next step, students will get additional information.
- Give each student a copy of Master 3.7, Soil Formation. Instruct them to use the information on this master to add information and detail to the descriptions they wrote on Master 3.6.
- Encourage students to talk with their team members as they work through this. It may be helpful if students use a different color pen or pencil to make their changes or additions. In this way, they can easily see how their initial observations compare with the new information they are adding. Students can also make notes on Master 3.7 to cross-reference the steps on Master 3.6.
- It may be helpful to point out to students that the cross-section of soil represented on Master 3.6 is the same total thickness throughout. Soil is formed through the breakdown of bedrock and addition of organic materials. It is not formed only by the addition of material on top of bedrock.
- Conclude the activity by reviewing the information on Masters 3.6 and 3.7. Explain to students that horizons in soils from different areas are likely to be different—some thicker or thinner.
- This is also an opportunity to go back to what students have learned about soil in Lesson 2. In that lesson, students learned about soil texture and the dependency that plants have on getting water and nutrients from the soil. In this activity, students have learned about the soil horizons in which roots grow.
Activity 4: The Dust Bowl
In this activity, students investigate the relationships between farming practices and protecting the topsoil.
- Remind students that in the previous activity they learned about the formation of soil and soil horizons. Explain that although it takes many years to form fertile soil, it can be destroyed in a relatively short time.
- Ask, “What factors do you think contribute to the disruption of the soil horizons?”
- Student responses will vary. Some will mention natural factors such as weather (hot and dry) while others may mention human influenced factors such as farming practices.
- Explain that students are going to investigate a severe example of soil destruction that occurred in the Great Plains region of the United States back in the 1930s.
- Ask students to refer back to their copies of Master 3.6, Where Does Soil Come From? and Master 3.7, Soil Formation, and ask , “Which soil horizon is the most important to plant health?”
- Students should recall that the topsoil (layer A) contains minerals and organic materials that are important to plant health.
- Pass out to each student 1 copy of Master 3.8, Disrupting the Soil Horizons: the Dust Bowl. Instruct students to read the handout and on a separate piece of paper describe how the dust bowl was related to:
- weather conditions
- economic conditions
- farming practices
- If appropriate, you can assign this for homework.
- After students have completed their assignment, reconvene the class . Ask volunteers to report how the weather, economy, and farming practices contributed to the creation of the Dust Bowl.
- Students should report the following:
- Explain that the most important government response to the Dust Bowl was to encourage farmers to change their farming practices . To conclude the activity, teams will look more closely at farming practices and their effects on the soil.
- Arrange students in teams of 2–3. Pass out to each team 1 copy of Master 3.9, Farming Practices. Instruct the teams to read the short descriptions of farming practices and to follow the directions on the handout.
- Give teams about 10–15 minutes to discuss the farming practices and write down their conclusions.
- Ask for volunteers to report their conclusions about each farming practice.
- Teams should report the following:
- Crop rotation addresses the problem of nutrient depletion. By rotating crops with different nutritional requirements, one crop can restore to the soil an essential element that was removed by the previous crop.
- Strip farming addresses the problem of soil erosion. The roots help hold the soil together. Positioning the cultivated strip perpendicular to the prevailing winds minimizes erosion from the bare strips.
- Contour farming addresses the problem of water runoff. The furrows made by the plow (perpendicular to the slope) serve as dams that slow water runoff during rainstorms.
- Teams should report the following:
- Explain that return of rain to the Great Plains, the end of the Great Depression, and the implementation of better farming practices helped the country recover from the Dust Bowl. Ask, “Do you think the US could ever experience another Dust Bowl? Why or why not ?”
- Student responses will vary. Some students may believe that the lessons learned from the 1930s will enable today’s farmers to avoid the mistakes of the past. Other students may be concerned that global warming may lead to another Dust Bowl.
- Project Master 3.10, Dust Study. Ask a volunteer to read the study aloud to the class. Ask, "does this data cause you to revise your thinking about a return of the Dust Bowl?”
- Some students may be surprised by these findings and think that another Dust Bowl is more likely than before.
- Explain that scientists believe that this increase in dust emissions may be due to several factors including increased windstorm frequency, drought cycles, and changes in land use patterns. This movement of dust can have significant effects on both the area where the dust is removed and the area where it is deposited.
Concept Elaboration and Evaluation
After conducting these activities, review and summarize the following key concepts:
- The events of the Dust Bowl contributed to changes in agricultural practices to prevent another similar disaster from taking place.
- Conservation Management practices are used in many areas of agriculture to minimize the negative impact of agriculture on the environment.
- Nutrients flow through our ecosystem through natural cycles.
Ask students to write a short paper that describes how the plant vascular system is similar and dissimilar to the human circulatory system. Students’ descriptions should include the following: Similarities Both systems use a series of tube-like structures to transport material throughout the organism. Both systems use diffusion to move nutrients and oxygen gas (O2) into cells. Plants have separate systems for moving water up the plant (xylem) and for moving food down the plant (phloem). Humans have a separate system for moving oxygenated blood (arterial system) and non-oxygenated blood (venous system). Dissimilarities The human circulatory system uses the heart to pump blood, while the plant vascular system lacks such an organ. Blood in the circulatory system contains cells, while the sap in the plant vascular system does not contain cells. Capillaries join the arterial and venous systems, but there are no similar structures in the plant vascular system.
If students are interested, they could do additional research on the Internet to learn more about topsoil depletion in different areas of the world and how topsoil depletion in one area can have consequences in a very different part of the world. For example, scientists can track dust traveling from Africa across the Atlantic Ocean. Some studies are finding wide-ranging effects, including damage to the health of coral reefs in the Caribbean.
This lesson is the third in a series of five related lessons. Refer to the following lessons for further depth.
- Nutrients for Life Foundation
- BSCS-Biological Science Curriculum Study
- Reviewed by Smithsonian Institution
Suggested Companion Resources
- Nutrients for Life eLessons
- Children of the Dust Bowl: The True Story of the School at Weedpatch Camp
- Dust Bowl Diary
- Out of the Dust
- Survival in the Storm
- The Great American Dust Bowl
- Dust Bowl: CBS 1955 Documentary
- FDR's Fireside Chat: Dust Bowl
- Hugh Hammond Bennett: The Story of America's Private Lands Conservation video
- Living Soil Film
- Soil, Not Dirt
- Dirt-to-Dinner: Food Matters
- Learn How To Compost
- Soil Health Education Resources
- Soil Science Society of America
- Unlock the Secrets in the Soil
- Web Soil Survey