Related National Standards
Grade Level: 7-12
Estimated Time: One class period to watch each of the videos. One class period to mix the soil and plant seeds. One month or more to collect data from the plants. One period to graph and analyze the data.
The difference between the soil of the Amazon and soil found in more temperate climates is striking. In this lesson, students will study plant growth in soils with different nutrients and physical compositions. By examining data, students will come to understand the effects of nutrient depletion in the Amazon and the impact of tree and plant removal from Amazon soils. Students will be able to describe slash-and-burn agriculture and its impact on the environment.
- Students will grow plants in soil representing that found in the Amazon basin, and compare results to plants growing in fertile soil and soil fertilized with leaf compost.
- Students will compare the growth of plants in simulated Amazon soil compared to soil prepared by the slash and burn method.
- Students will chart, average and graph plant growth by height.
- Students will compare leaf development in plants grown in different soils.
- "Journey To Amazonia" video
- Flats used by greenhouses to sprout plants or small plastic cups.
- Potting soil
- Agricultural clay (montmorillinte or laolinite is better)
- Decayed leaves
- Small pieces of wood sufficient to make a small fire
- Supply of seeds (FastPlants from Carolina Biological Supply Company or radishes work well)
- Florescent light or other light source
Begin the presentation of the unit on soils by taking the students outside to observe the soil horizons in your area. Dig a soil pit and identify the O, A, B and C horizons if possible. Have students examine soils taken from each horizon with a hand lens and describe what is found.
Describe the makeup of the soil in the Amazon and provide a short history of the continent. You may use short segments of "Journey To Amazonia" to show the layers of clay along the Amazon riverbanks. Discuss the lack of nutrients in the Amazon soil and have the students compare potting soil (or soil from your area) with the agricultural clay.
Break students into groups of two to four and distribute the planting flats. Planting flats are the containers used by greenhouses to sprout plants; each can accommodate between four and twenty plants, depending on the size of the flat. Students will fill all the sections in one flat with the same kind of soil and label the container. Ideally, each group will grow a flat of plants in each soil type, but soil types may be assigned to groups if space or monetary restraints dictate. One seed should be placed in the center of each section. Students will then take readings over the period of the experiment.
Teachers should explain that scientists take large numbers of readings in biology experiments and average results to better insure a correct interpretation of the results. Each group will take their own data and combine it with data from the rest of the class for the final result. Classes that have access to computers will find this is an excellent spreadsheet activity.
Students will conclude the exercise by writing a description of their findings in a lab report, or they may present their findings on a poster.
Soils are a mixture of the broken remains of rocks and decayed organic matter in a given area; soils provide the nutrients that sustain plant growth. Looking closely at the fragments in a soil sample under a magnifying glass or microscope will show parts of the base rocks. As time passes, these base rocks may become deeply buried by soils that have washed in from hundreds or thousands of miles away.
The Amazon River is an ancient river that may even have started when South America was joined with Africa. During the last 600 million years, the river has worn the land very flat and removed almost all of the soluble nutrients. The river meanders back and forth, constantly scouring the basin and moving nutrients downstream. It leaves behind almost inert clay as the soil.
Soils are divided into layers that are called horizons. The top horizon, the O horizon, is made of fresh and partially decomposed plant materials. In deciduous forests this layer may be fairly thick but in the rainforest, it's quite thin, usually under an inch. The next layer is the A layer which comprises a mixture of mineral particles and some decomposed organic material. In temperate regions this may be several inches to a foot thick, but in the Amazon it is usually no more than an inch. The B horizon is almost entirely rock particles, along with nutrients that have been washed out of the A horizon. This area is quite thick, running into the feet, in most of the United States while it is quite thin in the Amazon. The C horizon is composed of partially weathered bedrock and bedrock. It lies at various depths in the United States but is buried up to 4000 feet deep in the Amazon basin.
There are three main soil orders in the Amazon, though much of the basin has not been examined. Under the forest are the ultisols that are composed mainly of kaolinite clay and minerals that are washed from above. It is acidic, contains primarily aluminum compounds and has limited minerals for plant growth. The oxisols are found under some areas and are comprised mainly of kaolinite clay. This is thick, sticky clay that has almost no nutrients available for plants. Under the higher lands that do not flood, a soil type known as spoldsols is found. This is fairly sandy and acidic. Nutrients wash out of it easily, leaving little for plants to use.
Thus the people of the Amazon have multiple problems with farming the soil. The clays are relatively acidic, which stunts root development in plants. The clay contains a lot of aluminum that is toxic to plants. Last, the clay is thick, which makes it hard for plant roots and even water to penetrate.
Students will simulate the soils of the Amazon by using a mixture of the agricultural clay with a small amount of sand (about 5%) mixed in. Putting a 1 mm layer of decayed leaves across the top will represent the O and A horizons. Teachers should note that when this soil is watered, most of the water will stand on top of the clay rather than pass through. This is normal and explains why the rivers rise and flood in the rainy season since the water cannot soak into the soil.
The survival of the plant life in the Amazon depends on the recycling of nutrients. As leaves or plants die and fall, insects and fungus rapidly decompose them. New plants quickly spring up and use the available nutrients. This rapid succession has sustained the plant life for millions of years.
When humans come into an area, the land is often cleared of plants. Natives have cut the plants in small areas and burned them to concentrate the nutrients as a simple fertilizer for many years in a procedure called slash-and-burn farming. Given the lack of minerals, this produced a land that yielded crops for a short time before becoming depleted and forcing the people to move elsewhere. On a small basis, this produced no effect on the basin, but as the population has grown and as foreign corporations establish large-scale farms and ranches in the Amazon, much wider areas are being cut. The result of widespread cutting is to completely remove the nutrients from the area, making recovery impossible. It also allows erosion of the soft clay to occur, choking the waters with silt.
In this exercise, students simulate slash-and-burn fertilizer by building a small fire out of wood and other plants, then by mixing the ashes with the top of the clay in one of the flats.
- Dig a hole outside of the school and examine the soil horizons (this is best done away from the school where construction equipment did not effect the soil).
- Watch the "Journey Into Amazonia" videos or selected clips. Note the wide diversity of plants and animals in the Amazon basin.
- Episode One "Waterworlds" 4:00: Scenes behind the otters show the slippery clay and mud banks
- Episode One "Waterworlds" 18:30: Turtles dig in riverbanks to lay eggs
- Episode Two "Life On Land" 19:15: Aerial view of river with lower water showing depositional features
- Episode Two "Life On Land" 20:00: Rain with water rising
- Discuss the age of the South American continent and the effect of millions of years of rain washing the minerals to the ocean.
- Break students into groups of two to four. Distribute the flats and have each group of students fill the flats with potting soil. Place one seed in each section. Repeat using "Amazon" soil and again using "Amazon" soil that contains decaying leaves.
- Students will construct a chart sufficient to take growth readings for a month to 6 weeks depending on the plant seeds that were chosen.
- Water and take measurements of the plant growth every other day. Students should take descriptive notes of plant appearance at least once a week.
- Graph the final results and write a description of the results of the experiment.
Students should be assessed by performance criteria during the set up and data collection stages of this activity. A check-off sheet showing when each student took readings or watered the plants may be helpful. Students may be assessed objectively in the collection of data, construction of a chart and construction of the graph. A final written analysis of the data should be graded with a matrix that addresses the nutrients in the soil, the importance of the recycling of leaves, the impact of slash-and-burn farming and the effect of deforestation. An alternative to a written paper is to present students' findings on a poster for display in the class or the hallways.
- Soil test kits may be used to determine the fertility of the different soils. These kits may be obtained from science catalogs or from garden supply stores. The agricultural departments of many states will test soil samples professionally for little or no charge.
- Invite the district soil conservation specialist or agricultural agent to speak to the class.
- At this site or on other sites, investigate how many acres of rainforest disappear each day. Determine how much of that acreage is being lost to large scale agriculture (cattle, etc.) and what kinds of organizations are involved. Students might also compare deforestation in the Amazon to clear cutting in U.S. forests.
Related National Standards
This lesson addresses the following national content standards found in the McRel Standards Database at http://www.mcrel.org/standards-benchmarks/ :
- Knows how matter is recycled within ecosystems (e.g., matter is transferred from one organism to another repeatedly, and between organisms and their physical environment; the total amount of matter remains constant, even though its form and location change)
- Knows how the amount of life an environment can support is limited by the availability of matter and energy and the ability of the ecosystem to recycle materials
- Knows how the interrelationships and interdependencies among organisms generate stable ecosystems that fluctuate around a state of rough equilibrium for hundreds or thousands of years Knows ways in which humans can modify ecosystems and cause irreversible effects (e.g., human population growth, technology, and consumption; human destruction of habitats through direct harvesting, pollution, and atmospheric changes)
- Knows the composition and properties of soils (e.g., components of soil such as weathered rock, living organisms, products of plants and animals; properties of soil such as color, texture, capacity to retain water, ability to support plant growth)
- Knows components of soil and other factors that influence soil texture, fertility, and resistance to erosion (e.g., plant roots and debris, bacteria, fungi, worms, rodents)