Beneath Your Feet
- Identify the components and characteristics of soil
- Explain the importance of soil to living organisms
- Explain the role of soil organisms in decomposition
- Identify components of the soil foodchain
Soils, soil organisms, foodchain, decomposition
Pa. Standards Addressed Doing the Activity
- Materials Assessment and Evaluation
- Background Extensions
- Getting Ready Resources
Pennsylvania Environment and Ecology Standards Addressed:
- 4.6.4. Ecosystems and their Interactions
A. Understand that living things are dependent on nonliving things in the environment for survival.
B. Understand the concept of cycles.
Other Pennsylvania Standards Addressed:
- Science and Technology
- Reading, Writing, Speaking and Listening
Hands on, small group, investigation, data collection, simulation
- 1 soil equipment set per group – 1 trowel, 1 piece white paper, hand lens, 1 ruler, 1 soil compaction stick (1/2 inch dowel that has been sharpened at one end), 1 can with both ends removed (large enough to hold 1 cup of water), 1 measuring cup (1 cup size), small jug of water, stop watch or watch with second hand, pH kit or strips (optional)
- One copy of Soil Investigation Worksheet per student or group
- Flags to mark sampling areas
- See Cast of Characters and Their Roles pag
- Character identification tags
- Pictures of each character – plant, earthworm, nematode, arthropods & insects, fungi, bacteria, and protozoa
- Equipment for each character as described
- Plants - string, 2 cards marked energy, 2 cards marked carbon
- Earthworms - 2 water bottles, 2 vitamin bottles, 2 whisks, 2 trowels, 2 glue bottles
- Nematode -1 vitamin bottle
- Arthropods and Insects -1 each plastic millipede, spider, and beetle, 1 “Good Guy” sign
- Fungi - 1 water bottle, 1 vitamin bottle
- Bacteria - 1 vitamin bottle per bacteria
- Protozoa - 3 nitrogen signs
- 1 hand lens per student
- 1 set per group – trowel, plastic tray, piece of screen to cover tray, soil collecting jar, flashlight
- Identification guides for soil insects and arthropods
- Student journals
Wherever you stand, one of our most important and often overlooked resources lies just beneath your feet – the soil. Yet, our very survival is rooted in it. Without soil to provide food there would be little life on Earth.
Soil is more than just dirt. It is a mixture of minerals (sand, silt, and clay), organic matter (decaying plants and animals), air and water. The proportion of these ingredients determines how well the soil can support plant growth. Sand, silt and clay, which are tiny fragments of rock or minerals, make up nearly half of the material in soil. A typical soil is 45% mineral, 25% air, 25% water, and 5% organic matter or humus. The small percentage of organic matter is what supplies the nutrients that plants need.
The size of soil particles plays an important role in determining soil characteristics. The space between these particles influences oxygen availability, water retention and drainage. Larger particles have larger spaces between them. Sand particles are relatively large, so water often drains through them quickly. These soils often don’t hold enough water to support plants. Silt particles are smaller than sand, but larger than clay. Clay particles are the smallest and have the least amount of pore space. Clay particles also have the ability to attract water and hold onto it. Clay soils can become waterlogged and suffocate plants. The ideal soils for plants have a variety of pore sizes that allow enough drainage to provide space for oxygen.
Soils are often classified by texture, which is determined by the amount of sand, silt or clay they contain. Sandy soils have a gritty texture. Soils high in silt are smooth like flour and clay soils are sticky like Play-Doh. A fairly even mixture of all three is called loam.
Soil pH affects the availability of nutrients for plants to use. Plant roots absorb nutrients that are dissolved in water. If soils are too acid or too alkaline the nutrients won’t dissolve easily, so the roots can’t use them. Most nutrients that plants need are available at a pH of 6 to 7.5. Some plants, like rhododendrons, potatoes, and blueberries, prefer lower pH levels.
If you dig down into the soil, you will find several different layers. Leaf litter is a layer of dead plants and leaves that are found at the very surface. It prevents erosion by breaking the fall of rain. This is also where the decomposition process begins to turn dead plants and animals into humus. Beneath the leaf litter is a layer of topsoil. This layer contains minerals, air, water, humus, and living things. It takes hundreds of years to produce one inch of topsoil. The next layer, the subsoil, contains minerals and gravel, but little humus. The last soil layer is the substratum where rocks are still being broken into smaller pieces to form new subsoil. The deeper you dig the harder the earth becomes until you reach the bedrock. In the first 18 inches of soil it is possible to see all the way to the substratum. Each layer changes color, becoming lighter than the layer above it, because it contains less humus.
A healthy soil is full of living things. There are billions of bacteria in one cup of soil alone. Plant roots, animals, insects, bacteria, insects, fungi, nematodes and protozoa are some of the living parts of the soil ecosystem. These organisms help create the environment plants need to grow.
Many soil organisms power the decomposition process and nutrient cycle. Millipedes, sowbugs, bacteria, and fungi all feed on dead organic material and convert nutrients to a form plants can use. Earthworm castings, tiny lumps of worm droppings, are rich in nitrogen, potassium, and phosphorus. Earthworms and other soil animals are natural “tillers” who also mix and aerate the soil. One mature oak tree can drop over 100,000 leaves each year. Imagine what would happen without these amazing natural recyclers. The soil ecosystem also contains predators that hunt the plant eaters. Moles, centipedes, and predatory beetles search for prey in the soil and leaf litter. All of these organisms make up the soil foodweb.
A gardener’s goal is to provide an environment that supports soil organisms. These organisms will constantly decompose organic material to a form that will keep your garden growing.
Select and flag sampling areas. Choose locations in gardens, playgrounds, lawn, or under trees to get a variety of results from the soil tests. Have more than one group sample in garden locations, so they can compare conditions within the gardens as well as outside them.
Create a Soil Investigations master chart that lists each sample area – lawn, playground, garden, etc. – and each test, so students can compile their data on one sheet and compare results. See example:
- Garden A
- Garden B
- Soil ingredients
- Topsoil Depth
- Soil Texture
- Soil Compaction
- Soil Percolation
- Soil pH
Review the Cast of Characters sheet for student roles and equipment
Doing the Activity
In the classroom
- Ask students what they think soil is and why it is important. Explain that without soil there would be little life on Earth. Discuss the components of soil.
- Ask students if all soil is the same. Why are some soils good for growing plants, while others are not? Tell students they will be examining the soil to find this out.
- Review background information as necessary to help students understand the tests. Review the worksheet and explain how to use the soil sampling equipment for each test.
In the garden
- Divide the class into small groups. Give each group a set of soil testing equipment, a worksheet, and a pencil. Send each group to a sampling area.
- Have students run their tests and complete the worksheet.
- Bring the class together to share and compare results. Have students list the results for their area on the master chart.
- In which area did the water drain the fastest? The slowest? What do you think affected how fast the water drained? (Space between soil particles.) Which soils do you think had the most air spaces? Which soil was the most compact? The least compact? How does soil become compacted? Explain that plants need a combination of air and water in the soil for their roots to be healthy and help the plant grow. Which areas had soil that would provide this?
- Compare soil pH. Explain that most garden plants grow best at a pH between 6.5 and 7, because more nutrients are available for the plant to use. Which sites fit this requirement?
- What did the topsoil look like? What did you observe that made up the topsoil?
- What did the soil feel like? Based on the texture, do you think your soil is mainly sand, silt, or clay? Discuss some of the characteristics of sand, silt and clay and how they affect air and water movement.
- How deep was the topsoil layer? Explain the different layers of soil and how long it takes to form topsoil. Since it takes so long to make topsoil, what can we do to protect and keep what we have?
- Which area(s) had the best soil? Of the areas we sampled outside the garden, which had soil that would make a good site for a garden?
In the classroom
- Tell students that in addition to its non-living components, soil contains a whole world of living organisms. Ask students if they can name some organisms that live in the soil.
- Explain that these organisms make up the soil foodweb and have important roles in the soil ecosystem. Ask students what happens to leaves that fall to the ground in autumn. Discuss the importance of soil organisms in decomposition, nutrient cycling and soil improvement.
- Before doing the simulation show students pictures of the soil organisms. As you show the picture, discuss which organisms are plant eaters, decomposers, tillers, predators, etc. and review their role in the soil ecosystem. (See the Cast of Characters.)
- Tell students they will be acting out the parts of organisms in a soil ecosystem. Assign roles, handout props and review each participant’s action before starting the simulation. Have students do the simulation for about 30 seconds. Ask them to stop, look around and process.
- What happened here? How did the plants and animals help each other? What would happen if someone sprayed a pesticide that killed one group of animals? Simulate again, but this time without all of the insects and arthropods. How would this affect the soil foodchain? Is there a better alternative?
In the garden
- Tell students they will be examining some animals that live in the soil.
- Break students into small groups. Give each student a hand lens. Give each group a trowel, a plastic tray, a piece of screen, a soil collecting jar, and a flashlight. Have students use their trowels to collect soil and put it in their jar. They should place the screen over the plastic tray and turn the jar over onto it. Turn the flashlight on and place it on the top of the jar so that the light shines down onto the soil. Explain that the heat from the light will cause the soil animals to move deeper into the soil in search of moisture. When they get to the bottom they will fall into the tray.
- Have students examine the animals with their hand lens and try to identify some of them. Have them draw pictures of some of the organisms in their journals.
- Have students return animals and soil to the garden.
- Did you find many animals? Do you think we have a healthy population of soil organisms? What did some of the animals look like?
- Did you see any of the animals we talked about inside? Ask students to describe one of the animals they found and explain what they think its role is in the ecosystem or how it helps the soil? Ask what they think the soil provides for the animal? If students found enough animals, have them try to make a foodchain from their lists.
- Observation of student participation in soil testing activities.
- Review worksheets for understanding and completeness.
- Observation of student participation in foodweb activity and discussion.
- Evaluate student’s journal entries.
Have students make soil improvements by adding organic matter to the soil. Discuss how this might affect soil organisms.
- American Forest Foundation, Project Learning Tree, 1993
- Ball, Jeff, Rodale’s Garden Problem Solver, 1988
- Bradley, F.M. and Ellis, B.W., Rodale’s All-New Encyclopedia of Organic Gardening, 1992
- Pennsylvania Bureau of State Parks, Activities for Environmental Learning, 1989
- Lawless, Dawn, Create a Soil Aggregate Lesson, 2002
- Parrella, Deborah, Project Seasons, 1995
- Project Food, Land and People, 2000
- Silver, Donald M., One Small Square Backyard, 1993