Day 1: Optional: 5 min discuss soil observation homework from last night 10 min observe samples of different soil ingredients 30-40 min conduct soil quality tests
Day 2 (several days later): 15 min complete observations of Tullgren Funnel and soil separation tests 30 min discuss results and draw conclusions about "healthy" versus "unhealthy" soil
* Since the Tullgren Funnel and soil separation tests take several days, it is recommended to start Day 1 on a Friday and complete Day 2 on a Monday. *
For each group of students:
For whole class to share:
In an ecosystem, the soil and water form the foundation of the surrounding environment. Environmental impacts such as deforestation, the introduction of invasive species, climate change, acid rain and others all have profound implications on the soil and water in the area. Many changes can be readily detected and tracked by very simple tests that students can conduct with very simple materials. This lesson and the parallel water quality analysis lesson are opportunities to teach students the observational and analytical skills needed to apply these tests to real ecosystems in the real world.
The first test is a simple soil observation using all your senses. This allows students to practice observation and recording skills and helps kids notice that all soil is not the same.
The second test is a soil separation test to determine the relative proportion of clay, silt, sand and organic material. It draws on the principle of a density column where the largest, most dense particles (in this case sand) settle first. During the introduction of this portion of the activity, you can discuss density or not, depending on the background of your students. Students generally intuitively understand that in water, big heavy things will sink first. Clay soils tend to be sticky when wet and will hold together in a ball. When dry, clay soils harden to an almost rock-like density, holding very little air, thus making it difficult for critters to survive. Sandy soils tend to drain water rapidly. From a gardener's perspective, the ideal soil is a balanced mixture of sand, silt and clay with lots of air, water and organic material mixed in. The alum used during this test helps separate the particles of soil. Alum is relatively non-toxic but students should be warned not to put it in their mouths and should wash their hands after this station (and at the end of the period).
The third test measures the pH of the soil. pH is a measure of the chemical and mineral content of soil. Most plants prefer soil that is slightly acidic (between 5.5-7). However, some plants require very acidic soils (less than 4.5) to survive (such as blueberries and azaleas). Impurities and pollutants such as detergents, acid rain, and trash will alter the pH of soil. Major human disruption from mining, logging, and construction also changes the pH of soil by allowing the more neutral topsoil to erode, exposing the subsoil layers which tend to be more acidic.
The final test uses a Tullgren funnel (sometimes called a Berlese separator) to isolate bugs, worms and other critters from the soil. The basic principle is that the soil is placed in a funnel beneath a heat source. The mites, worms, and insects in the soil move downward through the soil to escape the heat and eventually fall out the bottom of the funnel into water. It can be absolutely fascinating for students to discover that soil is alive with critters large and small. Because it takes some time for the critters to fall through the funnel, (between 2-4 days) it is often ideal to allow the test to continue over a weekend. But make sure that there is enough water is the Petri dishes or the cup so that the water below the funnel does not dry out. For additional information and ideas about the Tullgren Funnel, see: The Open Door Website. How do you identify the critters? Great question and as of now, I have no idea. I have not found a good field guide which can be used to identify soil critters. If you find something, let us know!
Additional tests may be used or substituted for the 4 included here. For instance, you can measure the water-holding capacity of soil by adding fixed quantities of water (10 mls at a time for instance) to fixed quantities of dry soil in a funnel until water begins to drain out of the bottom. Water will continue to drain out the bottom until the soil reaches a steady state. Take the amount of water you added initially and subtract the amount of water that drained out and you have your water-holding capacity. This measurement relates closely to the composition of the soil. The more clay, the more water the soil holds. The more sand and gravel, the less water the soil holds.
What makes up healthy soil? In general, a balance of all soil ingredients with few pollutants and lots of organic material and living things makes healthy soil. Soil that completely lacks organic material or that is far outside the normal pH range is generally considered unhealthy. Soil that is completely dry cannot support life. A large number of soil critters is often a good indicator of healthy soil. However, this depends dramatically on the location and use of the soil. Soil in a garden will be different from forest soil which will be different from creek soil. Therefore, in the discussion of what makes healthy soil, expect and encourage differences in opinion.
It is recommended that students are familiar with pH - both understanding what pH is a measure of relative to household items and how to measure it using indicators. The previous lesson 2. Water Analysis provides a sufficient introduction to those ideas.
Optional: Comparing Soil Homework - This assignment asks students to observe and compare 2 samples of soil, one from home, one from school, and to devise original tests to look for differences between the soils.
- What did you observe about the school soil? about your home soil?
- What was similar? What was different?
- What ingredients make up soil?
- What test did you try?
- What did you discover?
- What other tests could we use?
- Were you surprised by any of the results?
- What do differences in pH mean?
- What might cause pH differences?
- How should we interpret differences in the relative amounts of clay, silt and sand?
- Was any layer missing?
- How would differences in soil composition affect the types of plants or animals that could live in that soil?
- Does knowing where the 2 soils came from help explain any of the results?
- Did everyone get the same results? Why or why not?
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Terrarium Habitats a GEMS guide by the Lawrence Hall of Science includes a great soil observation activity, including the soil separation test and Tullgren Funnel test.
The Open Door Website describes the Tullgren Funnel test with pictures and additional details.
Shaping Earth's Surface
2. Topography is reshaped by the weathering of rock and soil and by the transportation
and deposition of sediment.
Ecology (Life Sciences)
5. Organisms in ecosystems exchange energy and nutrients among themselves and with the environment. As a basis for understanding this concept:
e. Students know the number and types of organisms an ecosystem can support depends on the resources available and on abiotic factors, such as quantities of light and water, a range of temperatures, and soil composition.
5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
e. Students know how to determine whether a solution is acidic, basic, or neutral.
Investigation and Experimentation
7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations.