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3. Soil Analysis - Background
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.
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.