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Assessment
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Ask the students to look at the data they collected in the experiment. Ask them to rank the four materials from the one that allowed the most water to pass through to the one that allowed the least. Ask why they think this is so. Discussion should lead to an observation of the importance of particle size. Water passed most quickly through the large pieces of gravel and most slowly through the very fine powdered clay. Turn students’ attention to the natural soil that represents a mixture of particle sizes. Did it let as much water through as gravel and sand? [No.] How did it compare to clay? [More water passed through it than clay.) Ask the students to recall what else they found in their soil mixtures in the first activity. [They should recall seeing bits of plants and animals—organic material.] You can show your students that the organic material in the soil also contributes to its ability to retain water. Put a small amount of dampened (not soaked) peat moss or sphagnum moss in one of the cup set-ups and pour a measured amount of water through. Students will see that the organic material helps soil to hold more water.
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Revisiting Investigation Question 3: How does water move through soil?
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Complete this investigation by asking your students to reflect on the investigation question and how their answers may have changed as a result of what they have learned. Ask them why they think it’s important for water to move through soil. Remind the students that how well a soil holds onto water and how well it drains water helps plants to grow. Elementary students know the importance of watering plants with just the right amount of water—not too much and not too little. Tell students (or show photos to illustrate) that growing plants is not our only interest for measuring how well soil holds water. People building houses need to know how well the surrounding soil is able to drain water. In many areas, septic systems collect and drain household waste water away from the home and its water supply. For the systems to work properly, the soils must drain at a certain rate or the septic system backs up.
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The extent to which soil holds or releases water is dependent upon the types of particles in a soil sample. Soil is likely to have several kinds of rock and mineral particles. A few kinds are very common. The three most common kinds are quartz particles, feldspar particles, and small pieces of rock. A soil sample is very likely to have a lot of at least one of these three kinds of particles. Quartz particles have irregular shapes. They look gray and glassy. Their surfaces are often stained brown or orange, because they are coated with rust. Feldspar particles are usually white or cream-colored. Their surfaces are often flat, at least partly, rather than irregular. There are many kinds of rock particles. You can tell them apart from the mineral particles because rocks are made of many different particles of minerals, all stuck tightly together. The finest part of a soil sample is probably mostly very small flakes of clay. They are too small for you to see even with a hand lens. Sandy soils are loose and easy to dig. Soils with a lot of clay are harder to dig. Some plants like sandy soils and others like soils with more clay. Most soils have lots of organic matter. Some of the organic matter is in the form of living things, such as earthworms, insects, and microorganisms. Most soils are also rich in decaying plants. If the plant has decayed only slightly, you can usually recognize scraps of leaves, roots, and seeds. When the plant has decayed more, it turns into a soft, fine, dark material called humus. Humus is very important in soils. New plants can easily put their roots into humus. It is also good at holding water for later use by growing plants. |
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