My Notes Ka Hana Imi Na auao A Science Careers Curriculum Resource Go to: www.cds.hawaii.edu/kahana 1
My Notes Ka Hana Imi Na auao A Science Careers Curriculum Resource Go to: www.cds.hawaii.edu/kahana 2
SOILS ARE FILTERS: TEACHER S NOTES FOR POWERPOINT & DEMO Materials: Each pair or trio of students will need a bottle of water with cap, a tray and two sponges (best if 1 is quite dry). Slide 14 NOTES: Sponges show characteristics of soils & water Dryness GIVE STUDENT PAIRS A SPONGE & ASK: Is your sponge dry? ANSWER: No! Not even if dry to the touch! The sponge on the left is moist, but no more water can be removed by squeezing, so in one sense, it is "dry". Similarly, when plants use all the water they can extract from the soil, it is "dry" to them, yet it still contains water. The sponge on the right is air-dried. Though the sponge is dry to the touch, it is not "dry". It contains water. If the sponge is placed in an oven at 105 C, it will lose water, and therefore lose mass. Similarly, air-dried soils contain water. The definition of dry soil is soil that has been dried at 105 C until it reaches a constant weight, often 24 hours. Air-dried sands retain the least water, while air-dried clays retain the most relative to oven drying. Go to: www.wtamu.edu/%7ecrobinson/sponge/drysponge.html for more notes on Wilting Point and Hygroscopic Coefficient. Slide 15 NOTES: Sponges show characteristics of soils & water Infiltration & Run-off ASK STUDENTS: Hold your sponge over your tray and pour a little water through it from the side like in the photo on the left. What do you notice? ANSWERS VARY: The water filters through slowly, seeps out the bottom, some water runs off. Infiltration is the movement of water from the soil surface into the soil. The infiltration rate of soils is affected by many things, including texture, surface structure, surface cover (bare soil, growing plants or plant residues), hydraulic conductivity (rate of water movement through the soil), and the presence of limiting layers below the soil surface (compacted layers, bedrock, etc.). When water is added to the sponge faster than the infiltration rate, runoff develops. When the irrigation rate is excessive, or the precipitation comes rapidly, the infiltration rate is exceeded, resulting in runoff. Go to: www.wtamu.edu/%7ecrobinson/sponge/infiltrat.html for more info on soil forces (gravity, capillarity and matric or attraction) as well as agricultural management of soil and water conservation. Slide 16 NOTES: Sponges show characteristics of soils & water Wetting & Saturation ASK STUDENTS: Squeeze out your sponge & lay it in the tray in a very small amount of water. What do you notice? ANSWER: The water moves up into the sponge. Ka Hana Imi Na auao A Science Careers Curriculum Resource Go to: www.cds.hawaii.edu/kahana 3
ASK: If you add more water to the tray and apply force to the wet sponge, what do you notice? ANSWER: Bubbles come to the top. This means air is trapped inside the sponge. The factor controlling water movement in nature (and thereby in soil), is that it moves from areas that have higher potential energy to areas that have lower potential energy. This is the reason water flows downhill: It discharges its energy as it moves downhill. This is also the reason water moves upward in the sponge. Even soil that has been saturated for long time periods usually contains air in up to 10% of the pores (voids) between soil particles. Go to: http://www.wtamu.edu/%7ecrobinson/sponge/wetting.htm for film clips of these sponge models Slide 17 NOTES: Sponges show characteristics of soils & water Water Holding Capacity ASK STUDENTS: Hold up your sponge without squeezing. What do you notice about the water movement? Why does it do this? ASK: When you squeeze it out slowly, is it dry? ANSWERS: Gravity make the water move quickly down through large air pockets (soil pores). The small air pockets stay damp afterwards. ASK: If we put a plant seed in the damp sponge, would it sprout? ANSWER: If it had enough moisture in the capillaries (small soil pores). Drainage occurs through large soil pores. Small soil pores have the ability to hold the water against the pull of gravity through the process of capillarity. When all the water that can has drained from the soil by gravity, the soil is at field capacity. Capillary water is the water held against gravity in the small soil pores, or capillaries. This water can be extracted by the plants. All soils retain water that plants are unable to extract; clays hold the most unavailable water while sands hold the least - this is called the Wilting Point. Silt loams hold the most plant available water. Go to: http://www.wtamu.edu/%7ecrobinson/sponge/watholding.html for extensive notes and film clips of sponge models showing Gravitational Water, Field Capacity and Capillary Water. Slide 18 NOTES: Sponges show characteristics of soils & water Percolation & Drainage ASK: What happens when you slowly fill your sponge with as much water as possible? ANSWER: The sponge absorbs all the water at first, then when it is full (reaches capacity) the excess drains out the bottom. If infiltration continues for long periods (heavy rains), it is possible that the soil will reach field capacity. At that point, the soil can no longer hold water Ka Hana Imi Na auao A Science Careers Curriculum Resource Go to: www.cds.hawaii.edu/kahana 4
against the pull of gravity. The result is water begins to move through the soil profile and out of the plant root zone (called percolation or drainage). Go to: www.wtamu.edu/%7ecrobinson/sponge/percolat.html for more notes on this Slide 19 NOTES: Sponges show characteristics of soils & water Engineering & Bearing Capacity ASK: When you squeeze out your sponge, then place a little water in one end of the sponge in the tray, what happens when you try to place your water bottle at different places on the sponge? ASK: Why? ANSWER: The wettest part of the sponge is weaker, so the bottle tips over. (top right photo) The dry sponge gives slightly in response to the load applied. The height of the dry sponge is approximately 30 mm. When the sponge is wetted, it expands about 5 mm in height, as well as in length and width. This expansion changes the strength of the sponge matrix, weakening it substantially. Many clay soils, particularly those dominated by smectitic and vermiculitic minerals, expand when they wet and contract when they dry. This causes stresses in structures (houses, roads, etc.) that result in cracks in plaster, tiles, and even foundations. (left photo) Few buildings have equal load distributions, so the effect of placing a building on the soil is similar. When the building begins to settle unevenly, foundations or walls may crack to relieve the stress. This is not unlike buildings in Mexico City, Mexico, Amsterdam, Netherlands, Venice, Italy, New Orleans, LA, Houston, TX, and others, including the Tower of Pisa. Go to: www.wtamu.edu/%7ecrobinson/sponge/engineer.html for complete notes on Subsidence (lowering of the landscape surface) and other conditions also can cause structural problems in buildings and roadways Ka Hana Imi Na auao A Science Careers Curriculum Resource Go to: www.cds.hawaii.edu/kahana 5