Soil Evolution Field Biology CCTI Mrs. Conway Directions • Read through the following slides while completing your lab packet. • Be sure to answer all question in the packet. • Use this button to move forward • Use this button to move backward Where Does Soil Come From? • Read the Background Information on your handout. • Highlight the definitions the of new vocabulary words you encounter. What are the six components of soil? • Rocks and rock particles • Decaying plant and animal matter (organic) • Dissolved minerals/ elements • Soil air in porous spaces (aeration) • Fungus and bacteria • Bugs What are the two types of weathering? • Chemical weathering can be the result of plant growth, causing a chemical change. • Mechanical weathering occurs when water stands in the cracks of rock. • Read through the next slides while completing page 630 in your packet. • List and describe four types of chemical weathering and five types of mechanical weathering. Chemical Weathering What are some examples? Chemical Weathering Oxidation: Combines oxygen with another substance in the rock: this usually changes the color of the rock Chemical Weathering • Carbonation: happens in rocks that contain calcium carbonate (limestone); speeds up as the temperature decreases Chemical Weathering • Acids: formed by sulfur and nitrogen compounds and will cause rocks to fall apart Chemical Weathering • Hydration: the incorporation of H+ and OH- ions; this addition causes an increase in volume, adding stress within the rock Mechanical Weathering Two factors are very important for mechanical weathering, wind and water. What are some examples? Mechanical Weathering • Root Action: As roots grow they break rocks apart Mechanical Weathering • Thermal expansion: especially in deserts, rocks heat up in the day and, as the temperature drops greatly at night, the rocks can flake easily (exfoliate). Freeze/thaw will deposit water in cracks when this freezes the ice pushes outward and causes pieces of the rock to break off, like a layer of an onion. Mechanical Weathering • Hydraulic Action: Water rushes into cracks in rocks; but a tiny bit of air is compressed at the crevice of the crack: when the water recedes the air is forcefully released taking away fragments of rock with it. Mechanical Weathering • Organic Activity: Mosses and lichens attach to rocks and this causes physical as well as chemical breakdown of the rock. (plant roots exert extreme pressure on rock cracks and plants form an acid on the rocks that further breakdown the rock). Mechanical Weathering • Gravity: Large rocks often fall from mountainsides and as they hit bottom they break into smaller and smaller pieces. What is a Soil Profile? The soil horizons make up the soil profile. Now turn to Page 632 in your packet! Soil Profile: Click on the boxes to move forward! O (organic) A (Topsoil) B (Subsoil) E (Transition Area) C (Weathered Parent Material) R (Parent Bedrock) O Horizon • Colors: Black, dark brown • Structure and Composition: Loose, crumbly, well broken up litter (twigs, leaves) • Processes occurring: Decomposition • Some people might not realize that this layer is actually part of the solid ground • This layer is known as the twig-leaf layer! A Horizon • The A horizon is made of dark colored soil called humus. • Colors: Dark brown to yellow • Structure and Composition: Generally loose, crumbly, well broken up because roots are present and seeds germinate here; earthworms and bugs present; minerals present • Processes occurring: Leaching and nutrient center • Most roots of vegetation are found here! This is the best soil for growing crops! B Horizon • Colors: Brown, reddish to orange in color • Structure and Composition: Zone of larger chunks, may be dense, crumbly but mostly cement-like; clay and oxidized materials and organic matter accumulates from A horizon (topsoil) • Processes occurring: Accumulation • This layer is less suited for growing crops! E Horizon • Colors: Minerals are moved out from A to B (translocated) so the soil is light in color because the soil silica remains; this is also known as leaching. • Structure and Composition: Depends upon the mass of the horizons above E; gets more compact as soil evolution develops • Processes occurring: Transition between A and B (eluviation) where the leaching is predominant C Horizon • Colors: depends upon the color of the parent bedrock • Structure and Composition: Dense • Processes occurring: Weathering or disintegration of parent material or parent bedrock R Horizon • • • • Colors: generally solid (gray) Structure and Composition: Dense Processes occurring: Weathering This layer is the parent material from which soil is made! How much did you learn? • Complete the questions located on page 627 and 628. • Be sure that all answers are complete. • Use the back or forward arrows to review material. Congratulations! You have just completed the Weathering and Soil Formation/Evolution Tutorial!