Presentation on theme: "Weathering and Soil Formation"— Presentation transcript:
1 Weathering and Soil Formation Ch. 8Weathering and Soil Formation
2 Section 2: How Soil Forms What is Soil?Soil is the loose, weathered material on Earth’s surface in which plants can growBedrock is the solid layer of rock beneath the soilSoil CompositionSoil is a mixture of rock particles, minerals, decayed organic material, water, and airHumus is a dark-colored substance that forms as plant and animal remains decayHumus helps create spaces in soil for the air and water that plants must haveContains substances called nutrients, including nitrogen, sulfur, phosphorus, and potassiumThe fertility of soil is a measure of how well the soil supports plant growth
3 Section 2: How Soil Forms What is Soil?Soil TextureSand feels coarse and grainy, but clay feels smooth and silky; these differences are differences in textureSoil texture is important for plant growthSoil that is mostly clay has a dense, heavy textureSandy soil has a coarse textureSoil that is made up of about equal parts of clay, sand, and silt is called loam
4 Section 2: How Soil Forms The Process of Soil FormationSoil forms as rock is broken down by weathering and mixes with other materials on the surface. Soil is constantly being formed whenever bedrock is exposedA soil horizon is a layer of soil that differs in color and texture from the layers above or below itThe horizon is made up of topsoil, a crumbly, dark brown soil that is a mixture of humus, clay, and other mineralsThe rate at which soil forms depends on the climate and type of rock.
5 Section 2: How Soil Forms Soil TypesScientists classify the different types of soil into major groups based on climate, plants, and soil composition.Soil LayersThe C horizon forms as bedrock weathers and rock breaks up into soil particlesThe A horizon develops as plants add organic material to the soil and plant roots weather pieces of rockThe B horizon develops as rainwater washes clay and minerals from the A horizon to the B horizon
6 Section 2: How Soil Forms Living Organisms in SoilSome soil organisms make humus, the material that makes soil fertile. Other soil organisms mix the soil and make spaces in it for air and water.Forming HumusPlants contribute most of the organic remains that form humusAs plants shed leaves, they form a loose layer called litterHumus forms in a process called decompositionDecomposers are the organisms that break the remains of dead organisms into smaller pieces and digest them with chemicalsSoil decomposers include fungi, bacteria, worms, and other organismsExamples: fungi such as mold; bacteria such as microscopic organisms
7 Section 2: How Soil Forms Living Organisms in SoilMixing the SoilEarthworms do most of the work of mixing humus with other materials in soilAs earthworms eat their way through the soil, they carry humus down to the subsoil and subsoil up to the surfaceEarthworms also pass out the soil they eat as wasteMany burrowing mammals such as mice, moles, prairies dogs, and gophers break up head, compacted soil and mix humus through it; these animals also add nitrogen to the soil when they produce waste; they add organic material