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Chapter 5.2: Soils. Soil: A Product of Weathering. Weathering is normally thought of as destructive. However, if it were not for weathering, we would not have soils, which are an important natural byproduct of weathering .
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Soil: A Product of Weathering • Weathering is normally thought of as destructive. • However, if it were not for weathering, we would not have soils, which are an important natural byproduct of weathering. • Imagine a world without soils. Without soils, there would be virtually no life on land. • No terrestrial plants and animals.
Soil Composition • Soil is made up of four parts. These four parts are mineral matter, organic matter, water, and air. • The mineral matter of soil is the part that comes from broken rock. • The organic matter of soil is composed of dead leaf litter, dead vegetation, dead animals etc. • Another name for organic matter is humus.
Soil Texture and Composition • One way to determine a soil's composition and texture is to use a soil-texture diagram like the one shown here.
Soil Formation • The five most important factors that contribute to the way soils are formed are... • Parent material – the source of the soil material. Where did the soil come from? Did it come from granite, sandstone, glacial till? • Time – The longer a soil has been forming, the thicker it becomes; the more mechanical and chemical weathering it has been exposed to.
Soil Formation • Climate – The affect of climate on weathering is similar to the affect of climate on soils. • Hot, humid climates have the most intense chemical weathering and as a result the soils are generally unproductive (poor in nutrients). • Colder climates sometimes experience mechanical weathering (frost wedging, contraction/expansion etc.), but there is very little chemical weathering, so the soils are often more productive (nutrient rich).
Soil Formation • Organisms – Living organisms also contribute to soil formation. • Bacteria and fungi can help to break down the dead plants and animals and put nutrients into the soil. • Earthworms and other burrowing animals help to aerate and hydrate the soil with air and water. • Slope – The steepness of a slope can also play a factor in soil formation. Steeper slopes are affected more by erosion and have less soil than flat ground.
Soil Profiles • Some soils change gradually from the top to the bottom. Others have distinct regions or layers that vary in texture and composition. • One way to analyze soil layers is to look at a vertical section or column of soil called a soil profile. • Most typical soils have a top layer of mineral matter mixed with organic matter. This layer is called the A horizon or the topsoil layer.
Soil Profiles • The next layer down, located below the A horizon is called the B horizon. • The B horizon is often called the subsoil. • The B horizon contains fine clay sediment that washed out of the A horizon. • Sometimes the B horizon forms a hard impenetrable layer called hardpan.
Soil Profiles • The final layer is called the C horizon. • The C horizon barely resembles what you would typically think a soil would look like. • The C horizon is broken down solid bedrock.
Soil Types • Because there are so many different types of climates around the world, there are also many different types of soils. • The three main types of soils are... • Pedalfers – These soils form in temperate climates that receive moderate rainfall (63 cm). This is a common soil type found in the eastern half of the United States.
Soil Types • Pedocals – This soil type is found in the western United States that have grasses and brush vegetation. • Pedocals are typically found in dryer climates so chemical weathering is minimal. • Because chemical weathering is small, Pedocal soils contain less clay and more calcite. • Laterites – This soil type is found in hot, humid, tropical areas. Chemical weathering is severe.
