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Pg. 73 Soil is: ◦ Loose rock particles ◦ Decaying organic matter (called humus) Result of chemical and mechanical weathering Begins with mechanical weathering and then animals and bacteria live in weathered material and create the humus Forest soils contain more organic matter than desert soils Soil forms in layers as it develops The bedrock that the first weathered rocks come from is the parent rock Soil above the parent rock is called residual soil Topsoil lies above residual soil (subsoil) Chemical weathering can alter the components of soil The vertical sequence of soil layers is called a soil profile Poorly developed soils show little distinction between layers A distinct layer is called a soil horizon Three major horizons ◦ A –high organic matter; dark colored ◦ B- subsoils enriched with clay minerals ◦ C- subsoils that contain weathered parent material Subsoils are usually reddish brown Affects the thickness of soil Sloped areas coarser particles remain Soils in lower areas (like valleys) are thick and fertile South facing slopes receive more sunlight so they have more vegetation and thicker soils Soils vary greatly depending on climate Soils are classified based on the climates from which they form Four major types of soil: ◦ ◦ ◦ ◦ Polar Temperate Desert tropical Form at high latitudes and high elevations Greenland, Canada, Antartica Good drainage but no horizons because they are shallow Permanently frozen ground (called permafrost) is present under polar soils Vary greatly Annual rainfall greater than 50-60 cm Grasslands- abundance of humus; rich fertile soils Forests- less deep, less fertile soil; aluminum rich clays Prairies- support growth of grasses and bushes Low levels of precipitation High level of salts Can support a limited amount of vegetation Little to no organic matter but abundant nutrients High temps and heavy rainfall Infertile soil High bacterial activity that leaves the soil with few nutrients Characteristic red color because of the oxidation of iron Iron ore is mined from these soils (Brazil, Australia, and Jamaica) Classified according to size (clay, silt, or sand) Clay is smallest; sand is largest The proportions determine a soil’s texture Soil textural triangle is used to determine the texture of a soil sample Affects moisture and ability to support plant growth Measure of how well soil can support growth Factors that affect fertility: ◦ ◦ ◦ ◦ ◦ Availability of minerals Number of microorganisms Amount of precipitation Topography Level of acidity Use fertilizers to replace minerals Planting of legumes allows bacteria to grow Top soil is usually dark Red and yellow soils are the result of oxidation of iron minerals Yellow soils are an indication of environmental problems Grayish or bluish soils are usually found in regions where it is constantly wet and there is a lack of oxygen