Formation of Soil

Formation of Soil Pg. 55

Development of soil • Soil is: • Loose rock particles • Decaying organic matter (called humus) • Result of chemical and mechanical weathering • Beings 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 Composition • 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 • Transported soil has been moved away from the parent bedrock • Chemical weathering can alter the components of soil • The parent rock and climate determine how long it takes soil to form

Soil Profiles • The vertical sequence of soil layers is 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- subsoilsenriched with clay minerals • C- subsoils that contain weathered parent material Subsoils are usually reddish brown

Topography and soil profiles • 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

Soil Types • 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

Polar soils • 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

Temperate 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

Desert Soils • Low levels of precipitation • High level of salts • Can support a limited amount of vegetation • Little to no organic matter but abundant nutrients

Tropical Soils • 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)

Soil Textures • 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

Soil Fertility • 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

Soil color • 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

Formation of Soil