1. Formation of Soil
Pg. 73

2. Development of soil 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

3. 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
Topsoil lies above residual soil (subsoil)
Chemical weathering can alter the components of soil

4. Soil Profiles
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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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

13. 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

14. 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