Formation of SoilPg. 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

Development of soil

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

Soil Composition

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

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

Topography and soil profiles

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

Soil Types

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

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

Temperate Soils

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

nutrients

Desert 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)

Tropical Soils

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 Textures

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 Fertility

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

Soil color