Soil Formation and Soil Horizons

 SOIL 

·       Soil is a mixture of Organic matter, Minerals, Gases, Liquids and Organisms that together support life. It is essential for life on earth to exist.

·       Earth's body of soil, called the Pedosphere.

·       The scientific study of soil is called Pedology.

·       Soil has six importance

1)    Act as a medium for plant growth.

2)    Act as a means of water storage, supply and purification.

3)    Act as a modifier of Earth's atmosphere. 

4)    Act as a habitat for organisms.

5)    Act as nature’s recycling system for nutrients

6)    Act as an Engineering medium

FORMATION OF SOIL

1) Soil forming factors

·       The technical term for soil formation is Pedogenesis.

·       The five soil-forming factors are:

a)     Climate: Temperature, precipitation, and how they are distributed across the seasons.

b)    Biotic factors: Plants, animals, fungi, bacteria, and other microorganisms.

c)     Topography: Slope position, aspect, and shape.

d)    Parent material: Rock, alluvium (wind- or water-deposited material).

e)     Time: How long the soil has been forming.

2) Weathering

·       Weathering is the breakdown of rock into smaller and smaller pieces.

·       Three types of weathering are recognized: a) Mechanical (physical) weathering, b) Chemical weathering and Biological weathering

a)     Mechanical weathering is the breakdown of rock due to physical factors such as temperature fluctuations and freeze. An example would be Quartz breaking down to fine sand-sized particles (Quartz is resistant to chemical weathering).

b)    Chemical weathering refers to the breakdown of rock due to chemical reactions. For example, limestone (CaCO₃) and gypsum (CaSO₄) dissolve in water and become smaller and smaller compounds. Micas can lose potassium ions and become vermiculite. Vermiculite, in turn, can lose more potassium and become smectite. Feldspars lose potassium and become kaolinite. In these cases, rock weathers to a microscopic or even elemental state.

c)     Biological weathering – Lichens

COMPOSITION OF SOIL

               Minerals                          -             45 %

               Organic matter             -             5 %

               Soil and Air                    -             25 %

               Water                                -             25 %

SOIL HORIZONS OR HORIZONATION

·       Soils consist of one or more distinct layers called Horizons.

·       Soil horizons are typically parallel with the ground surface.

·       Soil horizon layers are referred to as O, A, E, B, C and R depending on their position and nature.

a)    Horizon O

ü  Top layer dominated by organic materials.

ü  Saturated with water for long periods.

ü  Saturated but are now artificially drained.

b)    Horizon A

ü  Mineral layers that formed at the surface or below an O horizon.

ü  Generally called Topsoil in agriculture.

ü  It has more organic carbon than underlying layers and is the best environment for plants and microbes to grow.

ü  Sometimes this layer is missing or reduced due to erosion or topsoil removal.

c)     Horizon E

ü  Horizon E is characterized by Eluviation (removal of materials such as silicate clay, iron, aluminum, or organic matter).

ü  Usually more pale colored than the A horizon.

d)    Horizon B

ü  Horizon formed below an A, E, or O horizon that is dominated by loss of most or all of the original rock structure and shows evidence of soil formation such as Illuviation (concentration of the silicate clay, iron, aluminum, or humus from higher horizons).

ü  Horizon plays a major role in development of soil color and structure.

e)    Horizon C

ü  Horizons or layers, excluding hard bedrock, that are little affected by soil-forming processes (Pedogenesis) and thus lack characteristics of O, A, E or B horizons.

f)      Horizon R

ü  The underlying hard bedrock.