Soil Formation and Weathering

• Soil formation is a gradual process influenced by physical, chemical, and biological factors.
• Weathering breaks down rocks into smaller particles, which form the basis of soil.

Weathering

• Weathering is the process that disintegrates rocks into smaller pieces.
• The two main types of weathering are physical and chemical.

Physical Weathering

• Physical weathering is the disintegration of rocks without changing their chemical composition.
• It increases the surface area of rocks, making them more susceptible to chemical weathering.
• Temperature changes can cause rocks to expand and contract, which leads to cracks and eventual breakdown.
• Frost action is when water seeps into cracks, freezes, expands, and breaks the rock.
• Wind erodes rocks through abrasion, particularly in arid environments.
• Water flow in rivers and streams erodes rocks through abrasion and hydraulic action.
• Glaciers grind and crush rocks as they move, leading to physical weathering.
• Living organisms, like roots, can grow into cracks and exert pressure, causing rocks to break.

Chemical Weathering

• Chemical weathering involves the decomposition of rocks through chemical reactions, which alters their mineral composition.
• Water is a key agent, facilitating many chemical reactions in chemical weathering.
• Oxidation is when oxygen reacts with minerals, especially iron-containing ones, causing them to rust and weaken.
• Carbonation occurs when carbon dioxide dissolves in water to form carbonic acid, which can dissolve rocks like limestone.
• Acid rain, caused by pollutants like sulfur dioxide and nitrogen oxides, accelerates chemical weathering.
• Living organisms can secrete acids that dissolve rocks.
• Hydrolysis involves the reaction of minerals with water, which leads to the formation of new minerals and the breakdown of the original rock.

Biological Weathering

• Biological weathering involves living organisms breaking down rocks.
• Lichens and mosses secrete acids that dissolve rocks.
• Plant roots can grow into cracks in rocks and exert pressure, causing them to break apart.
• Burrowing animals expose rocks to weathering agents.
• Decomposition of organic matter can release acids that contribute to chemical weathering.

Soil Composition

• Soil is composed of mineral particles, organic matter, water, and air.
• Mineral particles come from weathered rocks.
• Organic matter consists of decomposed plant and animal material, also known as humus.
• Water and air fill the spaces between soil particles.

Soil Formation

• Soil formation begins with the weathering of rocks.
• Physical weathering breaks rocks into smaller particles.
• Chemical weathering alters the mineral composition of the rock fragments.
• Organic matter accumulates on the surface and mixes with the mineral particles.
• Decomposition of organic matter releases nutrients that support plant growth.
• Living organisms, such as bacteria, fungi, and earthworms, contribute to soil development.
• Over time, distinct layers or horizons develop in the soil profile.

Soil Horizons

• Soil horizons are distinct layers within the soil profile, each with different characteristics.
• The O horizon is the uppermost layer, consisting of organic matter.
• The A horizon is the topsoil, which is a mixture of organic matter and mineral particles.
• It is usually dark in color and rich in nutrients.
• The E horizon is the eluviated layer, from which minerals and organic matter have been leached.
• It is usually light in color and sandy in texture.
• The B horizon is the subsoil, where minerals leached from the E horizon accumulate.
• It is usually dense and clayey.
• The C horizon is the parent material, composed of weathered rock fragments.
• The R horizon is the bedrock, which is the solid rock beneath the soil.

Factors Affecting Soil Formation

• Climate, specifically temperature and rainfall, affects the rate of weathering and decomposition.
• Organisms contribute to physical and chemical weathering, as well as the decomposition of organic matter.
• Relief, such as the slope and aspect of the land, affects soil drainage and erosion.
• Parent material composition influences the mineral content of the soil.
• Time: Soil formation is a slow process that takes thousands of years.
• Human activity can significantly impact soil formation and health, often negatively.
• Deforestation removes protective vegetation cover, increasing erosion and reducing organic matter inputs.
• Agricultural practices such as intensive tillage can degrade soil structure and deplete nutrients.
• Pollution from industrial and agricultural sources can contaminate soils, affecting their fertility and biological activity.
• Urbanization covers soils with impermeable surfaces, preventing water infiltration and disrupting natural soil processes.