Rocks and Soil Formation

1. Rocks: Types and Properties

Rocks are the parent materials from which soil is formed. They are classified into three major types based on their mode of formation:

• Igneous Rocks: Formed from the cooling and solidification of molten magma or lava. Examples: Granite, Basalt, Pumice. They are usually crystalline and do not contain fossils.
• Sedimentary Rocks: Formed from the accumulation and cementation of mineral or organic particles (sediments) over time. Examples: Limestone, Sandstone, Shale. They occur in layers (strata) and often contain fossils.
• Metamorphic Rocks: Formed when existing igneous or sedimentary rocks are subjected to high heat and pressure, changing their form. Examples: Marble (from limestone), Slate (from shale), Gneiss (from granite).

2. Factors Affecting Soil Formation (CLORPT)

Soil formation (pedogenesis) is influenced by five main factors:

• Climate: Temperature and rainfall are the most important. Rainfall promotes weathering and leaching, while temperature affects the rate of chemical reactions.
• Living Organisms (Biotic factors): Bacteria, fungi, and earthworms decompose organic matter. Roots of plants can break rocks.
• Parent Material: The type of rock from which the soil is formed determines its mineral composition and texture.
• Topography (Relief): The slope of the land affects erosion and water drainage. Steep slopes have thinner soils.
• Time: Soil formation is a slow process; mature soils take thousands of years to develop.

3. Weathering of Rocks

Weathering is the breaking down of rocks into smaller particles. It occurs in three ways:

• Physical Weathering: Mechanical breakdown without chemical change (e.g., temperature changes, ice wedging, wind).
• Chemical Weathering: Change in the chemical composition of minerals (e.g., Hydration, Oxidation, Carbonation, Hydrolysis).
• Biological Weathering: Caused by activities of living organisms (e.g., root penetration, burrowing animals).

4. Physical Properties of Soil

Soil Profile

A soil profile is a vertical section through the soil showing its different layers called horizons:

• Horizon A (Topsoil): Rich in organic matter (humus), dark in color, and where most biological activity occurs.
• Horizon B (Subsoil): Accumulates minerals leached from Horizon A (process called illuviation).
• Horizon C (Parent Material): Partially weathered rock.
• Horizon D (Bedrock): Unweathered solid rock.

Soil Texture and Structure

Texture: Refers to the relative proportion of sand, silt, and clay particles. Structure: Refers to the arrangement of soil particles into aggregates (e.g., granular, blocky, platy).

5. Chemical Properties and Soil pH

Soil pH measures the acidity or alkalinity of the soil. It ranges from 0 to 14, where 7 is neutral. Below 7 is acidic, above 7 is alkaline. Soil acidity can be corrected by liming (adding Calcium carbonate or Lime).

6. Soil Components and Calculations

A typical soil consists of:

• Mineral Matter: 45%
• Organic Matter: 5%
• Soil Air: 25%
• Soil Water: 25%

Example Calculation: If a soil sample weighs 100g and after heating in an oven it weighs 80g, the moisture content is: ((100 - 80) / 100) * 100 = 20%.