Soil Genesis, Profile & Classification

1. Soil Genesis (Pedogenesis)

Soil is the uppermost layer of the earth's crust, composed of mineral particles, organic matter (humus), water, and air. The process of soil formation is called pedogenesis.

Factors Controlling Soil Formation (Dokuchaev & Jenny's Equation)

Soil = f (Cl, b, r, p, t)

1. Climate (Cl): The most dominant active factor, specifically temperature and precipitation. It dictates the rate of weathering and the type of vegetation.
2. Biosphere/Organisms (b): Plants contribute organic matter (humus) when they die. Microorganisms decompose this matter. Earthworms aerate the soil.
3. Relief/Topography (r): Steep slopes prevent soil accumulation resulting in thin soils. Flat valleys accumulate thick, fertile soils.
4. Parent Material (p): The underlying rock determines the soil's basic mineral composition, texture, and initial pH (e.g., shale produces clayey soils, sandstone produces sandy soils).
5. Time (t): Soil formation is extremely slow. It takes hundreds of years to form a single inch of mature (fully developed) soil.

2. Soil Profile

A vertical section through different layers of the soil, extending from the surface down to the unweathered parent rock. These layers are called horizons.

• O-Horizon (Organic): Uppermost layer of newly fallen leaves and partially decomposed organic matter. Absent in deserts and agricultural fields.
• A-Horizon (Topsoil): Rich in dark humus mixed with mineral matter. The zone of maximum biological activity and root growth. Also the zone of Eluviation (washing out of minerals by percolating water).
• E-Horizon: A light-colored, heavily eluviated (leached) layer below the A horizon, often sandy.
• B-Horizon (Subsoil): The zone of Illuviation (accumulation). Minerals, clays, iron, and aluminum oxides leached from above accumulate here.
• C-Horizon (Regolith): Partially weathered, loose parent rock. No biological activity.
• R-Horizon (Bedrock): Unweathered, solid parent rock.

3. Zonal Classification of Soils

Based broadly on climate:

• Zonal Soils: Mature soils with a well-developed profile, occupying an extensive area matching a major climate zone (e.g., Laterite in tropics, Podzol in cold regions).
• Intrazonal Soils: Soils where local factors (like poor drainage or specific parent rock like limestone) dominate over climate (e.g., Peat, Rendzina).
• Azonal Soils: Immature soils lacking a well-developed profile, usually recently deposited (e.g., Alluvial soils by rivers, Loess by wind).

4. Soil Degradation and Conservation

• Soil Erosion: The removal of topsoil by wind (deflation) or water (sheet, rill, and gully erosion). Causes include deforestation, overgrazing, shifting cultivation, and improper plowing.
• Degradation: Loss of fertility due to salinization (over-irrigation in dry areas drawing salts to the surface via capillary action), waterlogging, and excessive chemical fertilizer use.
• Conservation Methods:
  • Agronomic: Contour farming, crop rotation, strip cropping, mulching, and planting shelterbelts (windbreaks).
  • Mechanical: Terracing on steep slopes, building check dams, and gully plugging.