Biogeography and Soil

The upper layer of loose material covering rocks, containing both organic and mineral matter, that supports plant growth is known as soil. It serves as a dynamic natural body covering the Earth’s surface. Soil can be transported by erosion agents like water or wind, or it may remain in its original position (in-situ). The properties and fertility of soil are modified by various natural factors like climate, biological activity, and topography.

Learning Outcomes:

1. Understand the factors influencing soil formation.
2. Learn the characteristics and significance of soil horizons.
3. Identify soil types and classification based on chemical composition and formation processes.
4. Comprehend the process of soil erosion and conservation methods.
5. Explore the relationship between soil, vegetation, and biotic regions.

Factors Influencing Soil Formation

Several key factors influence the formation and properties of soil:

1. Parent Rock: Contributes to soil texture and fertility. The mineral composition of the parent rock determines soil’s physical and chemical characteristics.
2. Climate: Temperature and rainfall impact soil development. High temperatures increase bacterial activity and weathering, whereas low temperatures aid the formation of thick organic layers.
3. Biotic Activity: Plants and animals contribute organic matter (humus) and help form the soil profile. Decomposing plant material enriches soil fertility.
4. Topography: Slope and location affect soil formation. Steeper slopes result in thinner soils due to difficulty in retaining soil constituents.
5. Time: Soil formation rates vary based on rock porosity. Sandstones and less dense rocks form soil quicker than impervious, massive rocks.

Soil Fertility and Structure

Soil fertility is the soil’s capacity to support plant growth, dependent on organic substances and minerals.

1. Soil Profile: The vertical arrangement of soil layers from the surface to parent rock. Identifiable horizons include:

• O-Horizon: Composed of decomposing organic material.
• A-Horizon: Rich in minerals and organic matter.
• E-Horizon: Characterized by heavy leaching.
• B-Horizon: Accumulates minerals from upper layers.
• C-Horizon: Least weathered, consisting of loose parent material.

1. Soil Structure: The aggregation mode of soil particles influencing soil fertility and porosity. Affects water retention and root penetration.
2. Soil Texture: Classified by particle size, influencing water and nutrient retention. Gravel (>2 mm), coarse sand (2-0.2 mm), fine sand (0.2-0.02 mm), silt (0.02-0.002 mm), clay (<0.002 mm).
3. Organic Content: Amount of humus determined by vegetation, animals, and bacterial activity. Influences soil acidity or alkalinity.
4. Soil Acidity: Measured on the pH scale (0-14). A pH of 6.5 is favorable for cereal crops. Acidic soils result from lime deficiency and develop in high-rainfall areas.

Mineral Composition of Soil

1. Primary Minerals: Silicate compounds of silicon and oxygen with various elements (aluminum, calcium, sodium). Do not directly sustain plant life.
2. Secondary Minerals: Essential for soil fertility. Clay minerals determine base status, affecting fertility. The presence of humus colloids indicates high fertility.

Important Note: Soils with high humus content usually possess higher fertility due to enhanced base-holding capacity.

Soil Forming Processes

1. Weathering: Breakdown of parent rocks in-situ.
2. Translocation: Movement of soil materials within the profile.
3. Eluviation: Downward transport of fine particles.
4. Illuviation: Accumulation of materials in lower horizons.
5. Decalcification: Leaching of calcium carbonate in moist climates.
6. Calcification: Accumulation of soluble materials in dry regions.
7. Salinization: Salt accumulation due to capillary action in arid zones.
8. Desilication: Removal of silica, leading to ferralsol formation.
9. Podzolization: Mobility of iron and aluminum oxides, associated with coniferous vegetation.
10. Gleying: Occurs in waterlogged areas.

Soil Development Stages

1. Initial Stage: Unweathered parent material.
2. Juvenile Stage: Onset of weathering.
3. Virile State: Significant decomposition; increased clay content.
4. Senile State: Advanced decomposition; resistant minerals remain.
5. Final Stage: Completion of soil development.

Classification of Soil

Soil classification depends on factors like parent rock and formation location:

1. Azonal Soil: Immature soils, formed recently, lacking distinct horizons (e.g., volcanic soil, loess sand).
2. Intrazonal Soil: Influenced by local factors; not solely determined by climate (e.g., calcareous soils, hydromorphic soils).
3. Zonal Soil: Mature soils, distinctly influenced by parent rock and climate (e.g., Tundra soil, Mediterranean soil).

Specific Soil Types

1. Podzol: Formed in cool climates, highly leached, and acidic.
2. Grey Podzol: Found in sub-Arctic regions, low pH, eluviated A-horizon.
3. Laterite Soil: Formed in tropical regions; leached of humus; useful for building materials when hardened.
4. Chernozem: Highly fertile, found in Steppe regions, rich in humus.
5. Prairie Soil: Fertile, found in temperate tall grass regions.

Soil Erosion and Conservation

Soil erosion involves the wearing away of soil due to natural agents or human activities. Conservation techniques include:

1. No-Tillage Farming: Growing crops without disturbing existing plant cover.
2. Contour Cropping: Ploughing along land contours to minimize runoff.
3. Strip Cropping: Alternating strips of dense plants to slow erosion.
4. Terracing: Creating flat terraces to reduce runoff on steep slopes.

Important Concept: Soil conservation methods maintain soil health and prevent the loss of fertility, crucial for sustainable agriculture.

Natural Vegetation and Biotic Regions

Natural vegetation consists of plant species growing without human intervention. It includes forests, grasslands, and deserts:

1. Forest Biome: Varies from tropical rainforests to temperate deciduous and coniferous forests.
2. Grassland Biome: Dominated by herbs and grasses; includes prairies, steppe, and savanna.
3. Desert Biome: Sparse vegetation due to extreme aridity.

USDA Soil Classification

Soils are classified into 10 orders under the USDA system:

1. Alfisols: Marked by clay translocation; moderate fertility.
2. Andisols: Dominated by short-range minerals, high phosphorus retention.
3. Gelisols: Presence of permafrost, features related to freezing and thawing.
4. Aridisols: Found in arid regions, minimal water for mesophytic plants.
5. Histosols: High organic content in the upper 80 cm; no permafrost.
6. Inceptisols: Wide range of characteristics; found in various climates.
7. Entisols: Dominance of mineral soil materials; lack distinct horizons.
8. Vertisols: High clay content, significant volume changes with moisture.
9. Oxisols: Extremely weathered; found in tropical/sub-tropical regions.
10. Ultisols: Intensive leaching, low supply of bases in lower horizons.
11. Mollisols: Dark brown to black surface, fertile, found in grasslands.

Important Note: Soil types influence agricultural practices and ecosystem dynamics.

Soil Conservation Techniques

1. No-Tillage Farming: Prevents soil erosion by preserving crop residue.
2. Contour Cropping: Helps retain water and reduce erosion on slopes.
3. Strip Cropping: Alternating crop types to slow water runoff.
4. Terracing: Reduces runoff and erosion on steep slopes.
5. Crop Rotation: Improves soil fertility by alternating crops.
6. Gully Reclamation: Quick-growing plants and check dams to prevent erosion.
7. Windbreaks: Planting shrubs and trees to shield cropland from wind erosion.

Concept: Effective soil conservation ensures the sustainability of land for future generations.

**Natural Vegetation and Bi

omes**

1. Tropical Rainforest: Evergreen forests with high biodiversity, found near the equator.
2. Savanna: Combination of trees and grasslands, fire-resistant vegetation.
3. Taiga: Cold climate, conical trees, long winters, and short summers.
4. Tundra: Extremely cold, short growing season, dominated by low-lying vegetation.

Important Note: Different biomes are determined by climate, soil type, and vegetation patterns.

MCQ: Which soil type is characterized by high clay content and significant volume changes with moisture?

1. Entisols
2. Oxisols
3. Vertisols
4. Histosols
   Correct Answer: 3. Vertisols