Soil formation Factors - 2 | Agriculture Optional Notes for UPSC PDF Download

Relief or Topography

Relief and topography are often used interchangeably, both referring to the shape and configuration of the land surface. However, topography specifically addresses variations in land elevation on a broader scale.

According to FAO Guidelines from 1990, there are several categories to designate different types of topography based on the slopes of the land surface:

• Flat to Nearly Flat: 0-2%
• Gently Undulating: 2-5%
• Undulating: 5-10%
• Rolling: 10-15%
• Hilly: 15-30%
• Steeply Dissected: >30% with a moderate range of elevation (<300 m)
• Mountainous: >30% with a significant range of elevation (>300 m)

Soil formation on flat or nearly flat terrain

On flat or nearly flat areas, most of the rainwater that falls percolates through the soil. Soils formed under such conditions are representative of the regional climate. They have well-defined soil layers (horizons) and a normal soil profile. However, flat lands with minimal slopes often experience drainage issues due to poor water movement.

Soil formation on undulating terrain

Soils on steep slopes are typically shallow, contain rocks, and have less distinct soil horizons. This is because rapid erosion removes surface materials before they can fully develop. Reduced water percolation occurs due to surface runoff, leading to insufficient water for plant growth, which in turn hampers erosion control and soil formation.

Soil formation in depressions

Depression areas in semi-arid and sub-humid regions retain more moisture compared to flat terrains. This is due to additional water received as runoff. These conditions, found in areas like the Tarai region of Uttar Pradesh, support more vegetation growth and slower decomposition of organic matter. Consequently, this leads to the formation of darker soils rich in organic material, known as Mollisols.

Soil formation and exposure/aspect

Topography influences soil formation by impacting temperature and plant growth through slope exposures or aspects. Southern exposures, facing the sun, experience higher temperatures and greater temperature and moisture fluctuations. Northern exposures, on the other hand, are cooler and more humid. Eastern and western exposures fall in between these two extremes in terms of temperature and moisture characteristics.

Active Soil Forming Factors 

The active soil forming factors provide the energy needed for soil formation. These factors are primarily climate and vegetation (biosphere).

1. Climate

Climate is the most influential factor that determines the type and speed of soil formation.
Different dominant climates are recognized, including:

• Arid Climate: In arid regions, the amount of precipitation is significantly less than the water required. Consequently, the soils in these areas are mostly dry throughout the year.
• Humid Climate: Humid regions experience precipitation well in excess of the water demand. This surplus water leads to the leaching of salts and bases, followed by the translocation of clay colloids.
• Oceanic Climate: Oceanic climates feature moderate seasonal variations in both rainfall and temperature.
• Mediterranean Climate: Mediterranean regions receive moderate precipitation, with dry and hot winters and summers.
• Continental Climate: Areas with a continental climate have warm summers and extremely cold or cold winters.
• Temperate Climate: These regions have cold and humid conditions along with warm summers.
• Tropical and Subtropical Climate: Tropical and subtropical areas experience warm to hot and humid conditions, with isothermal conditions prevalent in the tropical zone.

Climate plays a dual role in soil formation, both directly and indirectly.

• Directly, climate impacts soil formation by providing water and heat, which interact with the parent material to initiate the process.
• Indirectly, climate influences the activities of plants and animals (fauna and flora), which serve as a source of energy in the form of organic matter. This energy, in turn, acts on rocks and minerals by producing acids that release salts. The indirect effects of climate on soil formation are most evident in their relationship with vegetation.

Two key climatic factors, precipitation, and temperature, have the most significant influence on soil formation.

Precipitation

Precipitation is the most significant climatic factor in soil formation. When rainwater percolates through the parent material, it transports various substances in both dissolved and suspended forms. These substances are either redeposited in different areas of the material or eliminated from it as water moves upwards due to surface evaporation, carrying the soluble substances along. Consequently, rainfall leads to the redistribution of both soluble and suspended substances within the soil.

Temperature

Temperature serves as another climatic factor that influences the soil formation process.

• Elevated temperatures can impede leaching and lead to the upward movement of soluble salts within the soil.
• Higher temperatures are conducive to the rapid decomposition of organic matter and increased microbial activity in the soil. Conversely, lower temperatures, by reducing evaporation, promote leaching and facilitate the accumulation of organic matter, slowing down the decomposition process.
• Temperature is a critical factor in controlling the speed of both chemical and biological reactions occurring in the parent material.

For instance, Jenney (1941) found that weathering processes in tropical regions advance three times more rapidly than in temperate areas and nine times faster than in arctic regions.

2. Organism & Vegetation 

Organism

• The soil ecosystem comprises active components such as plants, animals, microorganisms, and human activity.
• Microorganisms play a role in soil formation by contributing to the processes of humification and mineralization of organic matter derived from vegetation.
• Animals, particularly burrowing species, contribute to soil formation by excavating and mixing the soil, thus altering the parent material.
• Human activities, including the manipulation of natural vegetation and agricultural practices, can also influence soil formation.
• Human and animal-induced compaction of the soil can decrease the rate of water infiltration, leading to increased runoff and erosion.

Vegetation

• Plant roots have a dual impact on soil formation, mechanically and chemically.
• They contribute to improved percolation and drainage by enhancing mineral dissolution through the release of CO₂ and acidic substances.
• The decomposition and humification of organic materials by plants further aid in the solubilization of minerals.
• Forested areas have the effect of lowering temperatures, increasing humidity, reducing evaporation, and promoting higher precipitation.
• Grasslands help reduce runoff and encourage greater water penetration into the parent material.