Soil Forming Processes - 1 | Agriculture Optional Notes for UPSC PDF Download

Introduction

• Pedogenic processes, though slow in human terms, are more rapid than geological processes in transforming lifeless parent material into living soil.
• Pedogenic processes are highly intricate and dynamic, involving numerous chemical and biological reactions that often occur simultaneously.
• These processes can sometimes counteract each other or work together to achieve the same outcomes.
• Different processes or combinations of processes are at play under various natural conditions.
• The combined interactions of soil-forming factors in different environmental settings give rise to recognized soil-forming processes.
• Fundamental processes in soil formation include the addition of water (mostly from rainfall) and organic and mineral matter to the soil, losses of these materials from the soil, the transformation of mineral and organic substances within the soil, and the translocation or movement of soil materials from one point to another. This translocation can be divided into the movement of solutions (leaching) and the movement in suspension (eluviation) of clay, organic matter, and hydrous oxides.

Fundamental Soil forming Processes 

Humification

Humification is a complex process in which raw organic matter is transformed into humus. This process involves various organisms. It begins with the breakdown of simple compounds like sugars and starches, followed by the decomposition of proteins and cellulose. Finally, even highly resistant compounds like tannins are broken down, resulting in the formation of dark-colored humus.

Eluviation

Eluviation refers to the mobilization and movement of specific constituents, including clay, Fe₂O₃, Al₂O₃, SiO₂, humus, CaCO₃, and other salts, from one part of the soil to another. This process involves the washing out of these constituents. It occurs as percolating water moves from the upper layers to the lower layers, carrying the constituents with it. Eluviation leads to differences in soil texture, and the horizon created by this process is known as the eluvial horizon (A2 or E horizon). The extent of translocation depends on the relative mobility of elements and the depth of percolation.

Illuviation

Illuviation is the process by which soil materials that were removed from the eluvial horizon (the horizon where materials are washed out) are deposited in a lower layer or horizon that has the property of stabilizing the translocated clay materials. The horizons formed by illuviation are known as illuvial horizons, typically labeled as B-horizons, with Bt horizons being a common example. This process creates a textural contrast between the eluvial (E) and illuvial (Bt) horizons, often resulting in a higher fine clay to total clay ratio in the Bt horizon.

Horizonation

Horizonation is the process of differentiating the soil into distinct horizons as you move deeper into the soil. This differentiation occurs due to fundamental processes like humification, eluviation, and illuviation.

Specific Soil Forming Processes 

The basic pedologic processes provide a framework for later operation of more specific processes

Calcification

Calcification is the process of calcium carbonate (CaCO₃) precipitation and its accumulation in a specific part of the soil profile. This accumulation of CaCO₃ can lead to the formation of a calcic horizon. Calcium is easily soluble in soil water with high acidity or when there is a high concentration of CO₂ in the root zone.

The process of precipitation after mobilization under these conditions is called calcification and the resulting illuviated horizon of carbonates is designated as Bk horizon (Bca). 

Decalcification

It is the reverse of calcification that is the process of removal of CaCO₃ or calcium ions from the soil by leaching

Podzolization

Podzolization is a soil formation process that leads to the development of Podzols and Podzolic soils. In contrast to calcification, which concentrates calcium in the lower part of the B horizon, podzolization involves the leaching of calcium carbonates from the entire soil profile. This process not only removes calcium but also other basic elements, resulting in an overall increase in soil acidity. Essentially, podzolization is characterized by the acid leaching of the soil.

The process operates under favorable combination of the following environments:

• Podzolization is most favorable in regions with a cold and humid climate.
• Siliceous (sandy) parent material, which has limited reserves of weatherable minerals, supports podzolization as it allows water to percolate easily.
• Acid-producing vegetation, like coniferous pines, is a key factor.
• During the decomposition of organic matter, various organic acids are generated. These organic acids interact with sesquioxides and remaining clay minerals, forming soluble organic-sesquioxide and organic clay complexes. These complexes are carried by percolating water to lower horizons (Bh, Bs). Additionally, aluminum ions in a water solution undergo hydrolysis, making the soil solution highly acidic.
  

Iron and aluminum movement leads to the A horizon taking on a bleached, grey, or ashy appearance. The term "Podzols" was coined by Russians, combining "pod" meaning under and "zola" meaning ash-like, to describe soils with an ash-like horizon appearing below the surface horizon.

In conclusion, Podzolization is a soil formation process that occurs in cold and humid climates dominated by coniferous vegetation with acid-forming properties. This process results in the mobilization and leaching of humus and sesquioxides from upper horizons, which are then deposited in lower horizons.

Laterization

The term "laterite" originated from the word "later," meaning brick or tile. Initially, it was used to describe a group of highly clayey Indian soils found in the Malabar hills of Kerala, Tamil Nadu, Karnataka, and Maharashtra.

Specifically, it refers to a particular cemented horizon in certain soils that, when dried, becomes as hard as a brick. In tropical regions, when these soils are significantly saturated with sesquioxides (iron and aluminum oxides), making up about 70 to 80 percent of the total mass, they are referred to as laterites or latosols (Oxisols). The soil formation process associated with this is called Laterization or Latozation.

Laterization is the process that removes silica from the upper soil layers instead of sesquioxides, leading to the concentration of sesquioxides in the soil profile.
This process occurs under specific conditions:

• Climate: In contrast to podzolization, laterization predominantly occurs in warm and humid tropical climates with an annual rainfall ranging from 2000 to 2500 mm and consistently high temperatures (around 25°C) throughout the year.
• Natural vegetation: The lush rainforests found in tropical regions provide an ideal environment for the laterization process.
• Parent Material: Parent materials with a basic nature, containing a sufficient amount of iron-bearing ferromagnesian minerals such as pyroxene, amphiboles, biotite, and chlorite, which release iron during weathering, are well-suited for the development of laterite soils.