Soil Forming Processes

    Soil forming processes or pedogenic processes are very complex because of simultaneously operating various chemical, mechanical and biochemical reactions.

Two categories of pedogenic processes:

• Basic or fundamental processes, 
• Specific pedogenic processes.

Basic or Fundamental Processes:

    The basic or fundamental processes are,

1. addition of water, organic and mineral matter to the soil,
2. loss of these materials from soil,
3. translocation of soil materials within soil profile,
4. transformation of mineral & organic matter & formation of definite layers or horizon.

Specific Pedogenic Processes:

     The basic or fundamental pedogenic processes bring about specific processes by means of a variety of reactions like Calcification, Decalcification, Podzolization, Laterization etc.,

Some of the important Soil Forming Processes:

Fundamental Processes	Specific Pedogenic Processes
Humification.	Calcification, Gypsification, Decalcification.
Eluviation.	Silication, Desilication, Podzolization, Laterization(Ferritization).
Illuviation.	Salination, Desalination, Alkalization, Dealkalization.
Horizonation.	Lessivege, Pedoturbation, Argillation.

Humification:

   Decomposition of organic materials & synthesis of new organic substances.

Eluviation:

   Mobilization & translocation of clay, Fe2O3, SiO2, Al2O3, humus, CaCO3 & other soluble salts,etc., from one area of the soil body to another area.

Illuviation: 

   Immobilization & accumulation of eluviation constituents at a depth below the soil surface .

Horizonation:

   Differentiation of soil in the different horizons along the depth of the soil body.

Podzolization:

   Leaching the entire solum off calcium carbonate & other soluble salts resulting in the formation of distinct acidic soil. It leads to the formation of Podzols or Podzolic soils. 

Laterization:

   Leaching of silica & accumulation of sesquioxides(Fe2O3, Al2O3)  in the upper layers leaving it to concentrate in the solum. It leads to the formation of Plinthite.

 

Podzolization	Laterization
It leads to the formation of podzols.	It leads to the formation of laterites.
It is the process of acid hydrolysis.	It is the process of alkaline hydrolysis.
Removal Fe, Al and cementation of Si.	Removal of Si and accumulation of Fe, Al.
High rainfall and low temperature is favourable.	High rainfall and high temperature is favourable.
Dominant influence of vegetation.	Influence of vegetation is not noticed.
High SiO2/Al2O3 ratio.	Low SiO2/Al2O3 ratio

Salinization:

   Accumulation of soluble salts such as sulphates and chlorides of calcium, magnesium, sodium and potassium. It leads to the formation of Salic Horizon. It is common in arid and semi-arid regions.

Alkalization:

  Accumulation of sodium ions in the exchange complex of clay. It results in the formation of Sodic Soil ( Natric Horizon).

Calcification:

  Formation and accumulation of calcium carbonate in the soil. It results in the formation of Calcic Horizon. It is common in arid and semi-arid regions.

Decalcification:

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

Pedoturbation: 

  It refers to the natural processes that mix the soil. Types of Pedoturbation:

• Faunal Pedoturbation: Mixing of soil by animals.
• Floral Pedoturbation: Mixing of soil by plants.
• Argillic Pedoturbation: Mixing of soil by the churning process by shrink-swell clays(deep black cotton soil).

Melanization:

   Darkening process of soil(usually dark brown) due to the accumulation of organic matter. It is best known in Mollisols(dark crumb textured soil of grasslands).

Gleization:

   In the lower part of the soil profile above the parent material due to poor drainage & water logged conditions(anaerobic condition) iron compunds are reduced to soluble ferrous form. Iron exists as Fe++ organo complexes or ferric & ferrous hydroxides. It leads to the formation of green-blue coloured Gley Horizons.

Argillation: 

   The process by which dispersed clay particles are migrated from upper to the lower soil horizons resulting in textural horizons.

Reference:

       Introductory Soil Science( fourth edition), D. K. Das, Kalyani Publishers, 2015.