Soil Formation: Factors and Processes

Soil Formation:- Soil is formed weathering of rocks and minerals. Soil formation is a slow process of two continuous but overlapping stages:-

1. Weathering of rocks in to parent material.

2. The formation of the soil from parent material.

Weathering Soil forming processes

Rock ----------->Parent material ----------------------->Soil

Dokuchaev (1889) was the first scientist who studied and classified soil forming factors viz: parent material, climate and organism, which he put in the form of an equation:-

S = F(Cl, O, R ,P,T)

S = Soil 

Cl = Climate

B = Biosphere

R = Relief / topography

P = Parent Material

T = Time

According to Jenny (1941), "Soil property is determined by the relative influnce of these factors."

He kept all the factors of soil formation in three groups:-

(a). Passive Factors:- Parent Material, Topography

(b). Active Factors:- Climate, Organisms

(c). Neutral Factors:- Time or age of land

Soil Formation Processses

The basic pedogenic processes involved in soil formation, according to Simonson (1959) are:

(1). Additions of water (mostly as rainfall) organic and mineral matter to the soil.

(2). Losses of above material from soil.

(3). Translocation in solution(leaching) and movement in suspension(eluviation) of clay, O.M. and hydrus oxide.

(4). Transformation of minerals and organic material and formation of definite layers.

Fundamental Processes are:-

(1). Humification:-  Humification is the process of transformation (decomposition) of raw organic matter in to ‘HUMUS’. It is an extremely complex process involving various organisms such as bacteria, fungi, actinomycetes, earth worms and termites.

(2). Eluviation:-  Eluviation means “Washing out”.  It is the process of removal of constituents in suspension or solution by the percolating water from the upper to lower layers. Leaching refers to the movement and removal of material in solution from the soil.

(3). Illuviation:- The process of deposition of soil materials (removed from the elluvial horizon “E”) in the lower layer is termed as “illuviation”.  The horizons formed by this process are termed as illuvial horizons (B-horizon especially).

(4). Horizonation:- The development of all the horizons in soil is termed as horizonation. In the favorable conditions it take about 250 years but in adverse conditions take even 1000 years.

Specific Pedogenic Processes.     (a). Zonal Pedogenic processes:-

(1). Calcification:- The process of precipitation and accumulation of calcium carbonate in some part of the profile is called calcification. This is a common process in arid and semi-arid regions, which are low in rainfall.  

The illuviated horizon of CaCO₃ is designated as ‘calcic horizon’.

(2). Decalcification:- It is the reverse of calcification that is the process of removal of CaCO₃ or calcium ions from the soil by leaching.

This occurs mostly in high rainfall or humid regions.

(3). Podzolization:-  ( Russian term) Pod = under and zola = ash like. It is a process of soil formation resulting in the formation of podzols and podzolic soils.  It is the process of accumulation of silica and eluviation of sesquioxides. The podzols soils are low in fertility and  mainly used for forestry and pastures.

(4). Laterization:- The term laterite is derived from the word “later” means “brick” or “tile”. This soil forming process is called “laterization” or “Lotozation” Eg: Soils of Malabar hills of Kerala. In laterization, silica is removed leaving sesquioxides to remain in solum. Favourable conditions are warm and  humid climate. this type of soil mainly used for shifting cultivation, growing plantation crops.

(b). Intrazonal Processes:-

(5). Gleization:- “Glei” means blue, grey or green clay. The gleization is a process of soil formation resulting in the development of a glei (or gley) horizon in the lower part of the profile above the parent material due to poor drainage conditions or water logged conditions.  Such soils are called “hydromorphic soils”.

(6). Salination:- It is the process of accumulation of salts such as sulphates, chlorides of calcium, magnesium, sodium and potassium in soils in the form of salty (salic) horizon. The soils are called saline soils, which have ESP less than 15% and pH between 7 and 8.5.

(7). Solonization/Alkalization:- The process involves the accumulation of sodium ions on the exchange complex of the clay, resulting in the formation of sodic soils (solonetz) under arid and semi-arid conditions. This occurs when anions like carbonates and bicarbonates predominate in soil.

(8). Solodization/Dealakilzation:- This process refers to the removal of Na⁺ from exchange sites.  The Na+ can be eliminated by increasing the concentration of Ca²⁺ or Mg²⁺ in the water, followed by improved drainage facilities.

(9). Pedoturbation:- It is the process of mixing of the soils. Mixing to some extent takes place in all soils.

 (1). Faunal pedoturbation:- Mixing by animals such as      ants, earthworms, moles, rodents and man himself.

 (2). Floral pedoturbation:- Mixing by plants, as in tree tipping that forms pits and mounds.

(3). Argillopedoturbation:- Mixing of materials in solum by churning process caused by swell - shrink clays as is observed in deep black cotton soil.

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