🧫 Soil formation factors

Soil develops through weathering and pedogenic processes governed by interacting formation factors. This lesson covers foundational models and major parent-material and relief effects.

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Soil formation factors and processes

The soil formation is the process of two consecutive stages .

1. The weathering of rock (R) into Regolith

2. The formation of true soil from Regolith

The evolution of true soil from regolith takes place by the combined action of soil forming factors and processes.

 The first step is accomplished by weathering (disintegration & decomposition)

 The second step is associated with the action of Soil Forming Factors

Weathering

Factors

2

Dokuchaiev (1889) established that the soils develop as a result of the action of soil forming factors

S = f (P, Cl, O)

Further, Jenny (1941) formulated the following equation

S = f (Cl, O, R, P, T, …)

Where,

Cl – environmental climate O – Organisms and vegetation (biosphere) R – Relief or topography P – Parent material T- Time … - additional unspecified factors

The five soil forming factors, acting simultaneously at any point on the surface of the earth, to produce soil.

Two groups

Passive : i) Parent material, ii) Relief, iii) Time

Active : iv) Climate, v) Vegetation & organism

Passive Soil forming factors

The passive soil forming factors are those which represent the source of soil forming mass and conditions affecting it. These provide a base on which the active soil forming factors work or act for the development of soil.

Parent Material

It is that mass (consolidated material) from which the soil has formed.

Two groups of parent material

 Sedentary

Formed in original place. It is the residual parent material. The parent material differ as widely as the rocks

 Transported

The parent material transported from their place of origin. They are named according to the main force responsible for the transport and redeposition.

a) by gravity - Colluvial b) by water - Alluvial, Marine, Locustrine c) by ice - Glacial d) by wind - Eolian

Colluvium

It is the poorly sorted materials near the base of strong slopes transported by the action of gravity.

Alluvium

The material transported and deposited by water is, found along major stream courses at

the bottom of slopes of mountains and along small streams flowing out of drainage basins.

Lacustrine

Consists of materials that have settled out of the quiet water of lakes.

Moraine

Consists of all the materials picked up, mixed, disintegrated, transported and deposited

through the action of glacial ice or of water resulting primarily from melting of glaciers.

Loess or Aeolian

These are the wind blown materials. When the texture is silty - loss; when it is sand.

Eolian

The soils developed on such transported parent materials bear the name of the parent

material; viz. Alluvial soils from alluvium, colluvial soils from colluvium etc. In the initial

stages, however, the soil properties are mainly determined by the kind of parent material.

Endodynamomorphic soils

With advanced development and excessive leaching, the influence of parent material on

soil characteristics gradually diminishes. There are soils wherein the composition of parent

material subdues the effects of climate and vegetation. These soils are temporary and persist only

until the chemical decomposition becomes active under the influence of climate and vegetation.

Ectodynamomorphic soils

Development of normal profile under the influence of climate and vegetation.

Soil properties as influenced by parent material: Different parent materials affect profile

development and produce different soils, especially in the initial stages.

 Acid igneous rocks (like granite, rhyolite) produce light-textured soils (Alfisols).

 Basic igneous rocks (basalt), alluvium or colluvium derived from limestone or basalt,

produce fine-textured cracking-clay soils (Vertisols).

 Basic alluvium or aeolian materials produce fine to coarse-textured soils (Entisols or

Inceptisols).

 The nature of the elements released during the decaying of rocks has a specific role in

soil formation. (e.g.) Si and Al forms the skeleton for the production of secondary clay

minerals.

 Iron and manganese are important for imparting red colour to soils and for oxidation and

reduction phenomena.

 Sodium and potassium are important dispersing agents for day and humus colloids.

 Calcium and magnesium have a flocculating effect and result in favorable and stable soil

structure for plant growth.

Relief or Topography

The relief and topography sometimes are used as synonymous terms. They denote the

configuration of the land surface. The topography refers to the differences in elevation of the

land surface on a broad scale.

The prominent types of topography designations, as given in FAO Guidelines (1990) are:

Land surface with slopes of

1 Flat to Almost flat 0 – 2 % 2 Gently undulating 2 - 5 % 3 Undulating 5 – 10 % 4 Rolling 10 – 15 % 5 Hilly 15 –3 0 % 6 Steeply dissect > 30 % with moderate range of elevation ( <300 m) 7 Mountainous > 30% with great range of

elevation (>300 m)

Soil formation on flat to almost flat position

On level topographic positions, almost the entire water received through rain percolates

through the soil. Under such conditions, the soils formed may be considered as representative of

the regional climate. They have normal solum with distinct horizons. But vast and monotonous

level land with little gradient often has impaired drainage conditions.

Soil formation on undulating topography

The soils on steep slopes are generally shallow, stony and have weakly- developed

profiles with less distinct horizonation. It is due to accelerated erosion, which removes surface

material before it has the time to develop. Reduced percolation of water through soil is because

of surface runoff, and lack of water for the growth of plants, which are responsible for checking

of erosion and promote soil formation.

Soil formation in depression

The depression areas in semi-arid and sub humid regions reflect more moist conditions

than actually observed on level topographic positions due to the additional water received as

runoff. Such conditions (as in the Tarai region of the Uttar Pradesh) favour more vegetative

growth and slower rate of decay of organic remains. This results in the formation of

comparatively dark- coloured soils rich in organic matter (Mollisols).

Soil formation and Exposure/ Aspect

Topography affects soil formation by affecting temperature and vegetative growth

through slope exposures (aspect). The southern exposures (facing the sun) are warmer and

subject to marked fluctuations in temperature and moisture. The northern exposures, on the other

hand are cooler and more humid. The eastern and western exposures occupy intermediate

position in this respect.

Time

Soil formation is a very slow process requiring thousands of years to develop a mature

pedon. The period taken by a given soil from the stage of weathered rock (i.e. regolith) up to the

stage of maturity is considered as time. The matured soils mean the soils with fully developed

horizons (A, B, C). It takes hundreds of years to develop an inch of soil. The time that nature

devotes to the formation of soils is termed as Pedologic Time.

It has been observed that rocks and minerals disintegrate and/or decompose at different

rates; the coarse particles of limestone are more resistant to disintegration than those of

sandstone. However, in general, limestone decomposes more readily than sandstone (by

chemical weathering).

Weathering stages in soil formation

 The soil properties also change with time, for instance nitrogen and organic matter

contents increase with time provided the soil temperature is not high.

 CaCO

3 [content may decrease or even lost with time provided the climatic conditions are ]

not arid

concentration increases with time because of chemical

 In humid regions, the H

weathering.

B. Active Soil Forming Factors

The active soil forming factors are those which supply energy that acts on the mass for the

purpose of soil formation. These factors are climate and vegetation (biosphere).

Climate

Climate is the most significant factor controlling the type and rate of soil formation. The

dominant climates recognized are:

 Arid climate: The precipitation here is far less than the water-need. Hence the soils

remain dry for most of the time in a year.

 Humid climate: The precipitation here is much more than the water need. The excess

water results in leaching of salt and bases followed by translocation of clay colloids.

 Oceanic climate: Moderate seasonal variation of rainfall and temperature.

 Mediterranean climate: The moderate precipitation. Winters and summers are dry and

hot.

 Continental climate: Warm summers and extremely cool or cold winters.

 Temperate climate: Cold humid conditions with warm summers.

 Tropical and subtropical climate: Warm to hot humid with isothermal conditions in the

tropical zone.

Climate affects the soil formation directly and indirectly.

Directly, climate affects the soil formation by supplying water and heat to react with parent

material.

Indirectly, it determines the fauna and flora activities which furnish a source of energy in the

form of organic matter. This energy acts on the rocks and minerals in the form of acids, and salts

are released. The indirect effects of climate on soil formation are most clearly seen in the

relationship of soils to vegetation.

Precipitation and temperature are the two major climatic elements which contribute most to soil

formation.

Precipitation

Precipitation is the most important among the climatic factors. As it percolates and

moves from one part of the parent material to another. It carries with it substances in solution as

well as in suspension. The substances so carried are re deposited in another part or completely

removed from the material through percolation when the soil moisture at the surface evaporates

causing an upward movement of water. The soluble substances move with it and are translocated

to the upper layer. Thus rainfall brings about a redistribution of substances both soluble as well

as in suspension in soil body.

Temperature

 Temperature is another climatic agent influencing the process of soil formation.

 High temperature hinders the process of leaching and causes an upward movement of

soluble salts.

 High temperature favors rapid decomposition of organic matter and increase microbial

activities in soil while low temperatures induce leaching by reducing evaporation and

there by favour the accumulation of organic matter by slowing down the process of

decomposition. Temperature thus controls the rate of chemical and biological reactions

taking place in the parent material.

Jenney (1941) computed that in the tropical regions the rate of weathering proceeds three times

faster than in temperate regions and nine times faster than in arctic .

Organism & Vegetation

Organism

 The active components of soil ecosystem are plants, animals, microorganisms and man.

 The role of microorganisms in soil formation is related to the humification and

mineralization of vegetation

 The action of animals especially burrowing animals to dig and mix-up the soil mass and

thus disturb the parent material

 Man influences the soil formation through his manipulation of natural vegetation,

agricultural practices etc.

 Compaction by traffic of man and animals decrease the rate of water infiltration into the

soil and thereby increase the rate of runoff and erosion.

Vegetation

 The roots of the plants penetrate into the parent material and act both mechanically and

chemically.

 They facilitate percolation and drainage and bring about greater dissolution of minerals

through the action of CO

2 [and acidic substances secreted by them. ]

 The decomposition and humification of the materials further adds to the solubilization of

minerals

 Forests – reduces temperature, increases humidity, reduce evaporation and increases

precipitation.

 Grasses reduce runoff and result greater penetration of water in to the parent material.

Soil Forming Processes

The pedogenic processes, although slow in terms of human life, yet work faster than the

geological processes in changing lifeless parent material into true soil full of life.

• The pedogenic processes are extremely complex and dynamic involving many chemical

and biological reactions, and usually operate simultaneously in a given area.

• One process may counteract another, or two different processes may work

simultaneously to achieve the same result.

• Different processes or combination of processes operate under varying natural

environment.

The collective interaction of various soil forming factors under different environmental

conditions set a course to certain recognized soil forming processes.

The basic process involved in soil formation (Simonson, 1959) includes the following.

 Gains or Additions of water, mostly as rainfall, organic and mineral matter to the soil.

 Losses of the above materials from the soil.

 Transformation of mineral and organic substances within the soil.

 Translocation or the movement of soil materials from one point to another within the

soil. It is usually divided into

 - movement of solution (leaching) and

 - movement in suspension (eluviation) of clay, organic matter and hydrous oxides

A. Fundamental Soil forming Processes

Humification

Humification is the process of transformation of raw organic matter into humus. It is

extremely a complex process involving various organisms.

First, simple compounds such as sugars and starches are attacked followed by proteins and

cellulose and finally very resistant compounds, such as tannins, are decomposed and the dark

coloured substance, known as humus, is formed.

Eluviation

It is the mobilization and translocation of certain constituent’s viz. Clay, Fe

2 [O] 3 [, Al]

2 [O] 3 [, ]

SiO

2 [, humus, CaCO]

3 [, other salts etc. from one point of soil body to another. 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. The eluviation encompasses mobilization and

translocation of mobile constituents resulting in textural differences. The horizon formed by the

process of eluviation is termed as eluvial horizon (A

2 [or E horizon). ]

Translocation depends upon relative mobility of elements and depth of percolation.

Illuviation

The process of deposition of soil materials (removed from the eluvial horizon) in the

lower layer (or horizon of gains having the property of stabilizing translocated clay materials) is

termed as Illuviation. The horizons formed by this process are

termed as illuvial horizons (B-horizons, especially Bt) The process leads to textural contrast

between E and Bt horizons, and higher fine: total clay ratio in the Bt horizon.

Horizonation

It is the process of differentiation of soil in different horizons along the depth of the soil

body. The differentiation is due to the fundamental processes, humification, eluviation and

illuviation.

B. Specific Soil Forming Processes

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

processes

Calcification

It is the process of precipitation and accumulation of calcium carbonate (CaCO

3 [) in some ]

part of the profile. The accumulation of CaCO

3 [may result in the development of a calcic ]

horizon. Calcium is readily soluble in acid soil water and/or when CO

root zone as:

2 [concentration is high in ]

CO

2 [+ H]

2 [O → H]

2 [CO]

H

2 [CO]

3 [+ Ca → Ca (HCO]

3 [)] 2 [(soluble) ]

Temp.

Ca (HCO

3 [)]

2 [→ CaCO]

3 [+ H]

2 [O + CO]

2 [(precipitates) ]

CO

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

Temp.

CaCO

3 [+ CO]

2 [+ H]

2 [O → Ca (HCO]

3 [)] 2 [(soluble) ]

(insoluble) CO

Podzolization

2

It is a process of soil formation resulting in the formation of Podzols and Podzolic soils.

In many respects, podzolization is the negative of calcification. The calcification process tends to

concentrate calcium in the lower part of the B horizon, whereas podzolization leaches the entire

solum of calcium carbonates.

Apart from calcium, the other bases are also removed and the whole soil becomes distinctly

acidic. In fact, the process is essentially one of acid leaching.

The process operates under favorable combination of the following environments.

i) Climate: A cold and humid climate is most favorable for podzolization.

ii) Parent material: Siliceous (Sandy) material, having poor reserves of weatherable minerals,

favor the operation of podzolization as it helps in easy percolation of water.

iii) Vegetation: Acid producing vegetation such as coniferous pines is essential

iv) Leaching and Translocation of Sesquioxide: In the process of decomposition of organic

matter various organic acids are produced. The organic acids thus formed act with Sesquioxide

and the remaining clay minerals, forming organic- Sesquioxide and organic clay complexes,

which are soluble and move with the percolating water to the lower horizons (Bh, Bs).

Aluminium ions in a water solution hydrolyze and make the soil solution very acidic.

2Al +6H

2 [O 2 Al (OH)] 3 [+ 6H]

+

As iron and aluminium move about, the A horizon gives a bleached grey or ashy

appearance. The Russians used the term Podzols (pod means under, the zola means ash like i.e.

ash-like horizon appearing beneath the surface horizon) for such soils.

To conclude, the Podzolization is a soil forming process which prevails in a cold and humid

climate where coniferous and acid forming vegetations dominate. The humus and Sesquioxide

become mobile and leached out from the upper horizon s and deposited in the lower horizon.

Laterization

The term laterite is derived from the word later meaning brick or tile and was originally

applied to a group of high clay Indian soils found in Malabar hills of Kerala, Tamil Nadu,

Karnataka and Maharashtra.

It refers specifically to a particular cemented horizon in certain soils which when dried,

become very hard, like a brick. Such soils (in tropics) when massively impregnated with

sesquioxides (iron and aluminium oxides) to extent of 70 to 80 per cent of the total mass, are

called laterites or latosols (Oxisols). The soil forming process is called Laterization or

Latozation.

Laterization is the process that removes silica, instead of sesquioxides from the upper layers and

thereby leaving sesquioxides to concentrate in the solum. The process operates under the

following conditions.

i) Climate

Unlike podzolization, the process of laterization operates most favorable in warm and

humid (tropical) climate with 2000 to 2500 mm rainfall and continuous high temperature (

25°C) throughout the year.

ii ) Natural vegetation

The rain forests of tropical areas are favorable for the process.

iii) Parent Material

Basic parent materials, having sufficient iron bearing ferromagnesian minerals

(Pyroxene, amphiboles, biotite and chlorite), which on weathering release iron, are congenial for

the development of laterites.

Gleization

The term glei is of Russian origin 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 soil profile above the parent material due to poor drainage condition (lack of oxygen)

and where waterlogged conditions prevail. Such soils are called hydro orphic soils.

The process is not particularly dependent on climate (high rainfall as in humid regions) but often

on drainage conditions.

The poor drainage conditions result from:

 Lower topographic position, such as depression land, where water stands continuously at

or close to the surface.

 Impervious soil parent material, and.

 Lack of aeration.

Under such conditions, iron compounds are reduced to soluble ferrous forms. The reduction of

iron is primarily biological and requires both organic matter and microorganisms capable of

respiring anaerobically. The solubility of Ca, Mg, Fe, and Mn is increased and most of the iron

++

exists as Fe

hydroxides.

organo - complexes in solution or as mixed precipitate of ferric and ferrous

This is responsible for the production of typical bluish to grayish horizon with mottling of yellow

and or reddish brown colors.

Salinization

It is the process of accumulation of salts, such as sulphates and chlorides of calcium,

magnesium, sodium and potassium, in soils in the form of a salty (salic) horizon. It is quite

common in arid and semi arid regions. It may also take place through capillary rise of saline

ground water and by inundation with seawater in marine and coastal soils. Salt accumulation

may also result from irrigation or seepage in areas of impeded drainage.

Desalinization

It is the removal by leaching of excess soluble salts from horizons or soil profile (that

contained enough soluble salts to impair the plant growth) by ponding water and improving the

drainage conditions by installing artificial drainage network.

Solonization or Alkalization

The process involves the accumulation of sodium ions on the exchange complex of the

clay, resulting in the formation of sodic soils (Solonetz).

All cations in solution are engaged in a reversible reaction with the exchange sites on the clay

and organic matter particles.

The reaction can be represented as

++

Ca.Mg.2NaX Ca

++

+Mg

+

+2Na

-6

+ x

+3CO

2 [CO]

2-

3

Na

3 [+ MgCO]

3 [+CaCO]

3

(Where X represents clay or organic matter exchange sites)

Solodization or dealkalization

The process refers to the removal of Na

from the exchange sites. This process involves

dispersion of clay. Dispersion occurs when Na

ions become hydrated.

++

ions are concentrated in the water,

Much of the dispersion can be eliminated if Ca

and or Mg

++

which is used to leach the soonest. These Ca and Mg ion can replace the Na on exchange

complex, and the salts of sodium are leached out as :

2NaX + CaSO

4 [ Na]

2 [SO]

4 [+ CaX ]

(leachable)

Pedoturbation

Another process that may be operative in soils is pedoturbation. It is the process of

mixing of the soil.

Mixing to a certain extent takes place in all soils. The most common types of pedoturbation are:

 Faunal pedoturbation: It is the mixing of soil by animals such as ants, earthworms,

moles, rodents, and man himself

 Floral pedoturbation : It is the mixing of soil by plants as in tree tipping that forms pits

and mounds

 Argillic pedoturbation: It is the mixing of materials in the solum by the churning

process caused by swell shrink clays as observed in deep Black Cotton Soils.

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Summary Cheat Sheet

Quick Recall Points

• CLORPT = climate, organisms, relief, parent material, and time.
• Pedogenesis reflects the interaction of all five factors, not a single factor.
• Factor intensity and duration determine final soil characteristics.

Exam Traps

• Do not treat CLORPT factors as independent; they interact continuously.
• Similar parent material can still form different soils under different climate and relief.

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

FAO Soils Portal

Website