🦪 Soil Formation

• The soil formation is the process of two consecutive stages.
  • The weathering of rock (R) into Regolith
  • 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.

  

Factors

• 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 (P, Cl, O, R, 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.
• Factors can classified into three groups:
  • Active: i) Climate, ii) Organism & Vegetation
  • Passive: iii) Parent material, iv) Relief/Topography
  • Neutral: v) Time

A. 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.

1. Parent Material

• It is that mass (consolidated material) from which the soil has formed.
• Chemical composition of soil and physically weathered soils has some resemblance to that of parent material, texture determines the depth of soil profile. Chemical compositions of soils depends upon the types of rocks and their mineralogical composition.
• Two groups of parent material
  • Sedentary Soil: Soils formed in original place/in-situe. It is the residual parent material. The parent material differ as widely as the rocks. Soil formation takes long time from this parent material. Igneous rock, sedimentary rock and metamorphic rock are the examples of this category.
  • Biological patent material: Decomposed or partially decomposed biological matters are termed as biological parent material.
  • Transported: The parent material transported from their place of origin. They are named according to the main force responsible for the transport and redeposition.

    

• 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.
  • Maraine: 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 windblown materials.
  • When it is sand - aeolian.
  • When the texture is silty – loess.

Endodynamomorphic soils

• Soils whose properties are influenced mainly by the parent material.

Ectodynamomorphic soils

• Soils whose properties are influenced mainly by factors other than parent material.
• Development of normal profile under the influence of climate and vegetation.

Ectoendodynamorphic

• Soils whose properties are influenced by both parent material and other factors.
• 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 clay and humus colloids.
  • Calcium and magnesium have a flocculating effect and result in favorable and stable soil structure for plant growth.

2. 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:

  

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.

3. 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).

• The soil properties also change with time, for instance nitrogen and organic matter contents increase with time provided the soil temperature is not high.
• CaCO₃ content may decrease or even lost with time provided the climatic conditions are not arid.
• In humid regions, the H⁺ concentration increases with time because of chemical 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).

1. 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-needed. 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.
  • In scanty rainfall regions, salts are accumulated at the soil surface.
  • But in high rainfall regions, total salts are leached into the lower horizon due to which soil becomes acidic.
• 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 CO₂ 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, Fe₂O₃, Al₂O₃, SiO₂, humus, CaCO₃, 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 (A2 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.
• In the favourable conditions, development of different horizons in soil takes about 250 years but in adverse condition, it takes even more than 1000 years.

B. Specific Soil Forming Processes

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

(a) Zonal Process

1. Calcification

• It is the process of precipitation and accumulation of calcium carbonate (CaCO₃) in some part of the profile. The accumulation of CaCO₃ may result in the development of a calcic horizon.
• Calcium is readily soluble in acid soil water and/or when CO₂ concentration is high in root zone as:

  

• 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).

2. Decalcification

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

  

3. Podzolization

• 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 podzols are low in fertility and are mainly used for forestry and pastures.
• The process operates under favorable combination of the following environments.
  • Climate: A cold and humid climate is most favorable for podzolization.
  • Parent material: Siliceous (Sandy) material, having poor reserves of weatherable minerals, favor the operation of podzolization as it helps in easy percolation of water.
  • Vegetation: Acid producing vegetation such as coniferous pines is essential
  • 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 + 6H₂O → 2Al(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 horizons and deposited in the lower horizon.

4. 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.
• Laterites/ Latosols / Oxisols are rich in sesquioxides and poor in silica. It is mainly used for shifting agriculture, low intensity grazing and plantation of coffee, banana, pine apple and coconut.
• The process operates under the following conditions.
  • 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.
  • Natural vegetation: The rain forests of tropical areas are favorable for the process.
  • 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.
• Calcination, Podzolization and Laterisation are zonal soil forming processes where soils produced are under normal prevailing conditions of climate & Vegetation.

(b) Intrazonal Process

👉🏻 In the Intrazonal soils, the profile characteristics are more influenced by certain local conditions such as relief or parent material than the climate and vegetation, These processes are occurred primarily in arid and semi - arid regions except gleization.

5. 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 hydromorphic 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⁺² organo - complexes in solution or as mixed precipitate of ferric and ferrous hydroxides.
• This is responsible for the production of typical bluish to grayish horizon with mottling of yellow and or reddish brown colors.

6. 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.

7. 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.

8. Solonization or Alkalization

• It is accumulation of Na ions on exchange sites and pH more than 8.5. Soil colloids are dispersed and tend to “move downward resulting in poor physical condition".
• The Organic Matter forms black organo-clay coatings and hence called black alkali soils.

9. Solodization or Dealkalization

• The process refers to the removal of Na ions from the exchange sites.
• This process involves dispersion of clay.
• Dispersion occurs when Na ions become hydrated.
• Much of the dispersion can be eliminated if Ca++ and or Mg++ ions are concentrated in the water, 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 + CaSO₄ → Na₂SO₄ + CaX

10. 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.