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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 (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:

    CO2 + H2O –> H2CO3 
    H2CO3 + Ca –> Ca (HCO3)2 (soluble) 
    Ca (HCO3)2 –> CaCO3 + H2O + CO2 (precipitates) 
    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 
    CaCO3 + CO2 + H2O —> Ca (HCO3)2 (soluble) 
    (insoluble)              CO2

    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 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 weather able 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 +6H2O à2 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 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 Sesquioxide (iron and aluminium oxides) to extent of 70 to 80 per cent of the total mass, are called laterite 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.
    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 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:

    1. Lower topographic position, such as depression land, where water stands continuously at or close to the surface.

    2. Impervious soil parent material, and.

    3. 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++ 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: 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 à Ca++ +Mg++ +2Na+ + x-6 +3CO3 2- à Na2CO3 + MgCO3                                                                                                                                                                                    +CaCO3 
    (Where X represents clay or organic matter exchange sites)

    9. 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. 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 à 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:

    1. Faunal pedoturbation: It is the mixing of soil by animals such as ants, earthworms, moles, rodents, and man himself

    2. Floral pedoturbation : It is the mixing of soil by plants as in tree tipping that forms pits and mounds

    3. 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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