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SPECIFIC SOIL FORMING PROCESSES
♣ Salinization and Alkalinization
♣ Calcification and Decalcification
♣ Carbonation and Gleization
♣ Pedoturbation and humification
The basic pedologic processes provide a framework for later operation of more specific processes
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.
Salt accumulation may also result from irrigation or seepage in areas of impeded drainage.
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 acidic 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.
Gleization
The term glei is of Russian origin which means blue, grey or green clay.
The Gleization is a process of soil formation resulting in the development of a gley horizon in the lower part of the soil profile above the parent material due to poor drainage condition 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.
Under such conditions, iron compounds are reduced to soluble ferrous forms.
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 greyish horizon with mottling of yellow and or reddish brown colors.
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.
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.
Desalinization
It is the process of removal of excess soluble salts from horizons that contained enough soluble salts to impair the plant growth.
Leaching is done by pending 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+ + X (3CO3 2- ® Na2CO3 + MgCO3 + CaCO3)
(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.
Much of the dispersion can be eliminated if Ca++ and or Mg++ ions are concentrated in the water. These Ca and Mg ions can replace the Na on exchange complex, and the salts of sodium are leached out as:
2NaX + CaSO4 ------------ Na2SO4 + CaX (leachable)
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
(insoluble) (soluble)
Carbonation
It occurs when carbon dioxide interacts chemically with minerals. When carbon dioxide is dissolved in water, it forms weak carbonic acid.
When carbonic acid comes in contact with the surface of the earth it dissolves large masses of limestone, creating caves and caverns.
Other common terms associated with carbonation are sink holes, karst topography, stalactites and stalagmites.
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