Soil Forming Processes.

Soil Forming Process
 
Weathering is a natural process of breakdown and transformation of rocks and minerals into unconsolidated residues, called regolith (Soil). In other words, the process of transformation of solid rocks into soils is known as weathering. Weathering processes are two types: 
      1.  Physical weathering brought about by the mechanical action of the various weathering agents, is designated as disintegration, and
      2.  Chemical weathering is designated as decomposition.
Source - ICAR Education


PHYSICAL WEATHERING
 
It is a mechanical process, causing disruption of consolidated massive rocks in to smaller bits without any corresponding chemical change. Various weathering agents are:
  •  Temperature
  • Water
  • Wind 
        
 CHEMICAL WEATHERING

Chemical weathering takes place mainly at the surface of rock minerals with the disappearance of certain minerals and the formation of secondary products. This is called chemical transformation. No chemical weathering is possible without the presence of water. The rate of chemical reaction increases with dissolved carbon dioxide and other solute in water, and with increases in temperature. The principal agents of chemical weathering are described below.


(a) Solution:- Some substances (halite, NaCl) present in the rock are readily soluble in water.

(b) Hydration:- Hydration means chemical combination of molecules with a particular mineral.

(c) Hydrolysis:-  it is one of the most important processes in chemical weathering. It means break down of water.

(d) Oxidation:- Oxidation means addition of oxygen is more active in the presence of moisture and results in hydrated oxides.

(e) Reduction:- This means the removal of oxygen. Under condition of excess water (less or no oxygen), reduction takes place

(f) Carbonation:- carbon dioxide dissolved in water, it forms carbonic acid

SOIL FORMING PROCESSES (Pedogenic  processes)

Most natural processes, such as the upliftment of a mountain mass and the tilling of an island in sea, take place rather slowly. In contrast, the pedogenic processes, 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. The ultimate result of soil formation is profile development.

Source - ICAR Education
                                       
A. Fundamental soil forming processes

(1) Humification

Humification is the process of decomposition of organic matter and synthesis of new organic substances. It is the process of transformation of raw organic matter into formation of surface humus layer, called Ao- horizon. The percolating water passing through this layer dissolves certain organic acids and affects the development of the lower A-horizon and the B- horizon.

(2) Eluviation and illuaviation

Eluviation is the process of removal of constituents by percolation from upper layers to lower layers. This layer of loss is called eluvial and designated as the A-horizon. The eluviated producers move down and become deposited in the lower horizon which is termed as the illuvial or B-horizon. The eluviation produces textural differences. The process of illuviation leads to the textural contrast between A2 and B1 horizon.

B. Specific soil forming processes

(1) Podsolisation

It is a type of eluviation in which humus and sequioxides become mobile, leach out from upper horizons and become deposited in the lower horizons. This process is favoured by cool and wet climate. It requires high content of organic matter and low alkali in the parent material. The process increases the proportion of silica, sesquioxide in A-horizons and accumulation of clay, iron and aluminum in B-horizons.

(2) Laterisation

In this process, silica is removed while iron and alumina remain behind in the upper layers. Laterisation is favoured by rapid decomposition of parent rocks under climates with high temperature and sufficient moisture for intense leaching, such as found in the tropics. The soil formed in this process is acidic in nature.

(3) Clacification

In this process, there is usually an accumulation of calcium carbonate in the profile. This process is favoured by scanty rainfall and alkali in parent material.

(4) Gleization

The term gleiis of Russian origin, which means blue, grey or green clay. The gleizationis aprocess of soil formation resulting in the development of a glei (orgley horizon) in the lower part ofthe soil profile above the parent material due to poor drainage condition (1ack of oxygen) and where waterlogged conditions prevail. Under such condition, iron compounds are reduced to soluble ferrous forms. This is responsible for the production of typical bluish to grayish horizons with mottling of yellow and I or reddish brown colours.

(5) Salinization

Salinization is the process of accumulation of salts, such as sulphates and chlorides of calcium, magnesium, sodium and potassium, in soils in the form of salty (salic) horizons. It is quite common in arid and semi arid regions. It may also take place through capillary rise of saline groundwater and by inundation with seawater in marine and coastal soils. Salt accumulation may also result from irrigation or seepage in area of impeded drainage.

(6) Desalinization

It is the removal by leaching of excess soluble salts from horizons or soil profile by ponding water and improving the drainage conditions by installing artificial drainage network.

(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).

(8) Solidization (dealkalization)

The process refers to the removal of Na+ from the exchange sites. This process involves dispersion of clay. Dispersion occurs when Na+ ions becomes hydrated. Much of the dispersion can be eliminated if Ca+ and Mg++ ions are concentrated in the water, which is used to leach the solonetz. These Ca and Mg ion can replace the Na on exchange complex, and the salts of sodium are leached out.

(9) Pedoturbation

Another process that may be operative in soils is pedoturbation. It is the process of mixing of the soil e.g. argillipedoturbation is observed in deep black soils.


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