Agriculture for Engineers 4 (3+1)

Lesson 3. Soil Forming Processes

3.2 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:

1. Temperature: The alternate expansion and contraction of rocks due to variation in temperature produce cracks. The number of cracks slowly increases and the rock gets broken in to pieces. This phenomenon is referred to as ‘exfoliation’. The dark coloured rocks are subjected to fast changes in temperature as compared with light colored rocks. The cubical coefficient of expansion of feldspar and quartz present in most of the rock is 1 : 2.
2. Water: In cold region, water freezes in rock joints and cracks. On freezing, the water expands in volume by about 9.0 per cent with a force of 150 tons per square foot. Due to this tremendous pressure the rock splits and is broken up into a loose mass of stones. The moving water has a tremendous transport capacity which by rolling action grinds the rocks into pieces. A current moving at a speed of 15 cm, 30 cm, 1.2 m and 9 m / sec can carry fine sand, gravel, stone (1 kg) and boulders of several tons, respectively. Water through its erosion forces removes weathered parts of rock, thereby exposing fresh surface to weathering. The excavation and destructive action of water is called denudation.
3. Wind: Wind carrying particles in suspension and blowing constantly over the rock at great speed exerts a grinding action, thereby the rock gets disintegrated. Loosely balanced rock boulders sometimes roll down by the action of wind and break in to pieces. At a velocity of 5 m/sec particles of 0.25 mm size are transported, while at a velocity of 10 m/sec, the wind can carry particles of 1mm size.

3.3 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. When the soluble substances are removed by the continuous action of water, the rock no longer remains solid and falls to pieces very soon.

(b)  Hydration:

Hydration means chemical combination of molecules with a particular mineral. Soil forming minerals occurring in rocks undergo hydration when exposed to humid condition e.g.

Due to this reaction the minerals increases in volume and become soft and more readily weatherable.

(c) Hydrolysis:

it is one of the most important processes in chemical weathering. Hydrolysis depends on the partial dissociation of water in to H-ions. Increases in H-ion concentration resulting in the accelerated hydrolytic action of water. Water thus, acts like a weak acidon silicate, e.g.

The products of hydrolysis are either wholly or partially leached by pre-cooling water. They may also recombine with other constituents to form clay. In a way, hydrolysis may be considered as principal agent of clay formation.

(d)  Oxidation:

Oxidation means addition of oxygen is more active in the presence of moisture and results in hydrated oxides. Soil-forming minerals , containing iron, manganese etc. are more subjected to oxidation, e.g.

A rusty-looking (red) crust is formed on the surface of the rock. The crust thickens and then slowly gets separated from the parent rock. As process continues, the change produced in the mineral weakens the rock and ultimately the rock itself crumbles to pieces.

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

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

The carbonic acid or carbonated water attacks many rocks and minerals and brings them into solution. Limestone, which is insoluble in water, is dissolved readily by carbonated water and is thus, removed from the parent rock.

3.4 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.

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 A_o- 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.