Agents of chemical weathering, Biological weathering, physical properties of soil

Agents of Chemical Weathering

a. Hydration

Hydration mainly occurs in the humid region and consists of the chemical combination of the water with the mineral. The hydration causes the increase in the volume and the softness of the rock. As hydration is the non-pedagogic process, it occurs under conditions which are remote from the operation of ordinary atmospheric agencies. Example:

2Fe₂ O₃ + 3H₂O → 2Fe₂O₃.3H₂O

Hematite

b. Oxidation

Oxidation generally refers to the addition of the oxygen to the various minerals. The soil minerals like Fe, Mn, Sulphides are moreover subjected to the oxidation process. When the iron contents or the iron gets in contact with the moisture, it forms reddish brown precipitation and it is called rust.

$$\underbrace{FeO + O_2}_{ferrous\;oxide} \longrightarrow \underbrace{Fe_2O_3}_{ferric\;oxide}$$

c. Carbonation

Carbonation means the reaction of any compound with the carbonic acid. Mainly carbonation results in the formation of the carbonic acid. In this process, the carbon dioxide is used under high pressure and the release of the pressure slowly goes out the effervescent solution. Generally, it results in the formation of the bicarbonates.

$$H_2O + CO_2 \longrightarrow \underbrace{H_2Co_3}_{carbonic\;acid}$$

H₂CO₃ + CaCO → Ca (HCO₃)₂

d. Hydrolysis

Hydrolysis refers to the breakdown of the compound to the chemical cation or anion by reacting with the water. The water dissociates into the cation form as H⁺ and in anion form as OH‾and acidity or the alkalinity of the water is measured by H⁺ concentration or the PH value. The higher the level of the pH, the more it is dissociated so greater the action as the chemical weathering agent.

e. Solution

Many of the agents get dissolved in the water as it is known as the universal solvent. Similarly, some of the minerals get dissolved into the water and it is removed as the solution. For the different rocks, the rate of the solution is different. The maximum degree of the solution is found with the chloride contents and the halides (NaCl), sylvite (KCl) etc. The sulphates and the carbonates are less soluble in the water. the process of the solution is enhanced by the positive action of the CO₂ and H₂O. However, the silica being the stable mineral as well undergoes solution in the alkaline form.

The process of the removal of the solution form of the minerals from the rocks by percolating H₂O is known as leaching. Leaching process is used in agriculture as well as in metallurgy.

2. Biological weathering

Biological weathering generally refers to the weathering of the rocks due to the activities of the plants, animals, and organisms like bacteria. This process involves the role of the plants and animals in breaking down of the rocks through the mechanical ways as well as the deposition of the rocks. When the crevices of the rock are filled with the mud the plant begins to grow with the lifecycle of the algae, lichen, and mosses which kept the rock surface moist and also aid to the chemical weathering. In addition to this, the burrowing animals also help to expose the rocks and help in the surface weathering.

Physical properties of the soil.

1. Soil texture

Soil texture generally gives the meaning of the relative proportion of the particles of various sizes in the given soil sample. It can be regarded as the permanent character of the soil as the texture of the soil does not change for over the long period of the time. The soil texture determines the aggregate stability of the soil as well as the infiltration rate of the soil, compaction of the soil and moreover it is the permanent character of the soil that determines the water intake, water storage in the soil and the ease of the tillage operation and the aeration.

Classification of the soil separates

Soil separates are the group of the soil particles made according to the size of the sand, silt, and clay. It is determined as:

Soil Separates Diameter (mm)

Coarse Sand: 2- 0.2

Fine sand: 0.2- 0.02

Sil:t 0.02- 0.002

Clay: < 0.002

The particles whose size is greater than 2mm are excluded from the soil texture determination because they make no contribution to the basic properties of the soil like water holding capacity, capacity to store and supply the plant nutrients.

2. Soil Structure

The soil structure flows out the meaning that the arrangement of the soil particles like sand, silt, clay and their aggregates into the certain pattern. It includes the solid and the pore spaces.

Types of the soil structure

a. Spheroidal Structure

Spheroidal shape includes the granular and the crumb subtype. The relatively on- porous aggregates are termed as granular whereas when the granules are porous,then, it is called the crumb. Such aggregates are rounded and lie loosely and separately from each other and is also considered as the most desirable structure. The spheroidal structure of the soil is particularly found in the high organic matter. The aeration and the water-holding capacity of the soil are better in the thin type of the soil structure.

b. Platy Structure

The soil structure in which the aggregates are arranged in the thin plate-like structure is known as the platy structure of the soil. They are in the flat form which reduces the penetration of the air, water and the roots. Such structures are found in the subsoil layer .

c. Prismatic Structure

These prismatic structures are common in the arid and the semi-arid land which are the vertically oriented aggregates and the top of these vertical aggregates are plane , level and clean out. The erosion of such prismatic structure edge leads to the columnar pattern.

d. Blocky Soil Structure

The blocky soil structure refers to the soil that has cube shape and the soil having the sharp edge of the cube and includes angular blocky and the subangular block subtypes. If the cube is more or less curved or when the faces or the edges are removed, then it is called subangular blocky. Such type of the structure is commonly found in the subsoil and particularly in those humid regions.

References:

Santra, S. (2004). Environment Science. India: New Central Book Agency (p) Ltd.

Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Bell, and Howell Company, 1985.

Mahapatra, G.B.Textbook of Physical Geology. Shahadra, Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.