Hydration: In this case. H and OH ions of water react with different compounds. For example hematitite reacts with water to from limonite.
- Carbonation: Water reacts with water to form carbonic acid. Carbonic acid reacts with rock.
- Oxidation: Rocks containing iron (Fe) undergo oxidation It makes the rocks more porous and breakable.
Solvent Action: Water is universal solvent. It dissolves some of the mineral matter. Thus it causes chemical weathering.
(b) Derivation of parent soil material
Soil is classified into two types oil the basis of parent material.
- Residual parent Material
Residual parent material is formed by the breakdown of socks. Rock minerals are exposed to atmosphere at some places. The intensity of weathering is greatest at these places. Complete physical decomposition and chemical alteration take place at the surface of a residual parent material. The mineral particles become larger with the increase of the depth. Therefore, these particles are less altered chemically. They incorporate in the bed rocks.
- Transported Parent Material
They are composed of derived mineral particles. Various agents bring these derived particles from their places of origin.The transported soils vary according to the transporting agent. Most of the agent operates one after the other. Therefore, the parent material of this class does not resemble the underlying rocks. The soil may be:
(a) Colluviall: The soil transported by gravity or wind is known colluviall soil. .
(b.) Alluvial: Soil transported by water is called alluvial soil
(e) Glacial: Soil transported by glaciers is glacial soil.
(d) Aeolian: The soil transported by wind is called aeolian soil.
(c) Organic material
The weathered rock material forms fine earth. Organic matter is added to it. The lichens and higher plants die. Their dead parts get mixed up with rock material. The remains of minerals of plants and their waste products are also added to this mixture.
Relative proportion of the soil particles size is known as soil texture. The process of weathering forms the texture of soil. Soil consists of fragments of various sizes. Their volume is 45-50% of the total soil. Soil texture causes variations in the soil water and soil air. Therefore, it indirectly affects the plants. The texture or mechanical composition of the soil determines the amount of water hold by it.
Sizes of soil particles
Majority of the soil particles consist of silica and silicates. They are relatively insoluble. Soil particles have following sizes:
- Gravel: more than 2mm
- Coarse Sand 2.0— 0.2 mm
- Fine Sand 0.2— 0.02 mm
- Silt 0.0-0.002 mm
- Clay Less than 07002 mm
Soil particles are classified as gravel, sand, Ioam, silt and clay. Or it may be sandy loam, silt loam and sandy soils,
The characteristics of the individual soil particles are as follows:
It is composed of large mineral particles. Size of its particles is more than 2.00mm. It does not support much vegetation.
- Coarse Sand
The size of particles of coarse sand is 2.0— 0.2 mm. Thus it has large sand particles. They are insoluble in water and are chemically inert. Therefore, large pore spaces are present between them. It has good aeration. It is useful for the plants. But excessive drainage occurs in it. It causes deficiency of soil nutrients.
- Fine Sand
The size of particles of fine sand particles is 0.2— 0.02 mm. It has smaller sized sand particles. Its properties are similar to coarse sand. But it has power of coherence (attachment). Fine sand only allow slow water movement. But it does not retain much water. It has small sand particles. Therefore, they retard free movement of water and air.
The size of particles of silt is 0.0-0.002 mm. Silt is composed of fragments of rock mineral. The small particles retard the movements of air and water. Silt also exhibits the properties of colloids. Silt has tendency to absorb and retain water.
The sizes of particles of clay are less than 0.002 mm. Clay differs from coarse sand, fine sand and silt both chemically and physically. Clay has small particles. So it is colloidal in nature like gelatine. It can absorb and retain a large amount of water. It has small quantity of electrolytes. Therefore, some of the clay particles aggregated together to form large sized particles known as crumbs. This process of crumb formation of
clay is called flocculation. But certain minerals like sodium reverse this effect. It causes dispersion of crumbs. This process is known as deflocculation.
A soil in which the sand, silt and clay particles are present more or less in equal proportion is called loam. A loam soil with predominant clay particles is called clay loam. It has more classes. These are sandy, Loamy sand, Silt loam, Silty Sandy clay loam, Silty clay and Clay etc.
The soil structure includes the aggregation of soil particles. Soil structure plays role in the determination of soil characteristics. Only coarse sand and fine sand are single grained. All other soil particles aggregate into larger structural units. The individual soil particles coalesce (sticking together) with the help of soil colloids. They form various sizes soil structures. The diameter of these particles is 1-10 mm. They have different shapes and forms. Formation of soil structure depends upon following factors:
- Parent material
- The process of humus formation
- The accumulation of soil colloids
- The activity of microorganisms protozoa and worms (particularly earthworms). .
- Fluctuating temperatures
- Liming and gypsum of the soil Importance of soil structure
- Aggregation of the soil is essential for soil with fine texture. A structured soil has favourable features of both coarse and fine textured soils.
- Aggregated soil is more favourable for plant growth.
- Aggregation makes the soil permeable for water.
- It helps in heat transfer.
- It improves soil aeration and increases the water holding capacity of the soil.
- The aggregated soils are not easily eroded by wind and water.
Types of structure of soil
There are four types of structure of soil:.
- Piety: In this case, all the soil particles are arranged in a horizontal manner. They form small flat plate like structures. It is mostly present in virgin soils.
- Prism like: In this case, the soil particles are arranged in vertical aggregates. They are hounded by flat surfaces. Their lengths vary for different soils. Prism like soils is found in arid and semi-arid regions. It has two forms:
- Columnar: The prism like soils in which the top are rounded is called columnar.
- Prismatic: In this case the top is plane or level.
3. Blocky: In this case soil aggregates are present in the term of blocks. They have rounded or flat six faced structures. Blocky soils are common in heavy subsoil in humid regions. It has three forms:
- Angular Blocky: When the edges of the cubes are sharp with distinct rectangular faces. it is called angular blocky.
- Subangular Blocky: When the sub-rounded aggregates are present the aggregates are called Subangular.
4. Spheroidal: In this case, the aggregates are rounded in outline. They are not more than half an inch in diameter. Spheroidal soil is present in furrow s or slices. It undergoes many rapid changes. It has two forms:
- Granular: In this case, the spheroidal aggregates are non-porous
- Crumb: In this case the spheroidal soil is porous. Granular soil is the best soil for plant growth.