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In this article we will discuss about:- 1. Introduction to Soil Formation 2. Stages of Soil Formation 3. Development of Soil Profile 4. Factors 5. Factors Affecting Weathering of Rocks and Minerals.
Contents:
- Introduction to Soil Formation
- Stages of Soil Formation
- Development of Soil Profile
- Factors of Soil Formation
- Factors Affecting Weathering of Rocks and Minerals
1. Introduction to Soil Formation:
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Soil which is considered to be a decomposition products consist of two main components namely minerals derived from rocks by weathering and organic materials. These components undergo transformation at the influence of various agents which might be either physicochemical or biological. The two components ultimately become closely integrated as organo-mineral complex characterising the mature soil.
Dokuchayev’s teaching about soil formation is based on the following main idea:
a. Soil is an independent natural body which is formed at the boundary between the lithosphere and biosphere by the interactions of all factors involved in the soil formation both live and dead.
b. The composition and properties of a soil should be examined as they relate to their environment. The later determines the dynamics and evolution of the soil.
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c. The fertility of a soil is its basic and specific property. It evolves with the soil and can be improved by rational soil use and by properly changing the processes that develop the soils.
d. When studying a soil as a natural body, the whole sequence down to the rock on which it is formed must be studied. If the crust has been weathered by more than 3 m (as in tropics and subtropics), it is necessary to consider the sequence within which water actively migrates and where the living organisms penetrate.
2. Stages of Soil Formation:
A soil is three dimensional, transferable natural material which is found on earth crust and provide with natural medium for the growth of the plants. Soil is an independent, dynamic body of nature that acquires properties on accordance with the forces which act upon it.
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Soil is a product of parent rocks and minerals by the process of weathering i.e. physical, chemical and biological. In the process of soil formation, both destructive and constructive processes takes place simultaneously.
In the genesis of soil, a succession of two phases are recognized as follows:
(i) Rocks and minerals are disintegrated and decomposed into smaller pieces by physical, chemical and biological agencies. The products of weathering are called ‘regolith’ which are small particles of rocks materials. This is a destructive process.
(ii) The weathered particles are basic materials which under the influence of pedogenic or soil forming process finally develop into mature soil. This is constructive process.
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The second phase is associated with the action of biosphere on the product of first phase i.e. weathered materials which are obtained from original parent rocks or original bed rocks, known as ‘parent materials’. The soil may be formed from parent materials in situ or after their carried down to some other places by different agencies.
The first land plants anchor themselves to the mineral constituents in search of their water and nutrient. The growing plants bring the nutrient from the lower horizons and incorporated them into their body. With the death of the plants, decomposition sets in and most of the energy accumulated in the process of photosynthesis is released. Thus by the process of bio-chemical and biocolloidal action, a mixture substances are obtained which are quite different both physically and chemically from the original parent material.
These solid mixture is the ‘soil’. Soil that forms the outer covering of the earth have been directly or indirectly developed from the mineral constituents of rocks. Through the weathering process, both physical and chemical activised by the atmospheric agencies, the rocks disintegrate and decompose to produce the unconsolidated parent materials which in turn under the influence of pedogenic process (i.e. soil forming process) ultimately develop into soil.
The process of soil formation from rocks can be summarised as follows:
Weathering is the initial stage and the soil forming process or pedogenic stages comes after wards. Although in simple terms, it means that weathering precedes soil profile development, no distinct line can be drawn where weathering stops and soil forming process or pedogenesis commence.
Weathering processes leads to disintegration and decomposition of complex mineral substances that are locked up in the rocks and eventually simple compounds are formed. The pedogenesis or soil forming process on the other hand are constructive process and of bio-geochemical in nature in which biological influences play important role.
Soil is formed from rocks into two stages viz.:
(i) Decomposition of parent material by weathering of rocks.
(ii) The development of soil profile.
Scientists believe that the process of soil formation involves three successive stages as follows:
A. Decomposition of parent material.
B. Decomposition of organic material.
C. Formation of organo-mineral complex.
3. Development of Soil Profile:
The process of soil formation leads to the development of soil profile. In the process of soil formation, the parent material has gradually converted to soil and definite layers are developed depending on the type and effect of weathering. The texture, structure, colour and physico-chemical property of each layer is not same. A vertical section of soil from the soil surface to the parent material is called a ‘soil profile’.
The layer of soil which is parallel to the soil surface but differs from adjacent genetically related layers in physical, chemical and biological properties is called the ‘soil horizon’. The horizons above the parent material are called as ‘solum’.
Dokuchayev first identified the following horizons:
A. The accumulation of organic and humus matters (substances).
B. The intermediate between the humus ‘A’ and the parent rock.
C. Parent rock weakly altered by soil formation.
D. The bed rock (both hard and loose but unaffected by soil formation).
For convenience of study and description, the layers are grouped under three heads, A, B, and C. The subdivision of these are called ‘Horizon’. A horizon is subdivided into A1, A2 and A3 from the top to downward. Depending on the degree of decay, the organogenic horizons are subdivided into sub-horizons as A0 and A00 ‘B’ horizon is sub-divided as B1, B2 and B3. The A group are mineral horizons which lie at or near the surface and are characterized as zones of maximum leaching or eluvial horizon (eluvial from the Latin words ‘ex’ or ‘e’ meaning out and ‘luv’ meaning washed).
The ‘B’ group called ‘illuvial’, (illuvial from Latin words ‘il’ meaning in and ‘luv’ meaning washed) which is a zone of deposits and accumulation of materials such as iron and aluminium oxide and silicate clays. In arid regions, calcium carbonate, calcium sulphate and other salts may be accumulated in lower ‘B’ horizon. The ‘B’ horizons are sometimes referred to as “sub-soil”. The C group is called the zone of parent material or unweathered material. In young or azonal soil, the ‘B’ horizon is absent but in a developed mature soil (zonal soil) ‘A’ and ‘B’ horizons are prominent and at times ‘C’ horizon may disappear.
The brief description of soil profile is as follows:
A00 – This horizon consists of loose, relatively much of undecomposed organic debris, usually absent in grass land but present in forest land specially in temperate region.
A0 – A partially or fully humified organic horizon where the original plant and animal forms cannot be so distinguished. In forest land, this zone is further divided into ‘F’ (Fermentation) and ‘H’ (Humification) zones.
A1 – A topmost dark coloured mineral horizon, containing large amount of humified organic matter thoroughly mixed with inorganic layer. This horizon is darker in colour due to containing the most organic matter. This layer is thin in podsol soil but broad in chernozem soil.
A2 – A light coloured mineral horizon formed due to leaching and bleaching action (Eluviation). This horizon is well developed in podzols, but absent in chernozem soil.
A3 – A transition layer between A and B. This horizon is not present in all soil.
B1 – A transition layer between A and B with properties nearly like B than A. This horizon is not present in all soil.
B2 – A dark coloured horizon and a zone of maximum accumulation of clay and hydrous oxides, known as illuvial (i.e. zone of maximum accumulation). This horizon is made up of iron and aluminium compound. This horizon becomes reddish brown in true podzol soil.
B3 – A transition layer between B and C with properties more like those of B than those of C below.
C – This zone consist of unconsolidated parent material from which solum has been developed. This may have come from the bedrocks below or have been transported from elsewhere. Its upper layer may in time become a part of the solum as weathering and erosion continue.
4. Factors of Soil Formation:
According to Dokuchayev “the soil is the result of the combined activity and reciprocal influence of parent materials, plant and animal organism, climate, age of the land and topography.” These factors largely control the kind of soil that finally develops. The factors are shown by following equation –
S = f (cl, o, r, p, t …)
where, f = function of or dependent upon
cl = climate (mainly precipitation and temperature)
o = Living organism (specially vegetation, organism etc.)
p = Parent material
r = relief or topography of the area
t = time (age of the land)
The soil property is a function of collective effect of all soil forming factors. Joffy (Jacob. S. Joffy, Soil Pedologist) divided soil forming factors into two as follows:
(i) Passive factors – Parent material, relief and time are the passive factors which represents the source of soil forming mass and conditions affecting it.
(ii) Active factors – Climate and living organism are the active factors which represents the agent that supply energy which acts upon the mass for the process of soil formation.
5. Factors Affecting Weathering of Rocks and Minerals:
Soils are derived from parent rocks by the process called weathering. The products of weathering are called regolith which are small particles of rock materials. These particles are basic materials which under the influence of pedogenic or soil forming process finally develop into mature soil.
C.F. Marbut (1926) states that “a mature soil is one that has assumed the profile features characteristics of predominant soil on the smooth upland within the general climatic and botanic regions in which it is found.” There are so many factors responsible for weathering of rocks.
Among these factors, only three are most important as follows:
1. Environmental Condition:
If sufficient time is allowed for weathering process, the ultimate products of weathering process will be more influenced by the environmental condition. Under low rainfall area, the physical process of weathering only takes place resulting in the disintegration of rocks into small particles with very little chemical change. But if the moisture content is very high, both the physical and chemical process of weathering will take place at their full potentiality.
The rate of weathering increases at a very high temperature and it is further enhanced if there is sufficient moisture. The climate will also indirectly influence the weathering process. The case of weathering of different minerals will vary with the variation of environmental condition.
2. Physical Properties of Rocks:
The weathering of rocks and minerals depends on their size, hardiness and degree of cementation. Rocks composed of large crystals are easily disintegrated as the physical weathering of those rocks dominates. This is due to increase in stress develop within the rock due to fluctuation of temperature.
But when the rocks are composed of small crystals of minerals, the rate of physical weathering will be slow, but the chemical weathering of those particles will take place rapidly. This is due to the fact that smaller the size of the particles more surface area is available for a chemical reaction. Hence the chemical process of weathering dominates over the physical or mechanical process of weathering.
The silt size quartz decomposes more easily than many other minerals of similar size. The hardness and degree of cementation also influences the rate of weathering to a great extent. As for example, quartzite and sand stone becomes very hard due to cementation effect of some secondary weathering product. As a result of which, they become very resistant to physical weathering and the process of chemical weathering also become slower. On the other hand, volcanic ash and porous limestone are easily weathered by physical and chemical process.
3. Chemical and Crystalline Properties of Minerals:
The rate of weathering of mineral depends to a large extent on the chemical and crystalline properties of minerals. Black coloured ferromagnesian minerals which contain higher percentage of iron and magnesium are comparatively easily weathered than many other minerals, because ferrous ions are oxidised to ferric ions.
Presence of carbonic acid in water increases the solubility of more resistant minerals like calcite and dolomite. Gypsum (CaSO4, 2H2O) is soluble in water and they can easily leached out from the surface of the soil. The compactness of crystal units in a mineral will also influence the rate of weathering. As for example, biotite and olivine are more easily weathered than muscovite and zircon.
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