In this article we will discuss about:- 1. Stages of Soil Formation 2. Soil Forming Process 3. Factors.
Stages of Soil Formation:
A. Decomposition of Parent Material:
Soils are formed by disintegration and decomposition of rocks and vegetables and animal fossils imbedded in them. The conversion of parent material into soil is known as soil formation.
Decomposition or weathering of parent materials are manifested by three types of agents and accordingly the process are termed as follows:
1. Physical or mechanical weathering
2. Chemical weathering
3. Biological weathering
1. Physical or Mechanical Weathering:
The principal agents of physical or mechanical weathering are as follows:
(i) Heat and Cold:
The general rule of expansion and contraction due to heat and cold (i.e. variation of temperature) is true to rocks also. Rocks are composed of various mineral crystals which possess different coefficient of expansion and contraction. The repeated differential expansion and contraction of adjacent unlike mineral due to temperature changes of a day and night loosen the crystals causing the rocks to crumble. In this way, rocks are weathered and finally reduced to soil material by other agencies. This type of weathering is most common in dry climate.
(ii) Freezing and Thawing:
When water freezes, its volume increases by about nine per cent and the force exerted is 150 ton per square foot. In the crevices of rock, water frequently freezes and the crevices are enlarged by breaking of fragments and when thawing occur they may roll down the slopes.
Glacier is the drainage system of the region of perpetual snow. The moving of ice obeys the same laws as streams and does the same kind of work. As the ice is a solid body, it has a great grinding power.
Torrential rain dislocates solid particles from rock and puddle the surface of some parent materials. The streams of water with their loads of various materials have grinding capacity. The debris is then transported to a long distance by flood water of streams and rivers and deposited as a soil in low lying areas, mouth of rivers and under the sea. Water frozen in rock crevices in cold regions breaks them open. Flowing water, waves on the shores of the seas and large lakes and moving ice-glaciers in high mountainous region wear off rocks and cliffs thus helping in soil formation.
Sand storms in deserts, semi-arid and dry river beds and high winds on sea shore have both erosive and transporting action. Wind makes the action of sea waves stronger in causing weathering along the coasts. Wave action is confined to the shores of the sea and large lakes.
The soil formed by physical weathering is known as “skeletal soil”.
2. Chemical Weathering:
The physical disintegration produces a greater surface area of rock exposed to the influence of chemical weathering agents.
Chemical weathering involves two steps such as:
i Disappearance of certain minerals.
ii Formation of secondary products.
Chemical weathering is a complicated process. In the chemical weathering, the minerals in the parent rocks are decomposed and as a result, the new substances are originated.
The chemical weathering of rocks is brought about by the following ways:
Most of minerals in the parent rocks are more or less soluble in water. But the solvent capacity of pure water is less. The decomposing action of water is increased by the presence of carbon dioxide and organic acid formed from the oxidation of sulphur compounds or iron sulphide also play an important role in the chemical weathering of rocks. Gypsum and calcareous rocks which are easily soluble in water make the soil after dissolving in water.
Hydration is the chemical combination of water with other chemical substances. It occurs more commonly in humid region. The union of water with minerals softens the rocks which then result in the weathering of rocks and ultimately brings about their decomposition. Hydration is also sometimes accompanied with an increase in volume, which may bring physical disintegration of rocks. Example of hydration is as follows –
Hydrolysis is essentially an exchange of constituent part between water and mineral. The minerals which are affected by hydrolysis are the complex silicates of calcium, magnesium, potassium, sodium, aluminium and iron. Examples of hydrolysis is as follows –
After the decomposition, the oxides of aluminium and silicon may again combine under suitable condition to form new silicate minerals like kaolinite or they may remain in the soil or they may be leached down by water.
Oxidation mainly affects ferrous iron which is a constituent of many minerals such as olivine, hornblende and augite. Oxygen combining with various minerals produces soluble oxides which on dissolving in water weaken the rock and help its disintegration.
Under anaerobic conditions, when soil pores are saturated with water, oxygen may be absent and reduction may occur.
Under certain conditions, organisms causing decay of organic matter set up a reverse action (reduction) by taking up oxygen from simple hydroxides.
Carbonation is the chemical combination of carbon dioxide (CO2) which is present in atmosphere or released by the decomposition of soil organic matter and respiration of plant roots and microorganism, with other chemical compounds. When carbon dioxide (CO2) combines with water, it forms carbonic acid which increases the solvent power of water. Carbon dioxide or carbonic acid may also react with other chemical compounds resulting in soluble carbonates which dissolves in water and weakens rock and help its decomposition.
Carbon dioxide reacting with basic substance (e.g. Ca(OH)2) forms carbonate or bicarbonate, which remains stored in dry region and leached in lower region in high rainfall area.
3. Biological Weathering:
Joffe states, “strictly speaking there is no biological weathering. Essentially it is physical and chemical weathering by biological agencies.” Physical and chemical weathering brings about physical disintegration and chemical decomposition of rocks in which organic matter are added and true soils are formed. Both the plants as well as animals are also largely responsible for the further reduction of rock minerals’ particles into soil in which crop plant can grow.
Plants and animals are responsible for biological weathering of rocks as follows:
Lower plants such as mosses and lichens can grow on almost bare rocks cause gradual disintegration. Grasses, shrubs and trees growing in rock crevices help in extending cracks of the rocks by their root growth. The presence of vegetation accelerates weathering process by producing carbon dioxide in respiration and by providing with materials for humus and humic acid. When the plants die, they leave organic matter.
The soil organisms convert the organic matter into humus which is very suitable medium for the growth of more plant. Animals like earthworm, termites, ants, moles, rats and rodents burrow underground and loosen hard soil or change its character by mixing upper and lower layer of soil. The activities of plant and animal change mineral composition as well as the physical structures of rocks and their growth may cause cracking and flaking, exposing a greater area for further weathering.
B. Decomposition of Organic Materials:
The decomposition of organic material may progress through two distinct stages. As dead vegetation accumulates on the soil surface, it provide food for a variety of soil organism which fragment and chemically degrade it. The end products of these physical and chemical transformation is a complex substance called ‘humus’. The production of humus was largely controlled by soil flora and fauna although physical and chemical agents also contribute formation of humus is considered to be first phase of organic matters decomposition and slow breakdown of these relatively stable complex.
C. Formation of Organo-Mineral Complex:
The mechanisms involve in the formation of organo-mineral complex are of two types as follows:
(i) Electro-Chemical Bonding:
In this case, aggregation of negatively charged colloidal clay and humus particles is brought about through electro-static bonding consisting of bridges of water molecules and metallic ions particularly calcium.
It involve the action of substances adsorbed on the surface of soil particles which effectively glue them together.
The ultimate result of soil formation is the development of soil profile. The soil profile is formed by interaction of various pedogenic factors under a special set of condition.
The fundamental process of soil formation are as follows:
(i) Addition of mineral and organic matter to the soil.
(ii) Losses of mineral and organic matter from soil.
(iii) Translocation of mineral and organic matter from one point of soil profile and deposited at another horizon.
(iv) Transformation of mineral and organic matter in the soil and formation of definite layers.
The fundamental process that develops a profile are described as follows:
The top layer of soil, called ‘A’ horizon contains abundant dead remains of plants, animals and other products of microbial metabolism. These products undergo decomposition which may produce some suitable organic compounds and some amorphous incompletely decomposed black coloured organic residues, the humus.
The process of decomposition of organic matter and synthesis of new organic substances (i.e. humus) is called ‘humification’. The humus and organic compounds are mixed with fine particles of weathered rock. The percolating water passing from humus layer dissolves certain organic acid and affects the development of lower A horizon and B horizon.
(ii) Eluviation and Illuviation:
Water is important agent which help in the development of some profile. While water percolating downwardly through mineral and organic substances of A horizon or top soil, it removes a number of chemical substances from the top soil. The process of washing away of soil constituent by percolation from upper layers to lower layers is termed as eluviation (meaning wash out) and the surface layer from which components are lost is called eluvial layer or A horizon.
The eluviated substances move downwardly and are deposited in the lower zone or B horizon, which is termed as illuvial layer (meaning wash in) or B horizon. The process of accumulation of eluviated material is called illuviation.
There are some fundamental soil forming process involved in the development of soil profile as follows:
Gleization is the process of reduction, due to anaerobic condition, of ferric compounds like ferric phosphate and ferric sulphide in water logged soil in presence of organic matter with the ferrous compounds like ferrous phosphate or ferrous sulphide. In this process, there develops a compact structureless and stick surface layer. The submerged layer is blue green in colour, poorly aerated and has reduced content of iron compound. Gleization usually takes place in low lying areas where water accumulates.
Podzolization (In Russian, Pod means ‘under’, and zola means ‘ash’) is the process of eluviation of oxides of iron and aluminium and also of humus under acid condition (pH 4-5), removal of carbonates by organic acids formed by decomposition of organic matter and illuviation of sesquioxides and humus in subsurface horizon. It is a type of eluviation in which humus and sesquioxides (i.e. oxides of iron and aluminium) becomes mobile, leach out from upper horizon and becomes deposited in the lower horizons.
The eluviated horizon assumes a bleached grey appearance and is left in highly acid, siliceous condition. The soil is called podzol due to having grey colour and ashy appearance. This process is more effective in sand based poor parent materials under intense leaching and thick vegetational cover.
Laterization (In Latin, later means brick and obviously refers to its use as building material and not to its colour) is the process of desilication i.e. the removal of silica and accumulation of sesquioxides. In tropical and subtropical region when rainfall occurs, the organic matter and minerals particularly silica, are leached away and hydroxides of iron and aluminium are precipitated in the form of residue which is ‘laterite’.
Laterite are also formed from clayey sandy rocks when large amounts of iron are accumulated in them. The process is termed as laterization. Laterite do not show well differentiated horizon. Laterization is favoured by rapid decomposition of parent rocks under climate with high temperature and sufficient moisture for intense leaching.
Podzolisation and laterization produce soil that belong to ‘pedalfer’ (iron accumulating) group.
In sub-humid and dry regions, soil accumulates considerable amount of soluble materials due to lack of excessive moisture in the soil and carbonates of calcium and magnesium are deposited in the B. horizon. The soils having such features is called ‘Pedocol’ (Calcium accumulating soil).
Salinization is the process of accumulation of soluble salts in soils. In arid region, soluble neutral salts of calcium and magnesium are deposited on the surface of the soil when water evaporates from the surface of the soil. The water containing soluble salts moves from the deeper layer to the surface of the soil.
Salinization may also takes place through capillary rise of saline ground water and by inundation with sea water in marine and coastal soils. Desalinization is the process of leaching soluble salts from the soil by rain water or irrigation water. Drainage is essential for desalinization.
f. Alkalization (Solonization):
Alkalization is the process by which soils with high exchangeable sodium are formed. The amount of exchangeable sodium in great quantities in the soil makes the soil alkalinity. Dealkalization (solodization) is the process by which sodium ion is replaced from the clay and humic micelle by hydrogen ion and the silicate clay is decomposed to release silica which is deposited on the soil particles. This process is affected by intense leaching.
g. Hydromorphic Soils (Gleysoils):
Hydromorphic or gley soils are regarded as intrazonal soil and this soil evolves when the soil is over-moistened either from the surface or groundwater. Under such conditions, the regime in the soil is anaerobic, promoting the reduction reaction which stimulates the genesis of gley horizon or a gley stratum. Such soil forming process results in swamp, bog, marsh, muck and peat soils.
Factors of Soil Formation:
Climate is the most influential of all factors and determine the nature of weathering that occurs. The precipitation and temperature are the important elements of weather that affects various physical and chemical process in soil formation.
The climate influences the process of soil formation directly and indirectly as follows:
The two primary elements of climate namely rainfall (precipitation) and temperature supply water and heat respectively to react with parent materials. Rainfall primarily determines moisture which affect the decomposition of minerals. The soluble products of decomposition are removed along with percolating and run-off water. The increase in temperature also increases the rate of weathering of primary minerals and clay content and cation exchange capacity (C.E.C.) of young soil. Temperature influences the decomposition of organic matter and microbiological activity.
Climate control vegetation of a particular area. Forests are the dominant vegetation in humid climate. The climate determines the vegetation which furnish sources of energy in the form of organic matter. This energy acts on rocks and mineral material by means of acid and salts release in the process of organic matter decomposition. The vegetative cover protects the soil from wind and water erosion.
The effects of precipitation and temperature in the formation of soil are as follows:
Precipitation affects leaching and percolation which in turn affect soil formation. As the percolating water come in contact with parent material, some constituent go into solution. They are translocated and deposited at another point i.e. depriving one part of the constituent and enriching another part. Large particles of soil material are translocated mechanically both vertically and horizontally from one place to another with the soil body or out of its bounds into sub-soil and ground water. Moisture is one of the determining factors of plant growth which in turn harnesses the energy of sun to act on mineral matter.
In arctic and sub-arctic region, temperature is of little significance in soil formation with the same precipitation in two different isothermal (equal temperature) belts, different soil profile will develop. Temperature affects velocity of chemical reaction. It also influences the organic matter decomposition and microbiological activities. In subarctic (cold temperature) region, organic matter accumulates in the soil due to restricted microbial activity at low temperature.
2. Living Organism (Biosphere):
Living organisms render an indirect effect on soil formation.
There are two components of biosphere which influences the soil formation as follows:
(i) Plants (Phytosphere):
Mosses and lichens which are symbiotic association of algae and fungi can grow on bare rocks and respire to produce carbon dioxide which react with water to form carbonic acid. This carbonic acid dissolves primary minerals and releases the nutrient contained in them for the growth of plant. Algae fix atmospheric nitrogen which is released in the soil upon the death of algae.
Plant grow in an environment having nutrients and water. After the death of these plants, the organic matter of rocks increases. Carbon dioxide produced from the decomposition of organic matter and respiration of microorganism and plant roots combines with primary minerals to form more clay and convert insoluble minerals to soluble ones.
Plant roots penetrate into the rocks and minerals and thus open channels for the movement of water and air and creates favourable environmental conditions for biological activity. As the roots and other Subterranean Plants die and decompose, a good number of organic and inorganic acids are released which may initiate different chemical reactions. Vegetation is the basic supplier of organic remains.
(ii) Animals (Zoosphere):
The contribution of animal kingdom to the process of soil formation is primarily mechanical in nature. Burrowing animals like rodents, termites, earthworm etc. when present in large number help to change the character of the soil as they harbour in soil, dig into soil body and mix the materials of different horizons. The earthworm have been reported to cause constant mixing of materials within the soil profile. Man through his land use activities causes both deleterious and beneficial effect on soil.
Man converts the forest areas into agricultural land. The human interference accelerates erosion of soil through some activities such as burning of forests, shifting cultivation and indiscriminate grazing. The agricultural practices such as cultivation, puddling, cropping system, use of manures, fertilizers, pesticides, soil amendment etc. drainage, irrigation etc. alter the general character of soil profile.
3. Parent Materials:
In pedology, all rocks from which soils are formed are called soil forming materials or parent materials. This concept was introduced by Dokuchayev, in an attempt to emphasize the great significance of rock in the formation of soils. The parent material has been defined by Jenny as “the state of soil system at time zero of soil formation.” C.P. Marbut (1923), states that “the geological deposit, the glacial till, the sandy deposit of sand plains, the lake laid or marine clays, sands and gravels, the residual earth resulting from rock decay constitute soil materials or parent materials of soil.”
A rock is an aggregate of one or more (usually more) minerals and solid materials which form the crust of the earth. Rocks may be classified according to the mode of origin and composition of rock.
The rocks are classified on the mode of origin as follows:
1. Igneous Rocks:
Igneous rocks are those rocks which at one time were in fluid molten state before their solidification. These rocks are formed by solidification of molten lava when they are coming out from the interior of earth surface.
They are classified into three categories as follows:
(i) Organic Rocks:
Organic rocks are formed by the solidification of magma or lava on the surface of the earth. As the solidification takes place rapidly, no big crystal can be formed in this type of rock. Therefore organic rock possess a glassic structure. This is generally a volcanic origin.
(ii) Plutonic Rocks:
Plutonic rocks are formed by the solidification of magma at great depth (about 3 miles) under high pressure. As a result of high temperature, the magma is solidified at a very slow rate forming big crystal in the rock. Therefore plutonic rock possess crystalline structure.
(iii) Intrusive Rocks:
Intrusive rocks are formed by solidification of fluid on moderate depth i.e. on their way to earth surface. So the position of this rock is in between organic rock and plutonic rock. The solidification takes place at moderate rate and the big crystal is not found in this rock.
Igneous rocks contain common soil forming rocks such as granite and diorite. They are made up of primary minerals such as quartz, feldspar etc. and dark coloured minerals including biotite, augite hornblende, gabbro and basalt. Rocks containing high proportion of quartz are called acidic rocks and the rocks containing basic elements like iron (Fe), aluminium (Al), calcium (Ca) and magnesium (Mg) are known as basic rocks. The most common igneous rock found in India are granite (acidic) and basalt (basic).
2. Sedimentary Rocks:
Sedimentary rocks are derived from igneous rocks and are formed from the deposition and re-cementation of weathering products of igneous rocks. These rocks develop due to gradual accumulation and consolidation of weathering products or mineral particles brought by water or wind on the surface of the earth. Such rocks are characterized by the presence of distinct sediment or layers in them.
(i) Mechanical Sediment:
The weathering products are carried down by wind or water and glacier in arctic region to the sea or lakes. In this way, the deposited weathering products form the beds of sea or lakes. Ultimately the deposited materials may be harden to form rock which is known as mechanical sediments.
(ii) Organogenic Sediments:
Plants grow by taking soluble minerals of the fresh or salty water of lake and sea. The small marine animals survive by taking these plants which in turn provides food for the bigger animals. After the death of these animals, the organic skeleton is left which in combination with mechanical sediment will form a type of rock which is known as organogenic sediment.
(iii) Chemical Sediment:
When water is evaporated completely from an isolated portion of sea or lake, the mineral portion of water is deposited on the beds of sea or lakes. This solid deposit may be harden to form a type of rocks which are known as chemical sediment. Limestone, sandstone, siltstone, conglomerate, shale etc. are important example of sedimentary rock.
3. Metamorphic Rocks:
Metamorphic rocks are those rocks which have undergone some sort of metamorphism or change. These rocks are formed by metamorphism or change, in form of other rocks. Igneous and sedimentary masses subjected to tremendous pressure and high temperature have succumbed to metamorphism. During the metamorphosis, the rocks may be harden, some new substances may be added or it may be stratified.
The common instances of metamorphic rocks are gneisses, derived from granite, syenite, diorite etc. quartzite from quartz, sandstones or conglomerate, talc, serpentine and soapstone derived from siliceous magnesian rocks, slates from shales (clay), marble derived from limestone and schist from sandy clay.
Rocks are also classified on the basis of their silica content as follows:
(i) Acid rocks – This rock contains 65-75 per cent silica, e.g. granite, sandstone, gneiss etc.
(ii) Basic rocks – This rock contains 40-55 per cent silica, e.g. basalt, lime stone, gabbro, diabase etc. Basic rocks are also rich in calcium, iron, magnesium and sodium.
(iii) Intermediate rocks – This rock contains 55-65 per cent silica, e.g. andesite, diorite, syenite etc.
Sedimentary rocks are more resistant to weathering than igneous and metamorphic rocks. Lime stone weathers more easily than sand stone. Basic igneous rock weathers more than acid igneous rock.
The order of decomposition of rock is as follows (Daikuhara):
Basalt > Gneiss > Granite > Hornblende > Andesite.
The effect of parent rock on soil formation are as follows:
(i) The nature of soil that develops depend in part upon the nature of the rock which influences the physical and chemical properties of resultant soil.
(ii) The effect of parent rock on soil is stronger in early stages of soil formation. Acid igneous rocks weather slowly and gives rise to coarser sandy soil with low base status and infertile soil. On the other hand, the basic igneous rocks and metamorphic rocks weather rapidly and lead to the development of relatively fertile soil with fine texture and with high base status.
Minerals are natural substances usually an inorganic body having definite chemical —composition and commonly a definite molecular arrangement which is expressed in geometric form. Soil forming minerals are principally alumino-silicates.
Soil forming minerals are of different kinds as follows:
Quartz is silicon dioxide consisting of a continuous framework of silica tetrahedra. All four oxygens of the tetrahedra are shared and there are no cleavage planes. It is also found in all crystalline rocks; also occurs in sands and the principal component of granite and sandstones. It is occasionally found in clay fraction also.
Feldspars are aluminium silicates with varying amount of silicates of potassium (K), sodium (Na) and calcium (Ca) and occasionally of other large cation such as Barium. The structure of feldspar consist of tetrahedra which are linked together by sharing each oxygen atom between adjacent tetrahedra. The feldspars make up an average of about 60 per cent by weight of igneous rocks and large fractions of sedimentary rocks.
(iii) Amphiboles and Pyroxenes:
The amphiboles and pyroxenes are heavy ferromagnesian minerals, the structure of which consists of long chain of linked silica tetrahedra. Amphiboles are more commonly found in rocks and includes olivine and hornblende.
Hornblende weathers fairly rapidly. Pyroxenes are more commonly found in heavy basic rocks, the principal mineral being augite.
Mica is silicate of aluminium (Al) and potassium (K), with silicates of iron (Fe), magnesium (Mg) and sodium (Na). Mica occurs extensively in soils. Well-ordered micas are inherited by soil mainly from igneous and metamorphic rocks. Biotite and muscovite are the two important type of mica.
There are some important non-silicate soil forming minerals as follows:
Calcite (CaCO3), dolomite [CaMg(CO3)2], gypsum (CaSO4.2H2O) and magnesite (MgCO3).
4. Relief or Topography:
The relief of the land refers to the difference of elevation within it. The topography (relief) influences soil formation through its effect on drainage, run-off, soil erosion and microclimate i.e. exposure of land surface to the sun and wind. The character of soil depends on the topography. In the smooth rock, water stands on the rock which helps to grow the plant and rock is modified in various forms to make the soil. A group of soil which has developed from same parent material in same climate but under different topographical condition is called “soil catena”.
The actual length of time that the materials are subjected to weathering plays an important role in soil formation. Time is also a condition governing various manifestation of soil formation. Jenny expressed soil age in terms of pedogenic factor, time (t). Time has been regarded as one of the factors influencing the soil formation because even chemical weathering of rocks to form soil requires sometimes to complete it.
Mohar and Van Baren (1959) recognized five stages of development of tropical soil as follows:
(i) Initial stage – Unweathered parent material.
(ii) Juvenile stage – Weathering just started, but much of the original material is still unweathered.
(iii) Verile stage – Easily weatherable minerals have been decomposed for the greater part, the clay content has increased and a certain mellowness is discernible. The content of soil components less susceptible to weathering is still appreciable.
(iv) Senile stage – Decomposition arrives at a final stage and only the most resistant minerals have survived.
(v) Final stage – Soil development has been completed and the parent material is fully weathered.
A mature soil represents a steady state in respect of parent material. Mature soils are formed due to principal factor acting for longer period of time than in young soil. The age of the soil is judged in terms of the maturity stage of development of the profile rather than the geological age of parent material.