Soil Formation: How is Soil Formed [with Factors and Processes for Class 7, 8 ,9, 10]

Soil Formation: How is Soil Formed [with Factors and Processes for Class 7, 8 ,9, 10]!

Soil formation is a long slow process. It is estimated that an inch of soil takes 500 to 1000 years to form. Soil is constantly being formed and eroded. The soil formation is the process of weathering of rocks and minerals into regolith and subsequently regolith is transformed to true soil.

Soil formation is constructive process which is slow and continuous. Pedogenesis is the study of soil formation. Soil formation is a process of two clear stages i.e., weathering of rocks and minerals (destructive process) and development of true soil (constructive process). Soil formation is the result of biological process.

Factors of Soil Formation:

Soil is formed from weathering of rocks and minerals. Soil formation is a slow process. Formation of one inch soil needs 800-1000 years.

Dokuchaev –  “Soil is the result of the combined activity and reciprocal influence of parent material, plant and animal organisms, climate, age of land and topography”.

S = f (cl,o,r,p,t,….)

S = Soil formation,

cl = climate

o = organisms

r = relief/topography.

p = parent material

t = time or age of land.

According to Jenny- “Soil property is determined by the relative influence of these factors”.

The factors of soil formation may be kept under three groups:

(a) Passive Factors:

i. Parent material

ii. Relief or Topography

(b) Active Factors:

i. Climate

ii. Organisms

(c) Neutral Factors:

Time or Age of land.

1. Parent Material:

It is the unconsolidated mass from which the solum develops. The physical, chemical and mineralogical properties of par­ent material have significant effect on soil formation.

These parent materi­als are grouped into three classes:

(i) Residual Parent Material:

Such parent materials are remained at same place for a longer time. Soil formation takes long time from this parent material. Igneous rock, sedimentary rock and metamorphic rock are the examples of this category.

(ii) Transported Parent Material:

The breakages and pieces of rocks and minerals are transported from one place to the other by the agencies viz. water, ice, air, gravity etc.

(iii) Biological Parent Material:

Decomposed or partially decomposed biological matters are termed as biological parent material.

Chemical composition of soil and physically weathered soils has some resemblance to that of parent material, texture determines the depth of soil profile. Chemical compositions of soils depends upon the types of rocks and their mineralogical composition.

2. Topography/Relief:

Influences through its effect on- drainage, runoff, soil erosion and micro-climate i.e. exposure of land surface to sun and wind. The depth of soil solum influenced by the degree of slope.

The soils of the upper slope are less clayey, lower in P^H, soluble salts & organic matter contents. Lighter in colour and well drained in comparison to the soils at lower slope. Soil of the valley is generally deep and clayey.

Variation in aspects and elevation influence the climatic factor. Vegetation differs if slope faces a certain direction.

3. Climate:

It includes precipitation, temp, humidity, and wind.

Rainfall and temperature are most important factors. Water acts both physically and chemically and moulds the morphological, chemical and physi­cal features of soil profile. Intensity, distribution and duration of rainfall are also important factors.

Soil properties affected by climatic conditions are P^H, base saturation of exchange complex, organic matter content, clay mineralogical composition amount and nature of clay.

Evaporation, transpiration and humidity modify the effect of precipita­tion by either reducing or increasing the quantity of water available for percolation and reaction.

In scanty rainfall regions, salts are accumulated at the soil surface. But in high rainfall regions, total salts are leached into the lower horizon due to which soil becomes acidic. High temperature fastens the weathering process. Soil formation process is activated under favourable temperature and rainfall.

4. Organism (Biosphere):

Flora and fauna. Flora includes plant roots, micro-organisms, forest and grasses. Fauna means earthworm, ants, termite, rodents etc.

Flora and Fauna plays active role in the formation of soil. Soil formed in the forest consists of relatively higher biological matter above the ‘A’ hori­zon which is called O’ horizon. Undecomposed biological matter is promi­nently found in the forest. Fauna plays an active role in the mixing and churning of such matters in the soil.

5. Time and Age of land:

Age is the span of period from inception of soil development to present stage. Soil formation is a very slow process. Conditions that hasten the rate of soil development are warm and humid climate, forest vegetation, permeable and unconsolidated parent material, low in lime content, flat topography with good drainage.

Time factor has no relevance after the soil reaches its maturity. If the soil solum is removed by erosion or deposited over by a fresh transported parent material, a new cycle of soil formation takes place over the buried soil.

Soil Forming Processes:

Basic process according to Simonson (1959) are:

(i) Addition of water, organic matter and minerals to soil.

(ii) Losses of water, O.M. and minerals from soil.

(iii) Translocation means movement in solution (leaching) and movement in suspension of clay, O.M and hydrous oxide.

(iv) Transformation of mineral and organic matter and formation of definite layers.

Fundamental Processes are:

1. Humification:

Process of decomposition of O.M. leading to the formation of humus.

2. Eluviation:
It means washing out, Eluviation is the process of removal of constituents in suspension or solution by the percolating water from the upper layers to lower layers. It refers to movement and re­moval of material in solution from entire solum.

3. Illuviation:

It means Washing in, It is the process of deposition of soil materials in the lower layers i.e. immobilization and accumulation of the eluviated constituents.

4. Horizonation:

The development of all the horizons in soil is horizonation. In the favourable conditions, development of different hori­zons in soil takes about 250 years but in adverse condition, it takes even more than 1000 years.

Specific Pedogenic Processes:

(a) Zonal Pedogenic Processes:

1. Calcification and Gypsification:

Calcification means depo­sition of CaCO₃ in soil profile, in arid and semi-arid regions, following reactions occur.

2. Decalcification:

Removal of CaCO₃ from soil by leaching.

3. Podzolization:

Calcification tends to concentrate CaCO₃ in the lower part of the B-horizon, whereas Podzolization leaches the entire solum. Apart from Ca, the other bases are also removed and the whole soil becomes distinctly acidic as the Sesquioxide (Fe & Al) move out, the A-horizon gives a bleached grey or ashy appearance hence the term podzol (Russian term pod=under, zola =ash like) and sesquioxide may accumulate in B -horizon.

A typical and mature podzol profile exhibits strongly contrasting B-horizon with 3 distinct layers i.e.:

(i) Precipitated humus

(ii) A reddish brown layer due to deposition of sesquioxides and

(iii) A yellowish horizon which gradually merge into the parent mate­rial. The podzols are low in fertility and ate mainly used for forestry and pastures.

Podzolization is a type of eluviation in which humus and sesquioxides become mobile, leach out from the upper horizons and are deposited in the lower horizons.

4. Laterization:

It is contrast to podzolization. Later means brick or tile. It refers specifically to a particular cemented horizon in certain soils which when dried, become very hard like a brick. Such soil (in tropics), when massively impregnated with sesquioxides (Fe & Al oxides) to the extent of 70-80% of the total mass, are called laterites or latosols (oxisols).

Laterization is the process that removes silica, instead of sesquioxides from the upper layers and there by leaving sesquioxides to concentrate in the solum. Laterites/Latosols/Oxisols are rich in sesquioxides and poor in silica. It is mainly used for shifting agriculture, low intensity grazing and plantation of coffee, banana, pine apple and coconut.

Favourable conditions are warm and humid climate with 2000-2500 mm rainfall and continuous high temp., throughout the year.

Calcination, Podzolization and Laterisation are zonal soil forming processes where soils produced are under normal prevailing conditions of climate & Vegetation.

(b) Intrazonal Process:

In the Intrazonal soils, the profile characteristics are more influenced by certain local conditions such as relief or parent material than the climate and vegetation, These processes are occurred primarily in arid and semi – arid regions except gleization.

1. Gleization:

Development of a glei (a Russian word meaning blue, grey or green clay) in the lower part of the soil profile above the parent material due to poor drainage and water- logging. Such soils are called hydromorphic soil. It is not dependent on climate but often on drainage conditions. Due to anaerobic condition, Fe is reduced with the formation of mottles and concretions of Fe & Mn.

2. Salination:

Accumulation of soluble salts in soil.

Factors affecting are:

(i) Arid/semi- arid climate associated with shallow and brackish ground­water.

(ii) Lower topography

(iii) Poor drainage

(iv) Old lake bottom which on drying leaves salts at its surface.

(v) Alluvial deposits along sea coasts

(vi) Saline irrigation water.

3. Solonization/Alkalization:

It is accumulation of high exchange­able Na⁺ ions and p^H more than 8.5. Soil colloids are dispersed and tend to “move downward resulting in poor physical condition. The O.M. forms black Organo-clay coatings and hence called black alkali soils.

4. Solodization or Dealkalization:

It means removal of Na⁺ ion from the soil.

2NaX + CaSO₄ → Na₂SO₄ + Ca X

5. Pedoturbation:

Process of mixing of soils.

It is of three types:

(a) Faunal pedoturbation – by animals

(b) Floral Pedoturbation – by plants

(c) Argillipedoturbation- by churning process caused by swell-shrink clays as observed in deep black cotton soils of central India i.e. Vertisois.

Soil-forming Factors:

Five basic factors control the formation of soils:

(i) Parent Material;

(ii) Topography;

(iii) Climate;

(iv) Biological Activity;

(v) Time.

In fact soil form­ing factors act in union and affect the action of one another.

(i) Parent Material:

Parent material is a passive control factor in soil formation. Parent materials can be any insitu or on-site weathered rock debris (residual soils) or transported deposits (transported soils). Soil formation depends upon the texture (sizes of debris) and structure (disposition of individual grains/particles of debris) as well as the mineral and chemical composition of the rock debris/deposits. Nature and rate of weathering and depth of weathering mantle are important considerations under parent materials.

There may be differences in soil over similar bedrock and dissimilar bedrocks may have similar soils above them. But when soils are very young and have not matured these show strong links with the type of parent rock. Also, in case of some limestone areas, where the weathering processes are specific and peculiar, soils will show clear relation with the parent rock.

(ii) Topography:

Topography like parent materials is another passive control factor. The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub­surface drainage over and through the parent materials. Soils will be thin on steep slopes and thick over flat upland areas.

Over gentle slopes where erosion is slow and percolation of water is good soil formation is very favourable. Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter giving the soil dark colour. In middle latitudes, the south-facing slopes exposed to sunlight have different conditions of vegetation and soils and the north-facing slopes with cool, moist conditions have some other soils and vegetation.

(iii) Climate:

Climate is an important active fac­tor in soil formation.

The climatic elements involved in soil development are:

(i) Moisture in terms of its intensity, frequency and duration of precipitation-evaporation and humidity;

(ii) Temperature in terms of seasonal and diurnal variations.

Precipitation gives soil its moisture content which makes the chemical and biological activities possible. Excess water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation). In climates like wet equatorial rainy areas with high rainfall, not only calcium, sodium, magnesium, potassium etc. but also a major part of silica is removed from the soil.

Removal of silica from the soil is known as desilication. In dry climates, because of high temperature, evaporation exceeds precipitation and hence groundwater is brought up to the surface by capillary action and in the process the water evaporates leaving behind salts in the soil. Such salts form a crust in the soil known as hardpans. In tropical climates and in areas with intermediate precipitation conditions, calcium carbonate nodules (kanker) are formed.

Temperature acts in two ways— increasing or reducing chemical and biological activity. Chemical activity is increased at high temperatures, reduced at low temperatures (with an exception of carbonation) and stops in freezing conditions. That is why, tropical soils with high temperatures show deep profiles and in the frozen tundra regions soil contains large mechanically broken materials.

(iv) Biological Activity:

The vegetative cover and organisms that occupy the parent materials from the beginning and also at later stages helps in adding organic matter, moisture retention, nitrogen etc. Dead plants provide humus, the finely divided organic matter of the soil. Some organic acids formed during humification aid in decomposing the minerals of the soil parent materials.

Intensity of bacterial activity shows up differences between soils of cold and warm climates. Humus accumulates in cold climate as bacterial growth is slow. With undecomposed organic matter because of low bacterial activity, layers of peat develop in sub­arctic and tundra climates. In humid tropical and equatorial climates, bacterial growth and action is intense and dead vegetation is rapidly oxidised leaving very low humus content in the soil.

Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants. This process is known as nitrogen fixation. Rhizobium, a type of bacteria, which lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant.

(v) Time:

Time is the third important controlling factor in soil formation. The time length in which the soil forming processes operate, determines maturation of soils and profile development. A soil becomes mature when all soil-forming processes act for a sufficiency long time developing a profile. Soils developing from recently deposited alluvium or glacial till are considered young and they exhibit no horizons or only poorly developed horizons. No specific length of time in absolute terms can be fixed for soils to develop and mature.

Factors of Soil Formation:

There are five major factors that influence soil formation viz. parent material, climate, living organisms (especially native vegetation), topography and time. The combined influence
of these soil-forming factors determines the properties of a soil and their degree of expression.

Hans Jenny (1941) expressed the relationship of these five factors by equation:

S = f (cl, r, o, p, t, …)

Where,

S = soil properties (organic matter content, pH, soil texture etc.)

cl = regional climate

o = potential biota

r = topography

p = parent material

t = time

1. Active Factors:

i. Climate:

Climate affects soil formation largely through precipitation and temperature. Water percolates and moves from one place of parent material to another, it carries with it substances in solution and in suspension. Rainfall affects profile development through erosion producing these soils on slopes and deposition of soil material downhill.

With increasing moisture, N and C content, clay content, aggregation, saturation capacity and exchangeable H tend to increase. The depth of CaCO₃ horizon (calcic) in pedocals soils increase with increasing moisture.

With increasing temperature, the depth of weathering and clay content show an increase. Under tropical and sub-tropical climate with high rainfall, hilly leached soils have developed i.e. laterite soil. Low temperature increases leaching by reducing evaporation and favours the accumulation of organic matter by slowing down the process of decomposition.

ii. Biosphere:

Biosphere consists of vegetation and organisms. Vegetation shows main effect on soil formation through the amount and nature of organic matter, it adds to the soils. Vegetation also helps to control erosion. Burrowing animals, rodents, earthworms, ants etc. are highly important in soil formation. Burrowing animals cause constant mixing within the soil profile. The role of microorganisms as soil formers is intimately related to humification and mineralization.

2. Passive Factors:

i. Relief or Topography:

With the same kind of climate and parent material , soils that have developed on steep hill sides have thinner A and B horizons due to the surface erodes quite rapidly and less water moves downward within the profile. Soil materials on gently sloping topography have more water passing through them and the profile is generally deeper.

ii. Parent Material or Development of Soil Profile:

The presence of limestone in parent material shows the development of acidity in humid regions. Trees growing in lime stone material produce leaf litter that is relatively rich in Ca.

iii. Time:

The length of time required for a soil to develop horizons depends upon many factors i.e. climate, nature of PM, burrowing animals and relief.

Soil Forming Processes:

Podzolization – When humus, sesquioxide and clay goes down in ‘B’ horizon by percolation from ‘A’ horizon. As a result, increases the ratio of silica- sesquioxide in ‘A’ horizon and accumulation of humus, sesquioxide and clay in ‘B’ horizon, the soil so formed is called podzol soil and process is known as podzolization.

Laterization – When mobile silica (SiO₂) is deposited in ‘B’ horizon and residue sesquioxide on ‘A’ horizon. As a result, it decreases the ratio of silica- sesquioxide (< 2) and ‘A’ horizon is brick like due to presence of Fe₂O₃ (Hematite). The soil formed in this process is called laterite soil or latosols and process is called laterization.

Pedoturbation – Inter mixing of soil horizon is called pedoturbation or it is the process of mixing in soil.

Calcification – Calcification is the process of precipitation and accumulation of carbonates in the solum or in some part of the profile.

Decalcification – Reactions that remove CaCO₃ from one or more soil horizons or movement of CaCO₃ out of a section of the profile is called decalcification.

Eluviation – Leaching or removing from upper horizon to lower horizon is known as eluviation. When salts, elements, organic matter, small particles etc. are dissolved or coincide in water or suspension with water and goes down in lower horizons by percolation from the upper surface, the process is called eluviation and the upper horizons are called eluvial horizon. Generally, ‘A’ horizon is also called eluvial horizon.

Illuviation – When precipitation and deposition of salts, elements, OM, small particles etc. occur in lower surface, the process is called illuviation and lower horizons are called illuvial horizon. Generally, ‘B’ horizon is also called illuvial horizon.

Soil Erosion – Soil erosion is defined as the detachment and transportation of soil mass from one place to another through action of water or wind or beating action of raindrops.

Leaching – Washing out soluble materials from the solum is called leaching. Leaching is possible when water table is down. Leaching process is very useful in salt affected areas.

Enrichment – Addition of material to a soil body is called enrichment.

Cumulization – Cumulization is the process of accumulation of mineral material on the soil surface either by air or water. Process occurs in where material eroded from upper slope has been accumulated in lower slope resulting heap the lower portion.

Clay Migration/Lessivage – Removal of fine clay in suspension form from the upper part of soil profile and its accumulation in the lower part is called clay migration. Physical downward movement of clay mineral is termed as clay migration.

Silication – Accumulation of silica is called silication.

Resilication – Formation of Kaolinite from Gibbsite or formation of Smectite from Kaolinite in presence of large amount of Si(OH)₄ (Silicic acid) at higher pH values.

Desilication – The chemical migration of silica out of a section of soil profile.

Decomposition – The breakdown of mineral and organic materials.

Synthesis – The formation of new particles of mineral and organic species.

Melanization – The process refers to changes in colour value in soil whether caused by addition or losses of organic matter. Melanization develops Mollic horizon due to incorporation of organic matter.

Littering – The accumulation of organic litter and associated humus on the mineral soil surface to a depth of <30 cm.

Humification – The transformation of raw organic material into humus. Humification is a complex process involving various organisms i.e., fungi, bacteria, actinomycetes, earthworms, termites. Humification helps in the formation of surface humus layer.

Mineralization – Decomposition of OM and subsequent release of oxide solids.

Gleization – The reduction of Fe under anaerobic waterlogged soil condition with the production of bluish-greenish grey colour with or without yellowish brown, brown and black mottles and ferric or maganiferrous concretions is called gleization. Gleization occurs in flood plain areas.

Braunification/Rubification – Release of Iron from primary minerals.

Salinization – The process of accumulation of soluble salts i.e. SO₄^–, Cl^– and NO3^– of Ca, Mg, Na, and K in soil is called salinization and the soil so formed is called saline soil.

Alkalinization – The accumulation of Na⁺ ions on soil colloidal complex is known as alkalinization and the soil so formed is called alkali or sodic soil. It develops natric horizon.

Desalinization – Removal of soluble salts from the surface soil is known as desalinization. It develops salic horizon.

Degradation – Leaching of Na⁺ and salts from surface soil and pH is about 6, the process is known as degradation and the soil so formed is called degraded sodic soil.

Brunisolic – Fulvic acid reacts with Fe and makes complex compound and may cause its eluviations which is known as brunisolic. The material is brown in colour.

Ferrugination – Iron sesquides adhere firmly to sand grains and gravels giving them red colour resulting cement like. It leads to development of brown, reddish brown or reddish soil colour.

Audification – Accumulation of H⁺ ions in soil body.

Chelation – Forming complexes with metals by organic agents.

Compaction – The physical reduction of the air content resulting in an increased BD.

Dehydration – Loss of water reverting the compound to the original status.

Deposition – Deposition is the sedimentation of transported material. Accumulation of soil particles, nutrients and enriched of medium sized particles.

Disaggregation – Breaking down of aggregates.

Induration – Hardening of a section of the profile caused by iron pans and plinthite and other cementing agents i.e. Si or Ca.

Decolouration/Leucinization – Lighting colour in a section of profile and formation of albic endopedon.

Oxidation – Formation of an oxide or the release of electrons.

Solifluction – Slow flow of saturated soil on a permanent frozen soil.

Upward Movement – Movement of dissolved or suspended matter by capillarity.

Carbonization – The conversion of OM into charcoal is called carbonization.

Horizonation – The development of all horizons in soil is called horizonation.

Paludization – Paludiation is the process of accumulation of a thicker mass of organic materials under poorly drained soil or anaerobic soil condition.

Sulfofication – The biological oxidation of sulphur and its compound.