VisionIAS - Video Classroom Lecture

Geography Optional Class 02

SOIL FORMATION (5:11 PM)

Soil profile:
The vertical section of soil showing all its horizons is called the soil profile.
The bedrock on which the soil develops by the process called weathering is the first horizon 'R' horizon.
This horizon is strongly connected to the bedrock-like basalt, granite, sandstone, etc.
Above the R horizon is the C horizon.
It is the weathered parent rocky material on which the soil forms.
B horizon:
This horizon is also called 'subsoil'.
In this layer the accumulation of clay, humus, and oxides of aluminum, iron gypsum, and some carbonates are present.
This horizon can be divided into:
B1 layer:
It is the transition to A but more like B.
B2 layer:
The maximum accumulation of the silicate clay material on other organic material in this layer is called a layer of accumulation or illuviation.
B3 layer:
It is the transition layer to C but more like 'B' so it is also called the BC layer.
A horizon:
It is the mineral horizon formed at the surface or just below the 'O' horizon.
This horizon shows the accumulation of organic matter closely mixed with minerals.
So it appears dark in color.
This horizon is further divided into A1, A2, and A3.
A1 layer:
A dark-colored horizon of mixed mineral and organic matter and more biological activity than the 'A2' layer.
This layer is also called the E layer.
Its main feature is the loss of silicate clay, iron, aluminum and so this layer is also called the eluviation layer.
A3 layer:
This is the transition layer to 'B' but is more likely than 'A'.
This layer is also called the 'AB' layer.
O horizon:
This layer is the organic layer.
It is mostly found in the forest and the grassland.
This horizon is missing in the arable lands.
It is further divided into two layers O1 and O2 layers.
O1 is the undecomposed organic layer whereas the O2 is decomposed organic layer.
The A horizon is also called 'topsoil' while the B horizon is also called 'subsoil.'
Together A+B is called 'solum' which decides the fertility of the soil.

SOIL FORMATION PROCESS (6:14 PM)

The process involves gains and losses of material to the soil profile.
The movement of material from one of the soil profiles to another and the chemical translocation within the horizons takes place.
Such a process is called 'pedogenesis'.
The principal soil formation processes include:
i) Weathering
ii) Translocation
iii) Podzolization
iv) Gleying
v) Organic changes
Weathering:
It is the disintegration or decomposition of rocks in situ by natural agents.
There are varieties of weathering like physical or mechanical weathering, chemical, and biological weathering.
Translocation:
This is a complex process.
It includes the movement of materials within the soil profile also the direction of movement based on climatic factors like leaching.
Eluviation:
The process of removing dissolved or suspended soil material from upper layers of soil to lower layers by water percolation.
It is driven by rainfall exceeding evaporation.
Leaching:
The process of removing soluble substances and colloids from the top layer of the soil by percolation.
(Colloids:
a mixture that has particles ranging between 1-1000 nm in diameter.
They are still able to remain evenly distributed through the solution.)
This process results in the loss of minerals and materials in the solution.
Illuviation:
It is the accumulation of the silicate clay and the humus in the sublayer.
Podzolization:
This is a process found in the acidic soils located at the upper latitudes.
It involves a situation in which the upper horizons of the soil become rich in silica and the lower layers appear ash gray.
Due to the illuviation of sesqui-oxides particularly of iron.
Podzol is a Russian term that means under ash gray layer.
Gleying:
In happens in waterlogged and anaerobic conditions (deficit of oxygen) favors the proliferation of specialized bacteria that use up organic matter.
They reduce iron to a soluble ferrous state in a process known as reduction.
This results in the creation of a sticky, structureless clay layer called gley horizon.
It appears bluish-gray.
Organic changes:
The organic accumulation in the topsoil decomposes and gradually percolates as humus into the top soils.
The extreme conditions in the climate may lead to the formation of the humus layer into a dark amorphous mass called peaty soil.
For a long period, the humus itself decomposes in a process called mineralization which releases nitrogenous compounds into the soil.
Degradation, humification, and mineralization help in making the soil suitable for the germination of the seeds.
Lateralization:
Laterite in Latin means brick.
This process occurs only under high rainfall and high-temperature conditions in alternating seasons.
During the rainy season as the raindrop is slightly acidic, it leaches down the important mineral salts from the top layer to the subsoil leaving behind insoluble minerals like iron and aluminum.
In the next season, the iron and aluminum in the topsoil under the influence of high temperature turn into a hard pan-like structure called lateralization.
Lateralization is opposite to podzolization.

CLASSIFICATION OF THE SOIL (7:15 PM)

The father of pedology Dokuchaiev gave this classification.
He recognized a strong relationship between climate, vegetation, and soil zones.
Later an American scientist presented a comprehensive soil classification in 1938.
It is as follows:
I) Zonal soils:
These are well-developed soils.
These soils are the result of an interplay between climate and vegetation over a long period.
These soils are mature soils with all the horizons in their profile.
These soils developed under good drainage conditions.
The zonal soils are divided into two types:
i) Pedalfar
ii) Pedocal
i) Pedalfar soils:
They usually occur in humid areas.
They dominantly are composed of iron and aluminum oxides.
These soils are widely distributed all over the globe and they are called by different names from the poles to the equator.
Tundra Soils:
These are located at the poles.
They are permanently frozen soils called permafrost.
The tundra soils are rich in nitrogen and phosphorus.
They also contain large amounts of lichens and mosses.
This soil stored methane and carbon dioxide.
Distribution: Arctic tundra in the northern hemisphere, Alpine tundra (in the elevated peaks of the mountains), Antarctic tundra.
Podzol/Taiga Soil:
They are located in the cool climates in the south of tundra soils.
Podzol soils have leached ash-gray soil in the 'E' horizon.
These soils are generally derived from quartz-rich sands and sandstone.
The coniferous vegetation is typically associated with the podzol soils.
These soils are generally poor for agriculture.
These soils can be of two types: humus podzols and iron podzols.
Humus podzols:
The humus is washed down the profile and it accumulates as humus humus-rich horizon.
Iron podzols:
There is a marked concentration of iron oxides in the subsoil.
Sometimes within the iron podzols, a hard pan of iron interrupts the water percolation resulting in the formation of gley podzols.
Laterite soils:
These soils are found in hot and humid conditions.
They are unfit for cultivation but the plantation can be supported.
These soils are found in alternating seasons of high temperature of high temperature and high rainfall.
The topsoil turns into a hard pan-like structure.
Grumusols:
They are also called Savannah soils.
They are dark clay soils formed under warm climatic conditions with dry and wet spells.
These soils are rich in bases like calcium.
They support the grass vegetation.
On proper fertilization and irrigation, they can yield high returns.
ii) Pedocal:
They are calcium-rich soils found under arid and semi-arid conditions.
They contain low humus.
The 'A' horizon is very thin but the 'B' horizon is thick and cemented layer with calcium carbonate.
Types of the pedocal soils:
Chernozems:
They are also called black soil and are dark in color and associated with a high percentage of humus and ammonium compounds.
They have high soil moisture storage capacity.
A little rainfall can result in high agricultural yield.
Chestnut soils:
These soils occur along the arid side of chernozems.
These soils are rich in carbonates and gypsum.
The 'B' horizon is very thick but contains low humus.
These soils are largely distributed in the Mediterranean climate.
Sierozens:
These are the desert soils.
They are rich in soluble salts and the soil is poor in nitrogen and humus.
It contains high levels of calcium carbonates.
On irrigation and supply of fertilizers, these soils can be brought under cultivation.
Prairies soils:
They occur as a transition soil between the chernozems and podzols.
These soils are high-yielding soils under proper irrigation and fertilization.
They are found in the Canadian and the American plains around the Great Lakes.

TOPIC OF THE NEXT CLASS: Azonal and intrazonal soils