Soil - Notes | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
      SOIL 
The soil is the topmost layer of the earth’s crust mainly composed of organic 
minerals and rock particles that support life.  
PROCESS OF SOIL FORMATION 
Soil formation or pedogenesis depends first on weathering (Weathering 
describes the breaking down or dissolving of rocks and minerals on the 
surface of the Earth. 
It is this weathering mantle (depth of the weathered material) which is the 
basic input for soil to form.  
First, the weathered material or transported deposits are colonised by 
bacteria and other inferior plant bodies like mosses and lichens. Also, 
several minor organisms may take shelter within the mantle and deposits.  
The dead remains of organisms and plants help in humus accumulation 
(Humus is the dark organic matter that forms in soil when dead plant and 
animal matter decays).  
Minor grasses and ferns may grow; later, bushes and trees will start growing 
through seeds brought in by birds and wind. 
 Plant roots penetrate down, burrowing animals bring up particles, mass of 
material becomes porous and spongelike with a capacity to retain water and 
to permit the passage of air . 
 Finally a mature soil, a complex mixture of mineral and organic products 
forms. 
SOIL-FORMING FACTORS 
Five basic factors control the formation of soils:  
Parental material 
Topography 
Climate 
Page 2


 
      SOIL 
The soil is the topmost layer of the earth’s crust mainly composed of organic 
minerals and rock particles that support life.  
PROCESS OF SOIL FORMATION 
Soil formation or pedogenesis depends first on weathering (Weathering 
describes the breaking down or dissolving of rocks and minerals on the 
surface of the Earth. 
It is this weathering mantle (depth of the weathered material) which is the 
basic input for soil to form.  
First, the weathered material or transported deposits are colonised by 
bacteria and other inferior plant bodies like mosses and lichens. Also, 
several minor organisms may take shelter within the mantle and deposits.  
The dead remains of organisms and plants help in humus accumulation 
(Humus is the dark organic matter that forms in soil when dead plant and 
animal matter decays).  
Minor grasses and ferns may grow; later, bushes and trees will start growing 
through seeds brought in by birds and wind. 
 Plant roots penetrate down, burrowing animals bring up particles, mass of 
material becomes porous and spongelike with a capacity to retain water and 
to permit the passage of air . 
 Finally a mature soil, a complex mixture of mineral and organic products 
forms. 
SOIL-FORMING FACTORS 
Five basic factors control the formation of soils:  
Parental material 
Topography 
Climate 
 
Biological activity 
Time 
SOIL-FORMING FACTORS  
PARENT MATERIAL 
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 
consideration 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.  
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 sub-surface 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. 
Page 3


 
      SOIL 
The soil is the topmost layer of the earth’s crust mainly composed of organic 
minerals and rock particles that support life.  
PROCESS OF SOIL FORMATION 
Soil formation or pedogenesis depends first on weathering (Weathering 
describes the breaking down or dissolving of rocks and minerals on the 
surface of the Earth. 
It is this weathering mantle (depth of the weathered material) which is the 
basic input for soil to form.  
First, the weathered material or transported deposits are colonised by 
bacteria and other inferior plant bodies like mosses and lichens. Also, 
several minor organisms may take shelter within the mantle and deposits.  
The dead remains of organisms and plants help in humus accumulation 
(Humus is the dark organic matter that forms in soil when dead plant and 
animal matter decays).  
Minor grasses and ferns may grow; later, bushes and trees will start growing 
through seeds brought in by birds and wind. 
 Plant roots penetrate down, burrowing animals bring up particles, mass of 
material becomes porous and spongelike with a capacity to retain water and 
to permit the passage of air . 
 Finally a mature soil, a complex mixture of mineral and organic products 
forms. 
SOIL-FORMING FACTORS 
Five basic factors control the formation of soils:  
Parental material 
Topography 
Climate 
 
Biological activity 
Time 
SOIL-FORMING FACTORS  
PARENT MATERIAL 
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 
consideration 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.  
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 sub-surface 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. 
 
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 of water helps in the downward transportation of soil 
components through the soil (eluviation) and deposits the same down 
below (illuviation).  
NOTE: 
Eluviation is the transport of soil material from upper layers of soil 
to lower levels by downward precipitation of water across soil 
horizons, and accumulation of this material (illuvial deposit) in 
lower levels is called 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 ground water 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 into a crust in the soil known 
as  hardpans.  
In tropical climates and in areas with intermediate precipitation 
conditions, calcium carbonate nodules (kanker) are formed.  
Page 4


 
      SOIL 
The soil is the topmost layer of the earth’s crust mainly composed of organic 
minerals and rock particles that support life.  
PROCESS OF SOIL FORMATION 
Soil formation or pedogenesis depends first on weathering (Weathering 
describes the breaking down or dissolving of rocks and minerals on the 
surface of the Earth. 
It is this weathering mantle (depth of the weathered material) which is the 
basic input for soil to form.  
First, the weathered material or transported deposits are colonised by 
bacteria and other inferior plant bodies like mosses and lichens. Also, 
several minor organisms may take shelter within the mantle and deposits.  
The dead remains of organisms and plants help in humus accumulation 
(Humus is the dark organic matter that forms in soil when dead plant and 
animal matter decays).  
Minor grasses and ferns may grow; later, bushes and trees will start growing 
through seeds brought in by birds and wind. 
 Plant roots penetrate down, burrowing animals bring up particles, mass of 
material becomes porous and spongelike with a capacity to retain water and 
to permit the passage of air . 
 Finally a mature soil, a complex mixture of mineral and organic products 
forms. 
SOIL-FORMING FACTORS 
Five basic factors control the formation of soils:  
Parental material 
Topography 
Climate 
 
Biological activity 
Time 
SOIL-FORMING FACTORS  
PARENT MATERIAL 
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 
consideration 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.  
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 sub-surface 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. 
 
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 of water helps in the downward transportation of soil 
components through the soil (eluviation) and deposits the same down 
below (illuviation).  
NOTE: 
Eluviation is the transport of soil material from upper layers of soil 
to lower levels by downward precipitation of water across soil 
horizons, and accumulation of this material (illuvial deposit) in 
lower levels is called 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 ground water 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 into 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 in higher 
temperatures, reduced in cooler temperatures (with an exception of 
carbonation) and stops in freezing conditions. That is why, tropical soils 
with higher temperatures show deeper profiles and in the frozen 
tundra regions soils contain largely mechanically broken materials. 
 
BIOLOGICAL ACTIVITY 
 The vegetative cover and organisms that occupy the parent materials from 
the beginning and also at later stages help in adding organic matter, moisture 
retention, nitrogen etc.  
 Dead plants provide humus, the finely divided organic matter of the soil.  
Some organic acids which form 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 climates as bacterial growth is slow. 
 With undecomposed organic matter because of low bacterial activity, layers of 
peat develop in sub-arctic 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, lives in the root 
nodules of leguminous plants and fixes nitrogen beneficial to the host plant.  
The influence of large animals like ants, termites, earthworms, rodents etc., 
is mechanical, but, it is nevertheless important in soil formation as they rework 
Page 5


 
      SOIL 
The soil is the topmost layer of the earth’s crust mainly composed of organic 
minerals and rock particles that support life.  
PROCESS OF SOIL FORMATION 
Soil formation or pedogenesis depends first on weathering (Weathering 
describes the breaking down or dissolving of rocks and minerals on the 
surface of the Earth. 
It is this weathering mantle (depth of the weathered material) which is the 
basic input for soil to form.  
First, the weathered material or transported deposits are colonised by 
bacteria and other inferior plant bodies like mosses and lichens. Also, 
several minor organisms may take shelter within the mantle and deposits.  
The dead remains of organisms and plants help in humus accumulation 
(Humus is the dark organic matter that forms in soil when dead plant and 
animal matter decays).  
Minor grasses and ferns may grow; later, bushes and trees will start growing 
through seeds brought in by birds and wind. 
 Plant roots penetrate down, burrowing animals bring up particles, mass of 
material becomes porous and spongelike with a capacity to retain water and 
to permit the passage of air . 
 Finally a mature soil, a complex mixture of mineral and organic products 
forms. 
SOIL-FORMING FACTORS 
Five basic factors control the formation of soils:  
Parental material 
Topography 
Climate 
 
Biological activity 
Time 
SOIL-FORMING FACTORS  
PARENT MATERIAL 
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 
consideration 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.  
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 sub-surface 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. 
 
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 of water helps in the downward transportation of soil 
components through the soil (eluviation) and deposits the same down 
below (illuviation).  
NOTE: 
Eluviation is the transport of soil material from upper layers of soil 
to lower levels by downward precipitation of water across soil 
horizons, and accumulation of this material (illuvial deposit) in 
lower levels is called 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 ground water 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 into 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 in higher 
temperatures, reduced in cooler temperatures (with an exception of 
carbonation) and stops in freezing conditions. That is why, tropical soils 
with higher temperatures show deeper profiles and in the frozen 
tundra regions soils contain largely mechanically broken materials. 
 
BIOLOGICAL ACTIVITY 
 The vegetative cover and organisms that occupy the parent materials from 
the beginning and also at later stages help in adding organic matter, moisture 
retention, nitrogen etc.  
 Dead plants provide humus, the finely divided organic matter of the soil.  
Some organic acids which form 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 climates as bacterial growth is slow. 
 With undecomposed organic matter because of low bacterial activity, layers of 
peat develop in sub-arctic 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, lives in the root 
nodules of leguminous plants and fixes nitrogen beneficial to the host plant.  
The influence of large animals like ants, termites, earthworms, rodents etc., 
is mechanical, but, it is nevertheless important in soil formation as they rework 
 
the soil up and down. In case of earthworms, as they feed on soil, the texture 
and chemistry of the soil that comes out of their body changes. 
TIME  
Time is the third important controlling factor in soil formation.  
 The length of time the soil forming processes operate, determines 
maturation of soils and profile development. 
No specific length of time in absolute terms can be fixed for soils to develop 
and mature. 
 A soil becomes mature when all soil-forming processes act for a sufficiently 
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. 
 
SOIL PROFILE  
ON THE BASIS OF PROPORTIONS OF PARTICLES OF VARIOUS 
SIZES: 
If soil contains greater proportion of big particles, it is called sandy 
soil.Water can drain quickly through the spaces between the sand particles. 
So sandy soil tend to be light, well aerated and dry. 
If the proportions of fine particles are relatively higher, then it is called 
clayey soil. Clay particles, being much smaller, pack tightly together leaving 
little space for air. 
If the amount of large and fine particles is about the same, then the soil is 
called Loamy soil. The best top soil for growing plants is Loamy soil, which 
is a mixture of sand, clay and another type of particle known as Silt. Silt 
occurs as a deposit in river beds. The size of the silt particles is between those of