SCIENCE OF SOIL
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Soil
A collection of natural bodies developed in the
unconsolidated mineral and organic material on the
immediate surface of the earth that serves as a natural
medium for the growth of land plants and has
properties due to the effects of climate and living
matter acting upon parent material, as conditioned by
topography, over a period of time.
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GENESIS OF SOIL
Rocks are chief sources for the parent material
over which soils are developed
Types of rocks Igneous
 Sedimentary
 Metamorphic
Genesis includes –weathering of rocks &
formation of soil
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Primary and Secondary
Minerals
 Primary Minerals: Minerals that have persisted with
little change in composition since they were
extruded in molten lava(eg. quartz, mica and
feldspars).They are most prominent in sand and silt
fractions.
 Secondary Minerals: Minerals such as the silicate
clays and iron oxides, have been formed by the
breakdown and weathering of less resistant
minerals as soil formation progressed.
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Weathering of rocks
It is physical and chemical disintegration and
decomposition of rocks. Weathering creates
the parent material over which the soil
formation takes place. Later weathering, soil
formation
and
development
proceeds
simultaneously.
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Physical weathering
 Temperature
 Water
 Wind
 Plants & animals
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Chemical weathering
 Solution
 Hydration
 Hydrolysis
 Acidification
 Oxidation
 Reduction
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Soil formation
The mineral weathering combines with the
associated physical and chemical phenomena
constitute the process of soil formation.
It includes1. The addition of organic & mineral materials
2. The loss of these materials from the soil
3. Translocation of materials from one point to
Another within the soil column
4. Transformation of minerals & organic
substances within the soil
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Two Approaches:
 Pedological
 Edaphological
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The origin of the soil ,its classification, and its
description are examined in pedology (pedon-soil or
earth in greek). Pedology is the study of the soil as a
natural body and does not focus primarily on the
soli’s immediate practical use. A pedologist studies,
examines, and classifies soils as they occur in their
natural environment.
Edaphology (edaphos means soil or ground in greek)
is the study of soil from the stand point of higher
plants. Edaphologists consider
the various
properties of soils in relation to plant production.
They are practical and have the production of food
and fibre as their ultimate goal.
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Composition of soil
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Soil Profile and its Layers(Horizons)
 Examination of a vertical section of a soil as seen
in a roadside cut or in the walls of a pit dug in the
field, reveals the presence of more or less distinct
horizontal layers. Such a section is called a
profile, and the individual layers are known as
horizons
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13
Topsoil and Subsoil
 When a soil is ploughed and cultivated, the
natural state of the upper 12-18 centimeters(5-7
inches) is modified. This manipulated part of the
soil is referred to as the surface soil or the topsoil.
 The subsoil is comprised of those soils layers
underneath the top soil.
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Mineral (inorganic) and
organic soils
 Mineral soils: Mineral or inorganic in
composition, low in organic matter ranges from 1
-6%.
 Organic soils: 50% organic matter by volume (at
least 20% by weight).
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Soil Texture and Soil Structure
 Soil Texture: Proportions of different sized
particles present in soil.
 Soil Structure: The arrangement of the sand
silt and clay particles within the soil.
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Table:General properties of three major inorganic soil particles
Property
Sand (0.052mm)
Silt (0.002-0.05mm)
Clay(<0.002
mm)
1. Means of observation
Naked eye
Microscopic
Electron
Microscope
2.Dominant minerals
Primary
Primary and
Secondary
Secondary
3.Attraction of particles for each
other
Low
Medium
High
4. Attraction of particles for water
Low
Medium
High
5.Ability to hold chemical nutrients Very low
and supply them to plants
Low
High
6.Consistency properties when wet
Smooth
Sticky,
plastic
Powdery, some
clods
Hard clods
Loose , gritty
7.Consistency properties when dry Very loose,
gritty
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Soil Organic Matter
 Soil organic
matter comprises an accumulation of
partially disintegrated and decomposed plant and animal
residues and other organic compounds synthesized by the
soil microbes as the decay occurs. Such material is
continually being broken down and re-synthesized by soil
microorganisms. Consequently, organic matter is a rather
transitory soil constituent, lasting for a few hours to
several hundred years.
 Organic matter binds mineral particles into granules that
are largely responsible for the loose. easily managed
condition of productive soils and increases the number of
water a soil can hold.
 It is also major soil source of phosphorus and sulfur and
the primary source of nitrogen (3 elements essential for
plant growth)
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 Organic matter, including plant and animal
residues, is the main source of energy for soil
organisms. Without it biochemical activity would
come to a near standstill.
 In addition to the original plant and animal
residues and to their partial breakdown products,
soil organic matter includes complex compounds
that are relatively resistant to decay. These
complex materials, along with some that are
synthesized by the soil microorganisms, are
collectively known as humus. This material is
usually black and brown in colour, is very
fine(colloidal) in nature.
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Soil Water
 Water is hold in the soil for varying degree of tenacity
depending on the amount of water present and the size of
the pores.
 Together with its soluble constituents, including nutrient
elements(eg. Ca, P, N and K), soil water makes up the soil
solution, which is the critical medium for supplying
nutrients to growing plants.
 The movement can be in any direction; downward in
response to gravity, upward as water moves to the soil
surface to replace that lost by evaporation, and in any
direction toward plant roots as they absorb this important
liquid. Although some of the soil moisture is removed by
the growing plants, some remains in the tiny pores and in
thin films around soil particles. The soil solids strongly
attract the soil water and consequently compete for it with
plant roots.
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Soil Solution
 The soil solution contains small but significant
quantities of soluble inorganic and organic
compounds, some of which contain elements
that are essential for plant growth
 Critical property of the soil solution is its
acidity or alkalinity. Many chemical and
biological reactions are dependent on the
levels of hydrogen ions and hydroxide ions in
the soil. These levels influence the solubility,
and in turn the availability to plants, of several
essential nutrient elements such as Fe, Mn, P,
Zn and Mo.
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 The concentration of hydrogen(H+) and hydroxide
ions(OH-) in the soil solution is commonly ascertained by
determining its pH. Technically the pH is the negative
logarithm of the concentration of hydrogen ion in the soil
solution. Thus each unit change in pH represents a
tenfold change in the activity of the H+ and OH- ions.
Acidity
Alkalinity
Very
strong
3-4
Strong
3 4-5
Moder
ate
Slight
Neutral
Slight
5-64
6-7
7
7-8
Modera
te
Strong
Very
stromg
8-9
9-10
10-11
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Clay and Humus
 The attraction of ions such as Ca2+, Mg2+, and K+ on
the surfaces of colloidal clay and humus is not as exciting
as is the exchange of these ions for other ions in the soil
solution. For example, an H+ ion released to the soil
solution by a plant root exchange readily with a
potassium ion(K+) adsorbed on the colloidal surface .The
K+ ion is then available in the soil solution for uptake by
the roots of crop plants. A simple example of such cation
exchange illustrates this point.
colloid H+ + K+(aq)
colloid K+ + H+(aq)
(adsorbed)
(in soil solution)
(adsorbed)
(in soil solution)
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-ve charge +ve charge
Al
Ca
Mg
Clay Micelle
K
Na
H
Ionic double layer
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Essential nutrient element and their sources
Use in relatively
large amounts
Use in relatively large Use in relatively small
amounts
amounts
Mostly from air and
water
From soil
From soil
Carbon(C )
Nitrogen(N)
Iron(Fe)
Hydrogen(H)
Phosphorus(P)
Manganese(Mn)
Oxygen(O)
Calcium(Ca)
Boron(B)
Magnesium(Mg)
Molybdenum(Mo)
Sulfur(S)
Copper(Cu)
Zinc(Zn)
Chlorine(Cl)
Cobalt(Co)
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Soil Air
Soil air differs from the atmospheric air in several
respectsFirst ,the composition of soil air is quite dynamic
and varies greatly from place to place within a
given soil.
Second, soil air generally has a higher moisture
content than the atmosphere; the relative humidity
of soil air approaches 100% when the soil
moisture is optimum.
Third, carbon dioxide in soil air is often several
times higher than the 0.03% commonly found in
the atmosphere, Oxygen decreases accordingly
and, in extreme cases 5-10%, or even less, as
compared to about 20% for normal atmosphere.
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Composition of soil air
Particulars
Percentage by
volume
Nitrogen
Oxygen
Carbon dioxide
Atmospheric air 79.00
20.95
0.03
Soil air
79.20
20.60
0.25
Sandy soil air
79.20
19.95
0.30
Loamy soil air
79.20
19.20
0.62
Clay soil air
79.20
19.69
0.66
Manured soil
air
79.20
18.23
1.85
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Soil Degradation
 Soil degradation is a concept in which the
value of the biophysical environment is
affected by one or more combination of
human-induced processes acting upon the
land. It is viewed as any change or disturbance
to the land perceived to be deleterious or
undesirable. Natural hazards are excluded as a
cause, however human activities can indirectly
affect phenomena such as floods and
bushfires.
It is estimated that up to 40% of the
world's agricultural land is seriously degraded.
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Causes
The major causes include:
 Land clearance, such as clearcutting and
deforestation
 Agricultural depletion of soil nutrients through
poor farming practices
 Overgrazing
 Inappropriate Irrigation and overdrafting
 Urban sprawl and commercial development
 Land pollution including industrial waste
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 Vehicle off-roading
 Quarrying of stone, sand, ore and minerals
Overcutting of vegetation
 Overgrazing
 shifting cultivation without adequate fallow
periods, absence of soil conservation
measures,
 Population pressure
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Effects
 The major stresses on vulnerable land include:
 Accelerated soil erosion by wind and water
 Soil acidification and the formation of acid sulfate soil





resulting in barren soil
Soil alkalinisation owing to irrigation with water containing
sodium bicarbonate leading to poor soil structure and
reduced crop yields
Soil salinization in irrigated land requiring soil salinity
control to reclaim the land
Waterlogging in irrigated land which calls for some form of
subsurface land drainage to remediate the negative effects
Destruction of soil structure including loss of organic
matter
Ultimately results into low vegetation cover, extensive soil
erosion which leads towards desertification
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 Every year 84 billion tonnes of productive top
soil are lost world wide through degradation.
 Degradation has already affected 1900 m ha
of land globally (De Man et. al. 2007).
 Additionally each year over 14 million acres of
productive lands are oversalted because of
improper water management.
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Soil Erosion
 Soil erosion is the process of detachment of soil
particles from the parent body and transportation
of the detached soil particles by wind or water.
Mechanism of Water Erosion:
a. Detachment
b. Transportation
Causes:
a. Natural
b. Anthropogenic
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Forms of Water Erosion
 Sheet Erosion: uniform removal of top soil in thin layer





from the field, least conspicuous.
Rill Erosion: channelization begins ,no longer uniform.
Gully Erosion: unchecked rills result in increased
channelization of runoff.
Ravines: manifestation of prolonged process of gully
erosion. Deepening & Widening of gullies used to form
ravines.
Landslides: occur in mountain slopes when the slope
exceeds 20 per cent and width 6 m.
Stream-bank Erosion: Seasonal streams or rivulets often
change their course from season to season due to blockage
of their previous course by transported rocks, clods of soil &
vegetation grown during lean periods.
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Gully erosion
Ravine erosion
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Forms of wind erosion
 Suspension- Most spectacular method of transporting
soil particles is by suspension. Dust particles of fine sand
( less than 0.1 mm dia) are moved parallel to ground
surface and upward. About 5-15 % of wind erosion
afftected soil is transported by this process.
 Saltation- Particles in the range 0.1-0.5 mm diameter are
lifted by the wind, then fall back to the ground, so they
move in a hopping or bouncing fashion. These particles
cause abrasion of the soil surface and as they hit other
particles they break into smaller particles, a process
called attrition. Depending on conditions, this process
may account for 50-70% of the total movement of soil.
 Surface creep- Rolling and sliding of larger particles
(more than 0.5 mm dia) along the surface. Surface creep
account to 5-25% of total movement due to action of
wind.
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Soil Conservation
Definition
Soil conservation is using and managing
land based on the capabilities of the land
itself.
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Soil Conservation Measures
Agronomic Measures
Cultivation – By ploughing and sowing
across the slope, each ridge of plough furrow and
each row of the crop act as an obstruction to runoff,
providing more opportune time for water to enter into
the soil and reduce soil loss.
 Tillage – Tillage alters soil physical characters like
porosity, bulk density, surface roughness and hardness
of pans. Conventional tillage includes ploughing
twice or thrice followed by some secondary
operations like harrowing and planking that smoothen
and pack the soil in seed-bed and/or control weeds.
 Mulching – Mulches are any material such as straw,
plant residues, leaves, loose soil or plastic film placed
on the soil surface to reduce evaporation, erosion or
to protect plant roots from extremely low or high
temperature.
 Contour
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Cropping Systems: It represents cropping patterns used on a farm
and their interaction with farm resources, other farm enterprises and
available technology. Cropping pattern indicates yearly sequence
and spatial arrangements of crops and fallow in an area. Practically,
it implies different crops grown either in combination or
sequentially in farm over the years. Therefore, growing a crop which
produces the maximum cover, in addition to principle crops like rice
or wheat, reduces runoff and soil loss. For example, cow pea and
green gram are important cover crops for the rainy season. Tobacco,
being a clean cultivated crop, allows higher runoff and soil loss.
These losses can be reduced by growing cowpea or green gram
during early monsoon before tobacco is planted.
Strip-cropping: long and narrow strips of erosion resisting crops
(e.g. groundnut, beans) are alternated with strips of erosion
permitting crops (e.g. maize). The strips are laid across the slope.
Use of chemicals: Soil stability can be increased by spraying
chemicals like polyvinyl alcohol at 480 kg/ha.
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Mechanical Measures
 Contour Bunding – Runoff from any given surface is along the line
of greatest slope and the velocity of runoff increases with the
vertical distance through which it is moved. The contour bund
being on the same elevation, assures that the depth of water against
the bund is uniform throughout its length. It ensures uniform
distribution of water above the bunds and therefore, better
cultivation possibilities than any other type of bund. As the bunds
are at regular intervals, they intercept the runoff from attaining
erosive velocity and causing erosion. The velocity of flowing water
is slowed down and water thus held on the field for a longer time,
soaks into the soils.
 Broad Base Terrace - A terrace is a combination of ridge and
channel built across the slope. These terraces have wide base and
low height of ridge and usually formed with machinery. BBTs are
constructed in soils with high clay content which develop deep
cracks in summer (e.g. Black soil).
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Bench Terracing - Bench terracing consists of transforming relatively
steep land into a series of level strips or platforms across the slope of
the land. It reduces the slope length and consequently erosion. The field
is made into a series of benches by excavating the soil from upper part
of the terrace and filling in the lower part. On steeply sloping and
undulated land, farming practices is possible only with bench terracing.
It is usually practiced on slopes ranging from 16 to 33%.
Trenching –Contour trenches are made in non-agricultural land for
providing adequate moisture conditions in order to raise trees or grass
species. The trenches are usually 60 cm × 48 cm in size. The spacing
varies from 10 to 30 m.
Vegetative Barriers – these are closely spaced plantations-usually a few
rows of grasses or shrubs --- grown along contours . They act as barrier
to check the velocity of overland flow and entrapment of silt load behind
them. Khus (Vettiveria zelanica) is the most suitable plant for this
purpose.
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Grassed Waterways – These are drainage channel either
developed by shaping the existing drainage ways or constructed
separately. Suitable perennial grasses that are not edible by cattle,
deep rooted and spreading type are established subsequently for
the stability of the waterway (e.g Panicum repens, Brachiara
mutica, Cynodon dactylon, Paspalum notatum). The objectives
are- 1. to provide drainage, 2. to convert gullies or unstable
channels into stable channels by providing grass cover, and 3. for
leading water at non-erosive velocity into a water body.
Gully Control – The basic approach to gully control involves
reduction of peak flow rates through the gully and provision of
stable channel for the flow that has to be handled. Temporary and
permanent structures such as check dams, drop-spill ways are
constructed.
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Agrostological Measures
 Grasses prevent soil erosion by intercepting rainfall, by
binding the soil particles and by improving soil structure.
A grass-legume association is ideal for soil conservation.
E.g Pennisetum pupureum, Cenchrus ciliaris, Setaria
sphacelata.
Forestry Measure
 Afforestation and re-forestation in wastelands
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AA
47
48
Semi-circular & triangular
contour bunds
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50
51
52
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Check dam
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