Ag Chem 1.1 Theory
Ag Chem 1.1 Theory
Ag Chem 1.1 Theory
SOIL
The word ‘soil’ as a verb, means ‘to make dirty’ as in the case of soiled dishes or clothing. The
noun ‘soil’ is derived through old French from the Latin solum, which means floors or ground. What,
a soil scientist calls soil, a geologist may calls fragmented rock, an engineer may calls earth, and an
economist may call land. In general, soil is defined as the more or less loose and crumby part of the
outer earth crust. Since time immemorial soils have been christened as a medium of plant growth to
meet basic food, fuel and fiber needs of humans. Soil health care was central to ancient farmers, amply
depicted in a quotation from Sanskrit believed to have been made in 1500 BC “Upon this handful of
soil our survival depends. Husband it and it will grow our food, our fuel and our shelter and surround
us with beauty. Abuse it and the soil will collapse and die taking man with it”.
Some of the famous ancient and contemporary proverbs listed below highlight the importance
of soil resource and its management:
✓ We know more about the movement of celestial bodies than about the soil underfoot - Leonardo
da Vinci
✓ A nation that destroys its soils destroys itself - Franklin Delano Roosevelt
✓ To forget how to dig the earth and to tend the soil is to forget ourselves ~ Mahatma Gandhi
✓ Soil is a rock on its way to ocean - Willard L. Lindsay
Soil is considered as a geomembrane of the Earth, protective filter, buffer, mediator of energy,
water, and biogeochemical compounds; sustainer of productive life, ultimate source of elements, and
the habitat for most biota; foundation that supports humans; and the dust to which humans would
finally return.
Generally soil refers to the loose surface of the earth as identified from the original rocks and
minerals from which it is derived through weathering process. The widely accepted definition
according to Buckman and Brady (1969) states “Soil is a dynamic natural body on the surface of the
earth in which plants grow, composed of mineral and organic materials and living forms.”
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Approaches of Soil Study
The soil is made-up of broken down rock materials of varying degree of fineness and changed
in varying degrees from the parent rocks by the action of various agencies in such a way that the
growth of vegetation is made possible. One treats soil as a natural body, weathered and synthesized
product in nature (Pedology) while other treats soil as a medium for plant growth (Edaphology).
The two approaches - pedological and edaphological can be used in studying soils.
(i) Pedological approach: The origin of the soil, its classification, and its description are examined in
pedology (from the Greek word pedon, which means soil or earth). Pedology is the study of the soil
as a natural body and does not focus primarily on the soil’s immediate practical use. A pedologist
studies, examines, and classifies soils as they occur in their natural environment.
(ii) Edaphological approach: Edaphology (from the Greek word edaphos, which means soil or
ground) is the study of soil from the standpoint 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 fiber as their ultimate goal. To achieve that goal, edaphologists must be a scientist to determine
the reasons for variation in the productivity of soils and find means of conserving and improving
productivity.
Soil Science
“The science dealing with soil as a natural resource on the surface of the earth, including
Pedology (soil genesis, classification and mapping), physical, chemical, biological and fertility
properties of soil and these properties in relation to their management for crop production.”
Soil Science has major six well defined and developed disciplines
1. Soil fertility: Deals with the nutrient supplying capacity of soil
2. Soil chemistry: Studies on chemical constituents, chemical properties and the chemical
reactions
3. Soil physics: Involves the study of soil physical properties
4. Soil microbiology: Deals with soil micro-organisms, its population, classification, its role in
nutrient transformations
5. Soil conservation: Dealing with protection of soil against physical losses by erosion or against
chemical deterioration i.e. excessive loss of nutrients either by natural or artificial means.
6. Soil pedology: Dealing with the genesis, survey and classification
Some of the soil facts highlighted by the International Union of Soil Sciences are as
✓ A handful of soil can contain billions of soil microorganisms
✓ Soil is one of the most complicated biological materials on our planet
✓ It can take more than 1,000 years to form a centimeter of topsoil
✓ In a handful of fertile soil, there are more individual organisms than the total number of human
beings that have ever existed
✓ Soil stores 10 per cent of the world’s carbon dioxide emissions
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✓ There are over 100,000 different types of soil in the world
✓ Soil carbon is the largest terrestrial pool of carbon
✓ Five tonnes of animal life can live on one hectare of soil
Soil provides biomass for food, fodder and renewable energy; filtering, buffering and
transformation for clean ground water and clean air; besides carbon sequestration and the maintenance
of a large variety of organisms, guaranteeing biodiversity. It emits greenhouse gases to atmosphere,
thereby contributing to global warming; transports solids to open water surfaces and to air by water
and wind erosion; and creates problems to human health through its ingestion, inhalation, and skin
contact.
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CHAPTER 2: SOIL FORMING ROCKS AND MINERALS
The earth’s crust and its composition
Classically there are three divisions of earth’s sphere corresponding to the solid, liquid and
gas, which constitute the Earth. The solid zone is the Lithosphere. The outermost 10 mile strata of the
lithosphere are called the ‘earth crust’. The incomplete covering of water forming seas and oceans is
the Hydrosphere and the gaseous envelope over the earth’s surface is the Atmosphere.
Atmosphere
An atmosphere (from Greek atmos, meaning 'vapour', and sphaira, meaning 'sphere') is a layer of
gases surrounding a planet or other material body, that is held in place by the gravity of that body. An
atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the
atmosphere is low.
The atmosphere of Earth is mostly composed of Nitrogen (about 78%), Oxygen (about 21%), Argon
(about 0.9%) with carbon dioxide and other gases in trace amounts. Oxygen is used by most organisms
for respiration, nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction
of nucleotides and amino acids and carbon dioxide is used by plants, algae and cyanobacteria for
photosynthesis. The atmosphere helps protect living organisms from genetic damage by solar
ultraviolet radiation, solar wind and cosmic rays. Its current composition is the product of billions of
years of biochemical modification of the paleoatmosphere by living organisms.
Hydrosphere
It is the layer of water surrounding the lithosphere. It is present in the form of seas and oceans.
It covers 70% of the earth leaving only about 30% above sea level. The surface of the waters of the
various seas is in one level in contrast with the surface of the land. This surface is known as the sea
level.
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The seawater has a higher specific gravity than terrestrial water due to the salts it contains in
solution. The average density is 1.026, but it varies slightly from place to place. Sea water contains
3.5% salts (minerals) It is least dense at the places where river enters the sea and very heavy at places
where evaporation is high.
Lithosphere
It is the inner most body within the gaseous and watery envelops. That portion of the
lithosphere, which rises above the seawater, is visible to us and is known as land. The land is only
about ¼th of the total surface of the earth. Most of this land is situated in the northern hemisphere.
The lithosphere consists of two portions, viz.,
1. The upper or outer cool solid surface.
2. The inner hot and molten mass.
It is the heaviest of the three spheres. Its mean density is 5.5 compared to that of water as one.
The outer crust has a density of about 2.5 to 3.0, while the inside core, consists of much heavier
materials. The outer solid layer, called as the earth’s crust is estimated to be about 10 to 20 miles thick.
It consists of the various rocks together with a more or less thin mantle of soil enveloping them. It is
on this crust that life, both animal and plant sustains. The inner mass, which forms the interior of the
earth, is in molten condition. According to one belief, the whole of the inner core is a molten mass of
materials, upto the centre. According to another view, the interior of the earth consists of a molten
magma, about 50 to 100 miles thick, surrounding a gaseous centre. A gradation exists from the central
gaseous nucleus, through the intermediate molten mass, to the outer solid crust.
Biosphere
The biosphere (from Greek bíos "life" and sphaira "sphere") which can also be termed as the
zone of life on Earth, all the living things in the planet are categorized under the biosphere. In this
view, the biosphere includes all of the animals, plants, and microorganisms of earth. Humans as well
belong to this group. The entire ecological communities within the physical surrounding of the earth
are within the umbrella of living things (biosphere). These ecological communities interact together
with the physical aspects of the earth including the hydrosphere, lithosphere, and the atmosphere.
ROCKS
A rock is consolidated mass of one or more minerals. Rocks are mixture of minerals and
therefore, their physical and chemical compositions vary with the characteristics of minerals present
in them. Rock is formed due to cooling and solidification of molten magma. It is mixture of one or
more minerals. First igneous rocks are formed. Then it is converted to sedimentary or metamorphic
rock.
Petrology is a science of rocks which consists of petrography-deals with the description and
petrogenesis– study of the origin of rocks.
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Classification
Rocks are divided into three natural groups based on mode of origin or formation. These are:
(i) Igneous rocks : formed from molten material on cooling
(ii) Sedimentary rocks : formed from sediments under pressure
(iii) Metamorphic rocks : formed from pre-existing rocks through action of heat and pressure
Igneous rocks, on the basis of the depth of formation, are classified into
(1) Plutonic - When the magma solidifies at great depth, about 2 to 3 mile deep under the surface of
the earth, the igneous rock formed is called plutonic e.g. granite, syenite, gabbro, norite etc. These are
crystalline rocks as the size of crystals are big.
(2) Intrusive or dyke - When the magma solidifies at moderate depth is called Intrusive or dyke e.g.
pegmatite, dolerite. The crystals are of smaller size in the rock. The rocks consolidated in vertical
cracks and formed wall like masses are known as dykes, whereas those consolidated in horizontal
cracks are known as sills. In some cases the molten material is consolidated in irregular and narrow
cracks is called a vein (Fig. 1).
(3) Extrusive or effusive - When the solidification takes place on the surface of the earth as a result
of volcanic activities, the igneous rock formed is called Extrusive or effusive e.g. rhyolite, pumis,
chalcidian, basalt, trap.
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On the basis of percentage silica content, igneous rocks are divided into
(a) Acidic: SiO2 content is more than 65% e.g. granite, pegmatite, rhyolite
(b) Intermediate: SiO2 content is between 55 and 65% (sub-acidic SiO2 60% to 65%, e.g. syenite;
sub-basic, SiO2 55 to 60 % e.g., diorite)
(c) Basic: SiO2 content is between 44 and 55 % e.g. basalt
(d) Ultra basic: SiO2 content is less than 44% e.g. picrite.
Fig.1: A schematic diagram of volcanic eruption showing the occurrence of plutonic (intrusive) and
volacanic (extrusive) igneous rocks.
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(3) Argillaceous: These rocks have individual grain size of clay in their formation. These may either
loose or consolidated rocks various clays are loose sediments. China clay (Kaoline) - formed from
decomposition of feldspar, Pipe clay - Iron free clay, Fire clay - Free from lime and alkalies and
Laterite - Reddish clay formed by decomposition of basalt and granite.
(iii) Metamorphic rocks (Meta means “change” and morph means “form”)
Metamorphic rocks are formed from igneous and sedimentary rocks through action of heat,
pressure and chemically active liquids and gases. Metamorphism may result in changes mainly
physical, chemical or both. Heat, pressure and water are called agents of metamorphism. The changes
due to result of water activity is called as hydro-metamorphosis and due to pressure is called dynamo-
metamorphosis.
Thus, when igneous and sedimentary rocks are subjected to tremendous pressure and high
temperature, metamorphism takes place and metamorphic rocks are formed. e.g.
(1) Sand stone Heat & Pressure Quartzite
(2) Shale Pressure Slate
(3) Lime stone Heat Marble
(4) Granite Pressure Gneiss (complete foliation, distinct separable layers)
(5) Basalt Pressure Schist (slight foliation, not distinct layers)
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The igneous and sedimentary rocks after they were first formed sometimes undergo a change.
When the change is considerable, the rock is said to have undergone metamorphosis and the new rock
is known as a metamorphic rock. The metamorphism is brought about by the action of water, heat or
pressure or by the combined action of any one of these or all. The change brought about by water is
hydro-metamorphism. The change brought about by heat is thermo-metamorphism. The change
brought about by pressure is dynamo- metamorphism. The changes that are brought about are both
physical and chemical in character. In some cases the metamorphism is so pronounced that the new
rock looks quite different from the original.
The action of water tends to remove some material or introduce new materials. By the
introduction of a cementing material like silica, lime or iron oxide, loose sand may be turned into
sandstone or sandstone into a quartzite. By the removal of certain constituents by percolating waters,
basalt or granite may be converted into a laterite.
The action of heat hardens the rock and develops new crystals in it. Crystalline marble is
produced this way from amorphous limestone by the action of heat and pressure. Due to pressure, the
crystals of the original rock get pressed or flattened and the new rock is foliated. When foliation is
slight, the layers are inseparable and it is called as gneiss. If foliation is complete with distinct and
separable layers, it is called as schist.
MINERALS
Minerals are naturally occurring inorganic solid homogeneous substances composed of atoms
having an orderly and regular arrangement with definite chemical composition and a characteristic
geometric form as quartz (SiO2), orthoclase (KAlSi3O8), calcite (CaCO3) , olivine [(Mg, Fe)2 SiO4]
and gypsum (CaSO4 .2H2O).
Minerals that are original components of rocks are called primary minerals. (feldspar, mica,
etc.). Minerals that are formed from changes in primary minerals and rocks are called secondary
minerals (clay minerals).
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Classification
Minerals can be classified on the basis of their amounts, mode of origin, composition and specific
gravity are given below:
(A) On the basis of origin and mode of formation
1. Primary minerals
When a mineral arises form the cooling and solidification of a molten mass is called primary
minerals. e.g. (i) orthoclase feldspar (ii) plagioclase feldspar (iii) anorthite feldspar (iv) quartz - SiO2,
(v) hornblende (vi) muscovite (vii) biotite and (viii) augite.
2. Secondary minerals
When it arises through the metamorphism or weathering of primary or other pre-existing
minerals, it is called secondary minerals e.g. (i) calcite - CaCO3, (ii) magnesite - MgCO3, (iii) dolomite
- CaMg(CO3)2, (iv) siderite - FeCO3, (v) gypsum - CaSO4.2H2O, (vi) apatite - Ca5(F, Cl) (PO4)3, (vii)
limonite - Fe2O3.3H2O, (viii) hematite - Fe2 O3 and (ix) gibbsite - Al2O3. 3H2O
Clay minerals
Clay minerals in the soils are formed from primary and secondary minerals due to weathering
processes. The clay fraction of the soil particles has a diameter less than 0.002 mm (2 µm). The mineral
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present in the clay fraction of the soil are called as clay minerals. Clay minerals are the most important
secondary minerals. They are colloidal and crystalline in nature. They carry a negative electrical
charge on their surface. Most of the physical, chemical and morphological properties of soils are
influenced by these clay minerals. The three most important groups of silicate clay minerals are
kaolinite, montmorillonite and illite. The relative occurrences of minerals in soil are given in Table 2.
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CHAPTER 3: WEATHERING AND THE SOIL FORMATION
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(1) Physical weathering
It is a mechanical process, causing disruption of consolidated massive rocks into smaller bits
without any corresponding chemical change.
(a) Temperature: The alternate expansion and contraction of rocks due to variation in temperature
produce cracks. Thy number of cracks slowly increases and the rock gets broken into pieces. This
phenomenon is referred to as ‘exfoliation’. The dark coloured rocks are subjected to fast changes in
temperature as compared with light coloured rocks. The cubical coefficient of expansion of feldspar
and quartz present in most of the rocks is 1: 2.
(b) Water: In cold regions, water freezes in rock joints and cracks. On freezing, the water expands in
volume by about 9.0 percent with a force of 150 tonnes/ft2 or 1465 Mg/m2. Due to this tremendous
pressure the rock splits and is broken up into a loose mass of stones. The moving water has a
tremendous transport capacity which by rolling action grinds the rocks into pieces. Water through its
erosion forces removes weathered parts of rock, thereby exposing fresh surface to weathering. The
excavation and destructive action of water is called denudation.
The amount material carried by water varies as the fifth power of its velocity, while the size of
material carried varies as the sixth power of its velocity. Thus doubling the velocity increases the
amount of material carried 32 times (25 = 32) and the size of materials by 64 times (26 = 64).
(c) Wind: Wind carrying particles in suspension and blowing constantly over the rock at great speed
exerts a grinding action, thereby the rock gets disintegrated. Loosely balanced rock boulders
sometimes roll down by the action of wind and break into pieces. At a velocity of 5 m/sec particles of
0.25 mm size are transported, while at a velocity of 10 m/sec, the wind can carry particles of 1 mm
size.
(d) Ice: The moving ice is an erosive detachment and transporting agency of tremendous capacity.
Snow received at higher elevations or polar regions accumulates and starts moving in the form of
glaciers. During their movement, glaciers cause great deal of cutting and crushing of the bedrocks.
Although glaciers are not so extensive in the present day environments; in the recent geological past,
they had transported and deposited parent materials over millions of hectares on this planet. At present,
glaciers are active in upper parts of the Himalayas.
(e) Plants and Animals: Some plants, like mosses and lichens, grow on the exposed rock. They
accumulate dust, which further encourages plant growth, and a thin film of highly organic material is
formed. Sometimes, roots of higher plants exert a prying effect on rocks (as the root girth increases
with plant growth), which results in some disintegration. Burrowing by rodents, movement of animals,
and human activities (cultivation, quarrying, land levelling, construction of roads, buildings, railway
lines, etc.) also result in physical weathering. Such influences, however, are of relatively limited
importance in producing parent material when compared to the drastic physical effects of water, ice,
wind, and temperature changes.
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(2) Chemical weathering
Physical disintegration is accompanied by chemical decomposition which produces changes
in the nature and composition of rocks and minerals. Chemical weathering takes place mainly at the
surface of rock minerals with the disappearance of certain minerals and the formation of secondary
products. This is called chemical transformation. No chemical weathering is possible without the
presence of water. The rate of chemical reactions increases with dissolved carbon dioxide and other
solute in water, and with increases in temperature. The principal agents of chemical weathering are
described below.
(a) Solution: Some substances (halite, NaCl) present in the rock are readily soluble in water. When
the soluble substances are removed by the continuous action of water, the rock no longer remains solid
and falls to pieces very soon.
(b) Hydration: Hydration means chemical combination of molecules with a particular mineral. Soil
forming minerals occurring in rocks undergo hydration when exposed to humid condition. e.g.
2Fe2O3 + 3HOH → 2Fe2O3 3H2O
Hematite (Red) Water Limonite (Yellow)
Due to this reaction, the minerals increase in volume and become soft and more readily
weatherable.
(c) Hydrolysis: It is one of the most important processes in chemical weathering. Hydrolysis depends
on the partial dissociation of water into H-ions and OH-ions. Increases in H-ion concentration resulting
in the accelerated hydrolytic action of water. Water thus, acts like a weak acid on silicate minerals,
e.g.
KAlSi3O8 + HOH → HAlSi3O8 + KOH
Orthoclase Water Acid silicate Potassium
(dissociated) Clay hydroxide
The products of hydrolysis are either wholly or partially leached by percolating water. They
may also recombine with other constituents to form clay. In a way, hydrolysis may be considered as
principal agent of clay formation.
(d) Oxidation: Oxidation means addition of oxygen to minerals. Oxidation is more active in the
presence of moisture and results in hydrated oxides. Soil-forming minerals containing iron, manganese
etc. are more subjected to oxidation, e.g.
4FeO + O2 → 2Fe2O3
Ferrous Oxygen Ferric Oxide
Oxide (Hematite)
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4Fe2O4 + O2 → 6Fe2O3
Magnetite Hematite
A rusty-looking (red) crust is formed on the surface of the rock. The crust thickens and then
slowly gets separated from the parent rock. As process continues, the change produced in the mineral
weakens the rock and ultimately the rock itself crumbles to pieces.
(e) Reduction: This means the removal of oxygen. Under condition of excess water (less or no
oxygen), reduction takes place e.g.
2Fe2O3 → O2 → 4FeO
Ferric oxide Oxygen Ferrous
(Hematite) oxide
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The parent materials transported from their place of origin are named according to the main
force responsible for the transport and redeposition. The material transported and deposited by water
is alluvium, found along major stream courses, at the bottom of slopes of mountains, and along small
streams flowing out of drainage basins, Colluvium is used for poorly sorted materials near the base of
strong slopes transported by the action of gravity. Lacustrine consists of materials that have settled
out of the quiet water of lakes. Glacial consist of all the materials picked up, mixed, disintegrated,
transported and deposited through the action of glacial ice or of water resulting primarily from melting
of glaciers. The wind blown materials are termed loess when the texture is silty and eolian sand when
these are primarily sand. The materials deposited by the melting glaciers vary widely in particle size
called as till or moraine. If the parent material is transported by water and fine sediments deposited at
the bottom of the sea and get exposed at the surface due to change in sea level named as marine. The
soils developed on such transported parent materials bear the name of the parent material, viz. alluvial
soils from alluvium, colluvial soils from colluvium (fig.3).
Fig.3: How various kinds of parent material are formed, transported and deposited.
(2) Relief or topography
The topography refers to the differences in elevation of the land surface. Topography largely
determines the drainage condition and the ground water level in the soil. On level topographic
positions, almost the entire water received through rain percolates through the soil. Under these
conditions, the soil formed may be considered as representative of the regional climate. They have
normal solum with distinct horizons. The soils on steep slopes are shallow, stony and have weakly-
developed profiles with less distinct horizons. Soils on steep terrain tend to have rather shallow, poorly
developed profile in comparison to soils on nearby, more level sites. On the steep terrain, water runoff
and soil erosion removes surface material before it has time to develop, hence no distinct horizon
observed at steep slopes. Red soils mostly developed at higher topographic position, while the black
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cotton soil developed at lower topographic position. In India black and red soils occur in close
proximity, it is observed that red soils occupy higher level while black soils are at lower levels.
(3) Time
Soil formation is a very slow process requiring thousands of years to develop a mature pedon.
The period taken by a given soil from the stage of weathered rock (i.e. regolith) up to the stage of
maturity is considered as time. By matured soils, we mean soils with fully developed horizons (A, B,
C). In soil formation nature works slowly. It has been reported that is takes hundreds of years to
develop an inch of soil. The time that nature devotes to the formation of soils is termed as pedologic
time (Fig.4).
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Organic matter accumulation, biochemical weathering, profile mixing, nutrient cycling, and
aggregate stability are all enhanced by the activities of organisms in the soil. Vegetative cover reduces
soil erosion rates, thereby slowing down the rate of removal of surface soil-minerals. Organic acids
produced from certain plants bring Fe and Al into solution by complexation and accelerate the
downward movement of these metal and their accumulation in the ‘B’ horizon.
Burrowing animals (fauna) such as moles, earthworms, ants, termites, and rodents are highly
important in soil formation. Old animal burrows in the lower horizons often become filled with soil
materials from the overlying ‘A’ horizon, creating profile features known as crotovinas. The
organisms cause constant mixing within the soil profile. Ants and termites, as they build mounds, also
transports soil materials from one horizon to another, and this mixing activities of animals called
pedoturbation.
The vegetation controls the erosion, thereby facilitates percolation as well as drainage and
brings greater dissolution of minerals. Their role as soil formers is related to humification and
mineralization.
Eluviation: It is the mobilization and translocation of certain constituent’s viz. Clay, Fe2O3, Al2O3,
SiO2, humus, CaCO3, other salts etc. from one point of soil body to another. Eluviation means washing
out. It is the process of removal of constituents in suspension or solution by the percolating water from
the upper to lower layers. The eluviation encompasses mobilization and translocation of mobile
constituents resulting in textural differences. The horizon formed by the process of eluviation is termed
as eluvial horizon (A2 or E horizon). The degree of translocation depends upon relative mobility of
elements and depth of percolation.
Illuviation: The process of deposition of soil materials (removed from the eluvial horizon) in the
lower layer (or horizon of gains having the property of stabilizing translocated clay materials) is
termed as Illuviation. The horizons formed by this process are termed as illuvial horizons (B-horizons,
especially Bt). The process leads to textural contrast between E and Bt horizons, and higher fine total
clay ratio in the Bt horizon.
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(B) Specific soil forming processes
(1) Podsolisation
It is a type of eluviation in which humus and sequioxides become mobile, leach out from upper
horizons and become deposited in the lower horizons. This process is favoured by cool and wet
climate. It requires high content of organic matter and low alkali in the parent material. The process
increases the proportion of silica, sesquioxide in A-horizons and accumulation of clay, iron and
aluminium in B-horizons.
(2) Laterisation
In this process, silica is removed while iron and alumina remain behind in the upper layers.
Laterisation is favoured by rapid decomposition of parent rocks under climates with high temperature
and sufficient moisture for intense leaching, such as found in the tropics. The soil formed in this
process is acidic in nature.
(3) Calcification
In this process, there is usually an accumulation of calcium carbonate in the profile. This
process is favoured by scanty rainfall and alkali in parent material.
(4) Gleization
The term glei is of Russian origin, which means blue, grey or green clay. The gleization is a
process of soil formation resulting in the development of a glei (or gley horizon) in the lower part of
the soil profile above the parent material due to poor drainage condition (lack of oxygen) and where
waterlogged conditions prevail. Under such condition, iron compounds are reduced to soluble ferrous
forms. This is responsible for the production of typical bluish to grayish horizons with mottling of
yellow and / or reddish brown colours.
(5) Salinization
Salinization is the process of accumulation of salts, such as sulphates and chlorides of calcium,
magnesium, sodium and potassium, in soils in the form of salty (salic) horizons. It is quite common in
arid and semi arid regions. It may also take place through capillary rise of saline ground water and by
inundation with seawater in marine and coastal soils. Salt accumulation may also result from irrigation
or seepage in area of impeded drainage.
(6) Desalinization
It is the removal by leaching of excess soluble salts from horizons or soil profile by ponding
water and improving the drainage conditions by installing artificial drainage network.
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(8) Solidization (dealkalization)
The process refers to the removal of Na+ from the exchange sites. This process involves
dispersion of clay. Dispersion occurs when Na+ ions becomes hydrated. Much of the dispersion can
be eliminated if Ca+ and Mg++ ions are concentrated in the water, which is used to leach the solonetz.
These Ca and Mg ion can replace the Na on exchange complex, and the salts of sodium are leached
out.
(9) Pedoturbation
Another process that may be operative in soils is pedoturbation. It is the process of mixing of
the soil. For example argillipedoturbation is observed in deep black soils. Mixing to a certain extent
takes place in all soils. The most common types of pedoturbation are:
✓
Faunal pedoturbation: Mixing of soil by animals such as ants, earthworms, moles, rodents,
humans
✓
Floral pedoturbation : Mixing of soil by plants as in tree tipping that forms pits and mounds
✓
Argillic pedoturbation: Mixing of materials in the solum by the churning process caused by
swell shrink clays as observed in deep Black Cotton Soils.
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Biocycli
Surficial ng of Surficial
gains of SOIL
materia losses of
material by Intersolum
ls material by
erosion TRASFORMATIO erosion
Intersolum
NS
TRASLOCATIO
(Pedogenic
N
Geochemicalweathering Leaching
weatheringetc.) losses of
supplying water and
materials other
materials
Soil profile
The vertical section of the soil showing the various layers from the surface to the unaffected
parent material is known as a soil profile. The various layers are known as horizons.
A soil profile contains three main horizons. They are named as horizon A, horizon B and
horizon C. The parent material from which the soil is formed is known as horizon C. The surface soil
or that layer of soil at the top which is liable to leaching and from which some soil constituents have
been removed is known as horizon A or the horizon of eluviation. The intermediate layer in which
the materials leached from horizon A have been re-deposited is known as horizon B or the horizon of
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illuviation. The Subordinate horizons may occur within a master horizon and these are designated by
lowercase following the capital master horizon letter, e.g., Bt, Ap, or Oi.
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Table 6. Lowercase letter symbols to designate subordinate distinctions within master horizons.
Letter Distinction Letter Distinction
a Organic matter, highly decomposed n Accumulation of sodium
b Burried soil horizon o Accumulation of Fe- and Al-oxides
c Concentration or nodules p Ploughing or other disturbance
d Dense unconsolidated materials q Accumulation of silica
e Intermediate decomposed OM r Weathered or soft bedrock
f Frozen soil s Illuvial OM and Fe/Al-oxides
ff Dry-permafrost ss Slickensides (shiny clay wedges)
g Strong gleying (mottling) t Accumulation of silicate clays
h Illuvial accumulation or OM u Present of human artifacts
i Slightly decomposed OM v Plinthite (high iron, red material)
j Jarosite (yellow sulphate mineral) w Structure without clay accumulation
ij Cryoturbation (frost churning) x Fragipan (high bulk density, brittle)
k Accumulation of carbonates y Accumulation of gypsum
m Cementation or induration z Accumulation of soluble salts
By way of illustration, Bt horizon is a ‘B’ horizon characterized by silicate clays accumulation (t from
the German ton, meaning clay). Similarly, in a Bk horizon, carbonates (k) have accumulated.
*****
27
CHAPTER 5: SOIL TAXONOMY
Classification is the grouping of objects in some Orderly and logical manner. It is based on the
properties of objects for the purpose of their identification and study. They are termed as differentiating
characteristics as they differentiate and serve to separate one class from the others; for instance, soils
are classified as sandy, loamy or clayey soils on the basis of their characteristics. For classifying the
individuals of a large and widely varying population, such as soils, it is useful to group individuals
into classes, and further into higher classes. This kind of grouping is called a multi-categoric or
hierarchical system of classification. The individual soils are grouped into classes of lower category
(e.g., soil series), which are further grouped into classes of higher categories (e.g., soil orders). The
lower categories are defined by a large number of differentiating characteristics and higher categories
by a few differentiating characteristics. Within each class, there is a central core or nucleus to which
the individual members are related in varying degrees. It is called the central concept or an idealized
individual which typifies the class.
SOIL CLASSIFICATION
The Russian scientist V. V. Dokuchaiev and his associates first conceived the idea that soils
exist as natural bodies in nature. Russian soil scientist soon developed a system for classifying natural
soil bodies, but poor international communications and the unwillingness of some scientist to
acknowledge such fundamental ideas, delayed the universal acceptance of the natural bodies concept.
In the United States, in 1920s, C. F. Marbut of the USDA, who grasped the concept of soils a natural
bodies, developed a soil classification scheme based on these principles.
Zonality Concept — The soils that have fully developed soil profiles, and are in equilibrium with the
environmental conditions, such as climate and vegetation, are termed as Zonal Soils, for instance,
Sierozem, Chestnut, Podzol** and Laterites.
The soils formed in regions, where time has been a limiting factor to produce fully-developed
horizons are termed as Azonal Soils, for instance, Alluvial soils and Regosols.
Still others, occurring within the zonal areas and having characteristics that are determined
largely by the local conditions, like topography, parent material, etc. are termed as Intrazonal Soils,
for instance, Calcimorphic and Hydromorphic Soils.
Joffey (1912) and Marbut (1936) classified soil on the basis of zonality, their own properties,
and morphological characters. They emphasized the concept of pedalfers* and pedocals. However,
Marbut’s classification was revised and elaborated by Baldwin, Kellogg, and Thorp (1938). The soil
classification of Baldwin et al. (1938) was notified by Thorp and Smith (1949), as they grouped the
soil into three orders as under:
*Pedalfers are the soils usually found in temperate areas receiving >60 cm of rain each year. Pedalfers
are very fertile, containing an abundance of Al and Fe and are a brown-black colour. Pedocals are the
soils usually found in dry, warm climate receiving <60 cm of rain per year, with an accumulation of
calcium carbonate.
**Podzol is a term given to the soils that are infertile and have a light-brown colour due to poor
humus.
Chernozem is a term given to the soils that are rich in humus and thus are fertile. In addition,
chernozem have a black colour because of the rich humus.
Dokuchaiev (1900) divided soils into three categories; Normal, Transitional and Abnormal.
These categories were later termed as Zonal, Intrazonal and Azonal soils, respectively.
i. Zonal soils (Normal): Well developed soil profiles reflecting the influence of climate and
vegetation.
ii. Intrazonal soils: Soil profile development in progress, Transitional stages in between Azonal and
Zonal soils.
iii. Azonal soils (Abnormal): poorly developed profiles because of time as limiting factor.
30
Serious problems arose when some soil series did not fit in any of the existing Great Soil
Groups. Therefore, as a stop-gap strategy, the system was revised in which three new Great Soil
Groups were introduced and three others were merged with the existing Great Soil Groups. The soils
were grouped in three Orders, viz. Zonal, Intrazonal and Azonal, following the Russians zonality
concept, as under:
Zonal Soils — The soils whose characteristics are determined primarily by the environment,
especially climate and vegetation (Figure 1).
Intrazonal Soils — These soils occur within a zone, but reflect the influence of some local conditions,
such as topography and/or parent material.
Azonal Soils — The soils that have poorly developed profiles because of time as a limiting factor, e.g.
young soils without horizon differentiation.
The three orders were further subdivided into nine suborders on the basis of specific climatic
and vegetative regions. Each suborder, in turn, was divided into Great Soil Groups, which are an
expression of more specific conditions. The Great Soil Groups were further subdivided into numerous
Soil Families, Series and Soil Types.
Salient Features
The Comprehensive System is a morpho-genetic system in which morphology of soil, that is an
outcome of soil genesis, serves as a guide (Smith, 1963). It is based on the properties of soils as they
exist today. Although one of its objectives is to group soils similar in genesis, the specific criteria used
to place soils in different groups are those of soil properties. The system has an edge over the earlier
systems in the following respects:
1. Unlike the Genetic Systems, the Comprehensive System is based on measurable soil properties
that exist today.
2. It considers all such properties which affect soil genesis or are the outcome of soil genesis.
3. The common definition of a class of taxonomic system is type or orthotype.
4. The nomenclature, using coined words, is derived mainly from Greek and Latin languages.
Although it appears difficult, once understood, it is the most logical nomenclature and helps in
relating the place of taxon in the system and in making interpretations.
5. A new category, viz. Subgroup, has been introduced to define the central concepts of Great
Groups and their intergrades in order to express and recognize more clearly that soils are in
continuum and show gradual change in many properties.
Unlike the Genetic System, it is an orderly scheme without prejudices, but facilitates easy recognition
of the objects.
Table 7. Major features of epipedons in mineral soil used for differentiation at the higher levels of
Soil Taxonomy
Diagnostic
horizons
Major features
(Typical genetic
horizon)
Surface horizons (Epipedons)
Human-modified mollic-like horizon, high in available P
Anthropic
Organic horizon saturated for less than 30 days per normal year
Folistic
Very high in organic content, wet during some part of year
Histic
Thick, black, high in organic matter (>6% Organic C), common in volcanic ash soil
Melanic
Thick, dark-coloured, high base saturation, strong structure
Mollic
Too light-coloured, low organic content or thin to be Mollic; may be hard and massive
Ochric
Human-made sod-like horizon created by years of manuring
Plaggen
Same as Mollic except low base saturation
Umbric
Sandy (loamy fine sand or coarser) horizon, 100 cm or more thick over an argillic
Grossarenic
horizon
33
Table 8. Major features of endopedons in mineral soil used for differentiation at the higher levels of
Soil Taxonomy
Diagnostic
horizons (Typical Major features
genetic horizon)
34
CATEGORIES (SOIL TAXONOMY)
Mollisols Order
Aquolls Suborder
Argiaquolls Great group
Typic Argiaquolls Subgroup
Family names in general identify subsets of the subgroup that are similar in texture, mineral
composition, and mean soil temperature at a depth of 50 cm. Thus the name fine, mixed, mesic, active
Typic Argiaquolls identifies a family in the Typic Argiaquolls subgroup with a fine texture, mixed
clay mineral content, mesic (8 – 15 C) soil temperature, and clays active in cation exchange.
Soil series are named after a geographic feature (town, river, etc.) near where they were first
recognized. Thus names such Fort Collins, Cecil, Miami, Norfolk, and Ontario identify soil series first
described near the town or geographic feature named. Approximately 23,000 soil series have been
classified in the US alone.
36
Differentiating characteristics and description
There are 12 Orders "A VAGAMI HOUSE*" phrase suggested to facilitate
naming of all the Orders. These are based largely on morphology, as produced
Order (12)
by soil-forming processes, and indicated by the presence or absence of major
diagnostic horizons.
There are 63 Suborders within 12 Orders. These emphasize genetic
homogeneity, wetness, climatic environment, parent material and vegetational
Suborder (63)
effects. The differentiate used vary, but most tend to emphasize wetness and
moisture regime.
Great Group There are 240 Great Groups within 63 Suborders of 12 Orders. The major
(240+ emphasis is on the diagnostic horizons (except in Entisols which have no such
approximately) horizon) and presence or absence of diagnostic layers, base status, soil
temperature and moisture regimes. There are more than 1,000 Subgroups. The
Subgroup Typic is used to define the central concept of a great group; the others are used
(1000+) to indicate intergraded to great groups, suborders and orders, and the extra-
graded to 'not soil'.
The soil properties that are most important for plant growth (like texture,
mineralogical class (dominant of solum), soil temperature class (based on MAST
Family
at 50 cm depth) and pH are used to differentiate families. They meet the need
for making practical prediction for land-use planning.
It is the lowest category in the system. The series is a collection of soil
Series individuals, essentially uniform in differentiating characteristics (like colour,
(Approximately texture, structure, consistence, pH and EC) and in arrangement of horizons. It is
200+ in India; the series which is most useful for making land-use plans of a small area. The
and 12,000 in series are named after the geographic name of the place where it was first
the USA) recognized or where they have wide extent of distribution, e.g. Jodhan loam,
Tigris silt loam. The textural name, along with the series name, suggest the
surface phase.
37
genetic processes, but it is because of the properties they have in common that they are included in the
same order: Mollisols.
Figure 11. Degree of weathering and soil development in different soil orders.
(Source: N. C. Brady, The Nature and Properties of Soil)
38
Table 3: Soil orders, its formative elements and their major characteristics
Suborders
Soils within each order are grouped into suborders (as mentioned above in each order detail)
on the basis of soil properties that reflect major environmental control on current soil-forming
processes. Many suborders are indicative of the moisture regime or less frequently the temperature
regime under which the soils are found. Thus, soils formed under wet conditions generally are
identified under separate suborders (e.g., Aquents, Aquerts, and Aquepts), as being wet soils. To
determine the relationship between suborder names and soil characteristics, refer to Table 11. Thus,
39
the Ustolls are dry Mollisols. Likewise, soils in the Udults suborder (from Latin udus, humid) are
moist Ultisols.
Great groups
The great groups are subdivisions of suborders. More than 400 great groups are recognized. They are
defined largely by the presence or absence of diagnostic horizons and the arrangement of those
horizons.
Subgroups
Subgroups are subdivisions of the great groups. More than 2500 subgroups are recognized. The central
concept of a great group makes one subgroup, termed Typic. Thus, the Typic Hapludolls subgroup
typifies the Hapludolls great group.
Families
Within, a subgroup, soils fall into a particular family, if at a specified depth, they have similar physical
and chemical properties affecting the growth of plant roots. About 8000 families have been identified.
The criteria used include broad classes of particles size, mineralogy, cation exchange activity of the
clay, temperature, and depth of the soil penetrable by roots. Table 12 gives examples of the classes
used.
Series
The series category is the most specific unit of the classification system. It is a subdivision of the
family, and each series is defined by a specific range of soil properties involving primarily the kind,
thickness, and arrangement of horizons.
40
6. Series: Name of the series is given on the basis where it described. Some examples of the soil series
identified and taxonomically grouped is shown below.
Soil Series Sub-group Great group Sub-order Order
Bodali Vertic Ustorthents Ustorthents Orthents Entisols
Eru Typic Chromusterts Chromusterts Usterts Vertisols
Ilav Typic Ustorthents Ustochrepts Orthents Inseptisol
Dandi Typic Halaquepts Halaquepts Aquepts Inseptisol
41
Soils of India
Soils of India have been divided into the following major groups (Figure 6.2):
(1) Alluvial soils
(2) Black soils
(3) Red soils
(4) Laterites and Lateritic Soils
(5) Desert Soils
(6) Tarai Soils
(7) Saline and Sodic Soils
(8) Acid Soils
It contributed the largest share to India’s agricultural wealth. Broadly this soil is divided into two
types: (i) Newer alluvium: Sandy, generally light coloured and less kankary, and (ii) Older
alluvium: More clayey in composition, generally dark and full of Kankar.
Formation of hard pans (impervious layer) is often observed in Indo-gangetic alluvial soils of Uttar
Pradesh and West Bengal. In Assam, old alluvium at hills is more acidic than the new alluvial soils
along the riverbanks, which are often neutral or alkaline. In general alluvial soils are low in N except
in Brahmaputra valley where they are moderate. Alluvial soils are found in Indo-gangetic plains of
Uttar Pradesh, West Bengal, Bihar and Brahmaputra valley of Assam. Alluvial soils are fertile and
suitable for most of the agricultural crops like lowland rice, pulses, cotton, banana etc.
42
• High swelling and shrinkage, plasticity and stickiness
• Impeded drainage and low permeability
• High content of exchangeable calcium and management
• Poor in organic matter, N and available P2O5.
• Suitable crops - Cotton, Sugarcane, Groundnut, Millets, Maize, Pulses, Safflower
43
Tarai Soils (Mollisols)
Tarai soils are derived from the materials washed down by the erosion of mountains (alluvial origin).
• Hard clay, coarse sand and gravel (parent material)
• Relatively high moisture content for the greater part of the year results in luxuriant vegetation
• Organic matter content is high
• Sandy loam to silty loam in texture
• Suitable cops – Tall grasses
High salt tolerant: Rice, Sugarcane, Oats, Berseem, Lucerne, Indian clover, Barley
Medium salt tolerant: Castor, Cotton, Sorghum, Cumbu, Maize, Mustard, Wheat
Low salt tolerant: Pulses, Peas, Sunnhemp, Gram, Linseed, Sesamum
Sodic/Alkali Soils
These soils contain high content of CO3 and HCO3 of Na. Hence, they are with high exchangeable
sodium percentage (ESP). Generally, they are non-saline and with dark encrustation hence called as
“black alkali”. These soils are rich in NaHCO3 and characterized by pH > 8.5; EC < 4 dS m−1; ESP
> 15. Use gypsum (CaSO4·2H2O) as amendment for reclamation of sodic/alkali soils. Iron pyrites,
(FeS2) bulky organic manures (especially, green manures) and crop residues which produce weak
organic acids are also used for reclamation. Crops having tolerance are grown in the soils.
Acid Soils
• Low pH with high amounts of exchangeable H+ and A13+.
• Occur in regions with high rainfall.
44
• Laterization, Podzolization in areas with
sub temperate to temperate climate.
• Significant amount of partly
decomposed organic matter.
• Kaolinitic and Illitic.
• Low CEC and high base saturation.
• Liming and judicious use of fertilizers
are the management measures suggested.
• Suitable crops – Acedophytes (like
potato).
Figure 6.2 Soil map of India
SOILS OF GUJARAT
Shallow: These residual soils have developed from different parent materials at different places in
Saurashtra and South regions. The depth varies from few centimeters to 30 cm. These soils lack
distinguished profile layering and are classified as Ustorthents.
Medium: These residual soils with medium depth of 30-60 cm are derived from basaltic trap parent
material in Saurashtra except in coastal belt, from granite and gneiss in some parts of Sabarkantha,
Panchmahals and Ahmedabad districts and a narrow strip form Chhotaudepur to Vansada and
Dharampur in South Gujarat. They are calcareous in nature in Saurashtra. Free lime content varies
between 15 to 45 per cent in Junaghadh district. The soil profiles have AC horizon with plastic, sticky
and hard consistency. Taxonomically these are classified as Ustochrepts.
Deep: These soils' are also known as ' regurs' or black cotton soils and have their origin in trap. The
depth varies from less than a meter to as much as 6 m. These soils occur mainly in Bharuch, Surat and
Valsad districts of South Gujarat. They contain 40 to 70 per cent clay dominated by smectite group of
clay minerals. They are natural to alkaline in reaction. These are grouped under Chromusterts.
(ii) Mixed red and black soils: These soils are localized in small areas in Junagadh district. These are
residual in nature and have developed from ferruginous limestone. The bedrock is porous. The soils
are shallow in depth, reddish or grayish brown in colour with A-C profile characteristics. They are
classified as Ustorthents.
(iii) Residual sandy soils: These soils occur in some parts of Kutch, Surendranagar and Rajkot
districts. They have been developed in-situ from the parent material originated from red sand stone
45
and shale. The soils are shallow in depth, reddish brown in colour with A-C profile characteristics.
These are classified as Ustorthents and Ustipsamments of Entisols order.
(iv) Alluvial soils: Alluvial soils are deep with undifferentiated horizon indicating their recent origin.
Illite is the main clay mineral. These soils are coarse in texture, nonca1careous and neutral to mildly
alkaline in reaction. The soils have good drainage but underground water is brackish. These are
classified as Ustifluvents, Haplaquents and Ustorthents of Entisols order and Ustochrepts of
Inceptisols order.
Coastal alluvial soils: These soils are situated along the sea coast extending in length to more than
1600 km. They are fairly deep with sandy clay loam texture. These are classified as Halaquents,
Fluvaquents of Entisols order and Halaquepts and Haplaquepts of Inceptisols order.
Saline / alkali soils: Generally, saline soils are encountered along the coastal area. Water logging for
a long time has created saline / alkaline conditions in the Bhal. The soils of the Bhal are clayey with a
high percentage of lime. These soils are classified as Salorthids, Ca1ciorthids and Natrargids of
Aridisols; Fluvaquerits, Ustipsamments, Halaquents and Haplaquents of Entisols and Halaquepts of
Inceptisols and Chromusterts of Vertisols orders. The irrigation projects have raised the water table
and have contributed to the problem of salinity in the areas of Kheda, Ahmedabad, Surat and Valsad
districts. The salt content varies form 1.5 to 5.7 per cent.
(vi) Desert soils: These soils are found in the Rann of Kutch. The texture of the soils varies from
sandy loam to clay loam. The salt content is very high.
(vii) Lateritic soils: There are no true laterites in Gujarat. In the Dangs, clayey soils developed from
trap have been washed down in the valleys. These have been produced by slow weathering of basalt
giving argillaceous material. The topography of the area is hilly and undulating.
(viii) Hilly soils: They are shallow in depth and composed of rock fragments. These soils are found in
the Dang and Panchmahals district and some part of Saurashtra region.
(ix) Forest soils: Forest soils of the Dangs and Junagadh districts are rich in organic matter. These
soils are clayey. Junagadh soils have higher percentage of lime and sand fraction but are low in silt
fraction as compared to the soils of the Dangs.
*****
46
CHAPTER 6: COMPONENTS OF SOIL
Components of soils (on volume basis)
Soil consists of four major components. They are: (i) Mineral matter, (ii) Organic matter, (iii)
Soil water, and (iv) Soil air (Figure 1.1). Soil contains about 50% solid space and 50% pore space.
Soil Mineral matter and organic matter occupy the total solid space of the soil by about 45% and 5%
respectively. The total pore space of the soil is occupied and shared by air and water on roughly equal
basis. The proportion of air and water will vary depending upon the weather and environmental factors.
2. Organic matter
Soil organic matter represents partially decayed and partially synthesized plant and animal
residues. Such material is continually being broken down by the action of soil microorganism.
Consequently, organic matter is a transitory soil constituent and renewed constantly by the addition of
plant residues.
Organic matter influences soil properties and consequently on plant growth. Organic matter
functions as a granulator mineral particles. It is responsible for the loose, easily managed condition of
productive soils. Organic matter improves the physical condition of soils, it also increases water-
holding capacity. It is a major source of plant nutrients like, N, P, K, S, Fe, Mn, Zn, Cu etc. Finally,
organic matter is the main source of energy for soil microorganisms.
3. Soil water
Soil water is the major component of the soil in relation to the plant growth. The water is held
within the soil pores. If the moisture content of a soil is optimum for plant growth, plants can readily
absorb the soil water. Not all the water, soils can hold is available to plants. Much of water remains in
the soil as thin film.
Soil water dissolves salts and makes up the soil solution, which is important as a medium for
supplying nutrients to growing plants. There is an exchange of nutrients between the soil solids and
the soil solution and then between the soil solution and plants.
4. Soil air
A part of the soil volume that is not occupied by soil particles, known as pore space, is filled
partly with soil water and partly with soil air. As the pore space is occupied by both water and air,
volume of air varies inversely with that of water. As the moisture content of the soil increases, the air
content decreases and vice-versa.
The soil air contains a number of gases viz., nitrogen, oxygen, carbon dioxide and water vapour.
Soil air differs from the atmosphere in several respects. First, soil air contains much greater proportion
of carbon dioxide and a lesser amount of oxygen than atmospheric air. Second, soil air has higher
moisture content than the atmosphere.
The content and composition of soil air is determined to a large degree by the soil-water
relationships. Following a rain, large pores are the first vacated by the soil water, followed by medium-
sized pores as water is removed by evaporation and plant utilization. Thus, the soil air ordinarily
occupies the large pores. As soil further dries up, medium size pore spaces occupy the soil air. Soil air
has marked effect on the plant and their root growth. It has also effect on soil microorganism, plant
nutrients formation and their availability.
*****
48
CHAPTER 7: PHYSICAL PROPERTIES OF SOIL
The relation between the diameter of a particle and its settling velocity is given below:
50
2 g (ds – dw) r2
V = ----- x ------------------
9 ŋ
Where, V = velocity of settling particle (cm/sec)
g = acceleration due to gravity (981 cm/sec2)
ds = density of soil particle (2.65 g/cc)
ŋ = coefficient of viscosity (0.0015 at 40C)
dw = density of water (1.0g/cc);
r = radius of spherical particle (cm)
To conduct a mechanical analysis, a sample of soil is crushed lightly in a wooden mortar. The
material is next passed through the sieve is taken for mechanical analysis. The organic matter and
other binding materials are removed from the soil before the mechanical separation.
There are several methods of mechanical analysis viz., sieve method, sedimentation method,
decantation method, centrifugal method, pipette method and hydrometer method. Pipette method is
universally employed for carrying out mechanical analysis of soil.
Textural classes
Textural names are given to soils based upon the relative proportion of each of the three soil
separates – sand, silt, and clay. Soil that are predominately clay, are called clay (textural class), those
with high silt content are silt (textural class), and those with high sand percentage are sand (textural
class). Three broad and functional groups of soil textural classes are recognized: (i) Sands → soils of
which the sand separates make up 70% or more of the materials by weight (ii) Loams → an ideal loam
soil may be defined as a mixture of sand, silt and clay particles which exhibits light and heavy
properties in about equal proportions. Note the loam does not contain equal percentages of sand, silt
and clay. It does, however, exhibit approximately equal properties of sand, silt and clay (iii) Clays →
a clay soil must carry at least 35% of clay separate and it most cases not less than 40%.
Based on the proportion of sand, silt and clay particles, classification was made and
standardized into twelve classes as shown in a triangular diagram. This triangle is known as USDA
(United States Department of Agriculture) soil textural classification triangle. The twelve classes are
as follows: (1) Sand (2) Silt (3) Clay (4) Loam Sandy (5) Clay silty (6) Clay (7) Clay-loam (8) Loamy
sand (9) Sandy loam (10) Silty loam (11) Sandy clay loam, and (12) Silty clay loam.
51
Figure 1.3
USDA soil textural triangle
USDA stands for United States Department of
Agriculture
For example, in a soil sample if the silt percentage is
20, sand percentage is 50 and clay percentage is 30,
then these proportions are intersecting at sandy clay
loam
(2) Soil structure
The arrangement of soil particles and their
aggregates into certain defined patterns is called soil
structure. Most soils are having a mixture of single grain structure or aggregate structure. The primary
soil particle sand, silt and clay usually occur grouped together in the form of aggregates. These groups
of soil aggregates are combined together by the binding effect of different available cementing agents
like organic matter and inorganic calcium carbonates, soil colloids etc. The formation of soil structure
is promoted by chemical (flocculation) processes, physical processes (e.g., freezing, drying), and
especially, biological processes (e.g., binding of particles by microbial glues and fungal hyphae, as
well as by fine plant roots).
Soil structure can be changed or improved by operations like ploughing, puddling, addition of
organic matter, etc. The management practices like proper land use, suitable tillage practice at
optimum moisture level, addition of organic matter, crop rotation, optimum fertilization, mulching,
drainage, controlled irrigation, soil conservation, protection against compaction and use of soil
conditioner may be tried for better soil structure management. The natural aggregates are called peds
whereas clod is an artificially formed soil mass. Soil structure is studied in the field under natural
conditions.
Types of soil structure
There are four principal forms of soil structure (see Figure 1.5).
• Plate-like (Platy): the aggregates are arranged in relatively thin horizontal plates or leaflets.
The horizontal axis or dimensions are larger than the vertical axis. When the units/ layers are
thick, they are called platy. When they are thin, called laminar. Platy structure is most
noticeable in the surface layers of virgin soils, and it may be present in the sub-soil. This type
of structure is inherited from the parent material, especially by the action of water/ice.
• Prism-like: The vertical axis is more developed than horizontal, giving a pillar like shape.
Vary in length from 1-10 cm; commonly occur in sub-soil horizons of arid and semi-arid
regions. When the tops of pillar are rounded, the structure is termed as columnar, and when
the pillar tops are flat plane, level and clear cut called prismatic.
• Block like: In this structure type, all three dimensions are about the same size. The
aggregates have been reduced to blocks. Irregularly six faced with their three dimensions
more or less equal. When the faces are flat and distinct and the edges are sharp angular, the
structure is named as angular blocky. When the faces and edges are mainly rounded it is
52
called sub-angular blocky. These types usually are confined to the sub-soil and
characteristics have much to do with soil drainage, aeration and root penetration.
• Spheroidal (Sphere like): All rounded aggregates (peds) may be placed in this category. Not
exceeding an inch in diameter. These rounded complexes usually loosely arranged and
readily separated. When wetted, the intervening spaces generally are not closed (by swelling)
as in the case with a blocky structural condition. Therefore, in sphere like structure,
infiltration, percolation and aeration are not affected by wetting of soil. The aggregates of
this group are usually termed as granular which are relatively less porous. When the
granules are very porous, it is termed as crumb. This is specific to surface soil particularly
high in organic matter/grass land soils.
53
Organic matter plays an important part in forming soil aggregates. The humic acid and other sticky
product produced during the decomposition of organic matter help in aggregate formation.
54
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑠 𝑀𝑠𝑜𝑙𝑖𝑑𝑠
𝑃𝑎𝑟𝑡𝑖𝑐𝑎𝑙 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑟 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑠 = =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑠 𝑉𝑠𝑜𝑙𝑖𝑑𝑠 × 𝑤𝑎𝑡𝑒𝑟
Where, w = density of water at 4 C. Since density of water = 1, this can be written as
𝑀𝑠𝑜𝑙𝑖𝑑𝑠
𝑃𝑎𝑟𝑡𝑖𝑐𝑎𝑙 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑟 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑆𝑜𝑙𝑖𝑑𝑠 =
𝑉𝑠𝑜𝑙𝑖𝑑𝑠
It is the mass per unit volume of soil solids. The chemical composition and crystal structure of a
mineral matter determines soils’ particle density. Particle density is not affected by pore space and
therefore is not related to particle size or to the arrangement of particles (Soil structure). Particle
density for most mineral soils varies between the narrow limits of 2.60 to 2.75 Mg m−3.
The particle density of soils with very high organic matter content may vary from 0.9 to 1.3 Mg m −3.
Particle density of soils is almost a permanent character which is not influenced by addition of
organic matter, tillage or depth.
B. Bulk Density
“It is the mass per unit volume of dry soil (volume of solid and pore spaces)”.
The bulk density of a soil is always smaller than its particle density. Loose and porous soils have low
bulk densities as compared to compacted soils. Bulk density is more important than particle density in
understanding physical behaviour of soils. Generally in normal soils, bulk density ranges from 1.0 to
1.60 Mg m−3. Finer the texture of the soil, lesser is the bulk density.
Numerical: If the bulk density of the cultivated field soil is 1.48 Mg/m3, what will be the weight of
hectare field (soil) up to the depth of 15 cm.
Volume of the soil (Field)
of hectare filed (soil) to the = Length x Width x Depth
depth of 15 cm. = 100 x 100 x 0 .15
55
= 1500 cubic meter
As the bulk density of soil is 1.48 Mg/m3, which means that weight of 1 cubic meter = 1.48 Mg
So, for 1500 cubic meter (1 hectare field soil up to 15 cm depth) will be
= 1500 x 1.48 (Volume of 15 cm Hectare furrow x Bulk density)
= 2220 Mg
1 Mg = 106 g = 103 kg = 1 tonne
So the weight of hectare field (soil) up to the depth of 15 cm will be 2220 Mg/ ha or 2220 t/ha up to
15 cm depth
Macropores are large sized pores (>0.06mm) invariably exist in between sand sized granules and
allow air and water movement readily. Micro or capillary pores are smaller sized pores (<0.06mm) in
which movement of air and water are restricted to some extent. These pores are very important for
crop growth. Generally clays and clayey soils have a greater number of capillary pores.
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The existence of approximately equal number of macro and micro pores would facilitate better
aeration, permeability, drainage, and water retention.
Calculation of porosity
𝑀𝑠𝑜𝑙𝑖𝑑𝑠
𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 (𝑫𝒑 ) = ∴ 𝑀𝑠𝑜𝑙𝑖𝑑𝑠 = 𝐷𝑝 × 𝑉𝑠𝑜𝑙𝑖𝑑𝑠
𝑉𝑠𝑜𝑙𝑖𝑑𝑠
𝑀𝑠𝑜𝑙𝑖𝑑𝑠
𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 (𝑫𝒃 ) = ∴ 𝑀𝑠𝑜𝑙𝑖𝑑𝑠 = 𝐷𝑏 × (𝑉𝑠𝑜𝑙𝑖𝑑𝑠 + 𝑉𝑝𝑜𝑟𝑒𝑠 )
𝑉𝑠𝑜𝑙𝑖𝑑𝑠 + 𝑉𝑝𝑜𝑟𝑒
𝐷𝑏 𝑉𝑠𝑜𝑙𝑖𝑑𝑠
∴ × 100 = × 100
𝐷𝑝 (𝑉𝑠𝑜𝑙𝑖𝑑𝑠 + 𝑉𝑝𝑜𝑟𝑒𝑠 )
𝑉𝑠𝑜𝑙𝑖𝑑𝑠 𝐷𝑏
× 100 𝑖𝑠 𝑛𝑜𝑡ℎ𝑖𝑛𝑔 𝑏𝑢𝑡 % 𝑆𝑜𝑙𝑖𝑑 𝑠𝑝𝑎𝑐𝑒 ∴ 𝑆𝑜𝑙𝑖𝑑 𝑠𝑝𝑎𝑐𝑒 = × 100
(𝑉𝑠𝑜𝑙𝑖𝑑𝑠 + 𝑉𝑝𝑜𝑟𝑒𝑠 ) 𝐷𝑝
𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦
∴ % 𝑃𝑜𝑟𝑒 𝑠𝑎𝑝𝑐𝑒 = 100 − × 100
𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦
𝐵𝑢𝑙𝑘 𝐷𝑒𝑠𝑛𝑡𝑖𝑦
∴ % 𝑃𝑜𝑟𝑒 𝑠𝑎𝑝𝑐𝑒 = 100 × [1 − ]
𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦
Exercise:
(1) Calculate the % pore space of a soil with a bulk density of 1.55 & particle density of 2.65 Mg/m3.
(2) Calculate the % pore space of a soil with a bulk density of 1.45 & particle density of 2.65 Mg/m3.
(3) Calculate the % pore space of a soil with a bulk density of 1.33 & particle density of 2.65 Mg/m3.
(4) Calculate the % pore space of a soil with a bulk density of 1.22 & particle density of 2.65 Mg/m3.
Note: After calculating the respective % pore space/ total porosity of above mentioned soils, we can
observe that with the decrease in bulk density of the soils with similar particle density, % pore space/
total porosity increases.
Soil compaction
Compaction is the dynamic behavior of soil. The degree of compaction depends upon the
nature of soil, amount of energy applied, water content and extent of manipulation of the soil.
Compaction is also associated with the rearrangement of the soil solid particles so that soil water and
soil air are compressed within the pore space. An increase in water content decreases cohesion between
the particles and thereby facilitating compaction. In many soils, a compacted layer is commonly found
at the bottom of the zone of ploughing. This layer is termed as ‘ploughed sole’. Age-old practices of
tillage using heavy farm equipments create the problem of surface and sub-surface compaction in soils.
These hard compact layers often restrict root penetration and growth and also reduce the water and
nutrient uptake by crops. Soil compaction changes soil moisture and thermal regimes and mechanical
resistance in soils. However, in highly coarse textured soils, loamy sand soils, compaction may be a
boon to water retention and reduce percolation loss of water.
Atterberg, a Swedish scientist, considered the consistency of soils in 1911, and proposed a
series of tests for defining the properties of cohesive soils. Strength decreases as water content
increases. At very low moisture content, soil behaves more like a solid. When the moisture contents
is very high, the solid and water may flow like a liquid.
Atterberg limits are the limits of water content used to define soil behaviour.
• Liquid Limit (LL) is defined as the moisture content at which soil begins to behave as a liquid
material and begins to flow.
• Plastic Limit (PL) is defined as the moisture content at which soil begins to behave as a plastic
material.
• Shrinkage Limit (SL) is defined as the moisture content at which no further volume change
occurs with further reduction in moisture content.
The plasticity index (PI) is the difference between the liquid limit and the plastic limit of a
soil. Plasticity index indicates the degree of plasticity of a soil. The greater the difference between
liquid and plastic limits, the greater is the plasticity of the soil. A cohesionless soil has zero plasticity
index. Such soils are termed as non-plastic. Clayey soils are highly plastic and possess a high plasticity
index.
A soil is in the liquid state when it acts as a viscous liquid when jarred. A soil is in the plastic
when it can be moulded in different shapes. A soil is friable when moist soil crumbles into aggregates
when crushed with only light pressure. A soil is loose when it has no consistency at all, typically of
dry, structure less sand. A soil is hard when it can only be crushed with difficulty between thumb and
forefinger, typically dry, structure less clay soils. The liquid limit is the limit between plastic and
liquid state. The plastic limit is the limit between the friable and plastic state.
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The colour of the soil is a result of the light reflected from the soil. Soil colour rotation is
divided into three parts:
Hue: It denotes the dominant spectral colour (red, yellow, blue and green)
Value: It denotes the lightness or darkness of a colour (the amount of reflected light)
Chroma: It represents the purity of the colour (strength of the colour)
The Munsell colour notations are systematic and letter designations of each of these three
variables (Hue, Value and Chroma). For example, the numerical notation 2.5YR5/6 suggests a hue of
2.5 YR, value of 5 and chroma of 6.The equivalent or parallel soil colour name for this Munsell
notation is red.
The colour signifies soil conditions and some important properties. A soil attains certain colour
depending on physic-chemical reactions. When soil is examined, colour is one of the first things which
we are noticing. The colour is determined by how much organic matter it contains, its drainage
conditions, and how much it has been changed by climate (extent of weathering). A rough indication
of soil colour is given in Table 1.3.
These different shades of colour are due to various reasons. One of the factors responsible is
the parent material e.g., red sand stone gives rise to red soil. The second factor is the soil organic
matter in surface soil which imparts dark brown to black colour. Third, the presence of certain minerals
in soils like titanium compounds impart darker colour. Iron compounds like haematite, give red and
limolite give yellow colour. Accumulation of salts makes soils white or black, depending on the type
of salts. If the soil is well drained like the lateritic soil, the ferric compounds of iron are commonly
formed and they give red colour to the soil, but if drainage is poor, the soil colour greenish or bluish
in waterlogged soils. Sometimes, in the subsoil layers, there is a colour variegation or mottling of soil
indicating alternate oxidising and reducing conditions at that particular depth mainly due to
fluctuations in water table.
Soil colour, besides being useful for classifying soils, is indirectly helpful in indicating many
other properties of soils. For example, dark brown soils indicate high organic matter and fertility, red
colour shows good aeration, white colour accumulation of salts etc.
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Light colours are a sign that the soil is low in organic matter. Light or pale colours in the
surface soil are often associated with coarse texture, highly leached conditions, and high annual
temperatures. Dark colours might indicate poor drainage, low annual temperature, and high organic
matter. Subsoil colours, in general, are indications of air, water and soil relationships and the extent
of mineral decay during breakdown of the soil’s parent materials.
*****
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CHAPTER 8: SOIL WATER
Water contained in soil is called soil moisture. The water is held within the soil pores. Soil
water is the major component of the soil in relation to the plant growth. If the moisture content of a
soil is optimum for plant growth, plants can readily absorb the soil water. Not all the water, soils can
hold is available to plants. Much of water remains in the soil as thin film. Soil water dissolves salts
and makes up the soil solution, which is important as medium for supplying nutrients to growing
plants.
Hydrogen Molecule
Hydrogen Molecule
Hydrogen Molecule
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(2) Hydrogen bonding
The phenomenon by which hydrogen atoms act as links between water molecules is called
hydrogen bonding. This type of binding accounts for polymerization and for the relating high boiling
point, specific heat and viscosity of water. It is also responsible for the structural rigidity of crystals
and some organic compounds like proteins. Hydrogen bonding also accounts for two basic forces viz.,
cohesion and adhesion, responsible for water retention and movement in soils and its utilization.
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SOIL WATER RETENTION AND POTENTIALS
The soils hold water (moisture) due to their colloidal properties and aggregation qualities. The
water is held on the surface of the colloidal and other particles and in the pores. The force responsible
for retention of water in the soil after the drainage has stopped are due to surface tension and surface
attraction and are called surface moisture tension. This refers to the energy concept in moisture
retention relationships. The strength with which water is held in the soil is called water potential. The
force with which water is held is also termed as suction.
Gravitational potential
The force of gravity acts on soil water the same as it does on any other body, the attraction
being toward the earth's center. The gravitational potential (Ψg) of soil water may be expressed
mathematically as Ψg = gh, where g is the acceleration of gravity and h is the height of the soil water
above a reference elevation. The gravitational potential of soil water above the reference point will
always be positive. It is independent of chemical and pressure conditions of soil water and dependent
on relative elevation. Gravity plays an important role in removing excess water from the upper rooting
zones following heavy precipitation or irrigation.
Matric potential
Matric potential is the result of two phenomena, adsorption and capillarity. The attraction of
soil solids and their exchangeable ions for water provide matric force that markely reduce the free
energy of soil water as compared to that of unabsorbed pure water. Consequently, matric potentials
are always negative. The matric potential exerts its effect not only on soil moisture retention but on
soil water movement as well.
Osmotic potential
The osmotic potential is attributable to the presence of solutes in the soil i.e. soil solution. The
solutes reduce the free energy of water, primarily because the solute ions or molecules attract the water
molecules. Unlike the matric potential, the osmotic potential has little effect on the mass movement
of water in soils. Its major effect is on the uptake of water by plant roots. The root membrane, which
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transmits water more freely than solutes, permits the osmotic effects to be exerted, a matter of
considerable importance if the solute content of soils is high. The osmotic potential also affects the
movement of water vapor since water vapor pressure is lowered by the presence of solutes. It is
negative.
2. Capillary water
Capillary water is held in the capillary pores (micro pores). Capillary pores (micro pores).
Capillary water is retained on the soil particles by surface forces (fig.12). It is held so strongly that
gravity cannot separate it from the soil particles. The molecules of capillary water are free and mobile
and are present in a liquid state. Due to this reason it evaporates easily at ordinary temperature.
Though, it is held firmly by the soil particle, plant roots are able to absorb it. Capillary water is,
therefore, known as available water.
Fig. 12
3. Hygroscopic water
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The water held tightly on the surface of the colloidal particles is known as hygroscopic water.
It is essentially non-liquid and moves primarily in the vapour form. Hygroscopic water held so
tenaciously by soil particles that plants cannot absorb it. Some microorganism may utilize hygroscopic
water. As hygroscopic water is held tenaciously by surface forces its removal from the soil requires a
certain amount of energy. Unlike capillary water which evaporates easily at atmospheric temperature
(fig.13), hygroscopic water cannot be separated from the soil unless it is heated.
Fig. 13
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used through wells for irrigation. Second, percolating waters carry plant nutrients down and often out
of reach of plant roots (leaching). Percolation is dependent on rainfall. In dry region, percolation is
negligible but in region of high rainfall, percolation is usually high. Sandy soils permit greater
percolation in comparison to clayey soils. Vegetation and high water table reduce the percolation
losses. Percolation losses of water are not harmful but the nutrients that are leached along with the
percolation water are of serious consideration. Losses of calcium and magnesium are the greatest but
phosphorus is lost by leaching in trace (very less).
(3) Permeability
Permeability indicates the relative ease of movement of water within the soil. The
characteristics that determine how fast air and water move through the soil are known as permeability.
The term hydraulic conductivity is also used which refers to the readiness with which a soil transmits
fluids through it. The permeability is basically dependent on the pore-size distribution in the soil.
The macro pores increases the permeability. The pore size distribution is greatly determined by the
extent of aggregation in the soil. The larger the aggregate, the greater is the amount of non-capillary
(macro) pores. The permeability of soil usually decreases with depth, as the sub-soil layers are more
compact reduces the macro pores. More content of organic matter and vegetation increase
permeability. Normally permeability decreases with increasing fine texture, but the extent of
aggregation in fine-textured (clay) soils may override the effects of texture. If irrigation water is high
in sodium content, it would cause dispersion of soil (soil becomes compact) and thus reduce
permeability.
Drainage
The purpose of drainage is to remove the excess water from the soil ensure proper soil aeration.
When soil gets saturated with water, the entire pore space is occupied by water and soil aeration is cut
off. Also, water ponded at soil surface cuts off gaseous exchange between atmospheric and soil air
even if there are enough air-filled pores in the soil below. Draining out the excess water to create air
porosity creates soil condition favorable for soil aeration.
Drainage is of two type’s viz., (i) surface drainage: To ensure adequate soil aeration, the
ponded water must be removed through gravity flow or pumping out and (ii) sub-surface drainage:
In order to ensure adequate soil aeration and prevent soil salinization, water table must be kept below
a certain level.
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(ii) Unsaturated flow of water: Soil pores contain some air as well as water is called unsaturated soil.
Under field conditions most soil-water movement occurs where the soil pores are not completely
saturated with water. The soil macro pores are mostly filled with air, and the micro pores with water
and some air. Water movement under these conditions is very slow compared to that occurring when
the soil is saturated. Movement will be from a zone of low suction (thick moisture film) to one of high
suction (thin moisture film). The prominence of finer (capillary) pores in the clay soil encourages more
unsaturated flow than in the sand.
(iii) Water vapour movement: There are two type of water vapour movement: (a) internal movement,
the change from the liquid to the vapour state takes place within the soil, that is, in the soil pores, and
(b) external movement, occurs at the land surface and the resulting vapour is lost to the atmosphere by
diffusion and convection (surface evaporation).
*****
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CHAPTER 9: SOIL AIR AND TEMPERATURE
A part of the soil volume that is not occupied by soil particles, known as pore space. Pore
spaces are filled partly with soil water and partly with soil air. As the pore space is occupied by the
both water and air, the volume of air varies inversely with that of water. As the moisture content of
the soil increases, the air content decreases and vice-versa. The constant movement of air in the soil
mass resulting in the renewal of gases is known as soil aeration. Thus, a well-aerated soil is one in
which gases are available to growing aerobic organism (including higher plants) in sufficient quantities
and in the proper proportions to encourage optimum rates of the essential metabolic processes of these
organisms.
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Fig. 14: Gaseous exchange between soil and atmosphere (diffusion)
Influence of soil air on plant growth
(1) Plant and root growth:
When the supply of oxygen is inadequate, the plant growth either retards or ceases completely
as the accumulated CO2 hampers the growth of plant roots.
(2) Microorganism population and activity:
The deficiency of air (oxygen) in soil slows down the rate of microbial activity, decomposition
of organic matter, nitrification, sulphur oxidation etc..
(3) Formation of toxic material:
Poor aeration results in the development of toxin and other injurious substances such as ferrous
oxide, H2S gas, CO2 gas etc. in the soil.
(4) Water and nutrient absorption:
A deficiency of oxygen has been found to check the nutrient and water absorption by plants.
(5) Development of plant diseases:
Insufficient aeration of the soil also lead to the development of diseases. e.g.wilt of arhar and
gram, dieback of citrus and peach.
SOIL TEMPERATURE
Agriculture is the exploitation of solar energy in presence of water and nutrients for plant
growth. The solar energy is the main source of heat for soil, which determines the thermal regime of
soil and growth of plants.
Soil temperature is one of the most important soil properties that affect crop growth. Soil
temperature controls the microbial activity and other plant growth process. It has been found that the
decomposition of organic matter and mineralization of organic form of nitrogen increases with
temperature. The soil colour, composition, and the water content in soils influence soil temperature.
Dark and fine textured soil absorb more heat during the day and lost it during night more rapidly that
coarse-textured soils, because later retain more water and the specific heat of water are 4 – 5 times
more than that of soil particles.
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(1) Solar Radiation: The main source of soil heat is the energy of sun’s rays (radiant energy) that
reach the earth after they pass through the atmosphere. The exposure of the earth after they pass
through the atmosphere. The exposure of the earth to the heat of the sun warms the surface of the soil
on which the rays fall.
(2) Conduction: The interior of the earth is very hot, the conduction of this heat to the soil is very
slow. Generally, during night, the surface soil becomes cooler than subsurface soil. Thus heat flows
from sub soil (warm layers) to soil (Cooler layers).
(3) Biology and Chemical reaction: Some amount of heat is liberated in the chemical and biological
process.
(4) Rain: The occurrence of warm rain during the winter months may raise the temperature of the soil.
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• For germination of different seeds requires different ranges of soil temperature e.g. maize
begins to germinate at soil temp of 7 to 10 C.
• Most of the soil organisms function best at an optimum soil temperature of 25 to 35 C.
• The optimum soil temperature for nitrification is about 32 C.
• It also influences soil moisture content, aeration and availability of plant nutrients.
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• Tillage: The cultivated soil has greater temperature amplitude as compared to the uncultivated
soil.
• Soil texture: Soil textures affect the thermal conductivity of soil. Thermal conductivity
decreases with reduction in particle size.
• Organic matter content: Organic matter reduces the heat capacity and thermal conductivity
of soil, increases its water holding capacity and has a dark colour, which increases its heat
absorbability.
• Slope of land: Solar radiation that reaches the land surface at an angle is scattered over a wider
area than the same amount of solar radiation reaching the surface of the land at right angles.
Therefore, the amount of solar radiation reaching per unit area of the land surface decreases as
the slope of the land is increases.
Effect of soil temperature on plant growth
There are various influences of soil temperature on soil fertility by changing the soil
environment which in turn affects plant growth.
(i) Germination of seeds: If the temperature is too low, the seed fails to germinate. On the other hand,
seeds may be injured if the temperature will be very high.
(ii) Physical properties of soil: Soil structure is greatly influenced by the temperature.
(iii) Microbial activity: The activity of micro-organisms is lowest when soil temperature is below 50C
and above 540C.The optimum temperature for the activity of most of the micro-organisms is in
the range of 25-350C.
(iv) Decomposition of organic matter in soil: At low temperature the rate of organic matter
decomposition is low resulting various toxic organic substances in soil and the high temperature
the rate of the same is very fast resulting beneficial products of organic matter decomposition and
hence influence the plant growth.
(v) Absorption of water: Variation in soil temperature affects the absorption of soil- water by the
plant roots.
(vi) Availability of nutrients: Temperature influences the solubility reactions of different nutrients and
releases larger amount of nutrient elements in the soil solution at higher temperatures.
(vii) Root growth: Low temperature encourages white succulent roots with little branching, while high
temperatures encourage a browner, finer and much more freely branching root system.
(viii) Plant diseases: At low temperatures, the soil contains many weekly parasitic fungi which will
grow actively and very rapidly and so those will kill the seedlings.
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CHAPTER 10: SOIL REACTION
Soil reaction
Soil reaction is the most important single chemical characteristic influencing many physical
and chemical properties of soil. Plant growth and microorganism activity depends upon soil reaction
and the factors associated with it. Three conditions possible in the soil are: Acidity, neutrality and
alkalinity.
The soil reaction describes the degree of acidity or alkalinity of a soil. The water molecule is
very stable and dissociates as:
H2O ---------------------→ H+ + OH-
The extent to which water ionizes can be expressed in terms of an ionization constant, Kw.
Thus,
Kw = (H+)(OH-) ………….. (Eqn. 1)
+ -
Where, (H ) and (OH ) are the concentration of hydrogen and hydroxyl ions expressed as
equivalents per litre. The value of Kw is 10-14 at 22°C; that is, the product of the concentrations of H+
and OH- ions is 10-14.
Eqn.1 can be written as,
Hence pH is the negative logarithm to the base 10 of its hydrogen ion concentration.
A normal solution of HCl contains 1 g hydrogen per litre, so its pH is log 101 = ° (since 10° = 1)
Since 0.1 N HCl contains 1 /10 g of hydrogen, so its pH is log 10 1/10 = log 1010 (since 101 =10)
Also 0.01 N NCI contains 1/100 g of hydrogen/litre, so its pH is log 10 1/100 = log 10100 =2 (since
102=100)
At neutrality, the H-ion concentration is 0.0000001 or 1 x 10-7 g of hydrogen/L
Substituting this concentration in the formula,
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Similarly at pH 6, there is 0.000001 g hydrogen/L,
pH = log10 ___1_____ = log10 (106) = 6 (Since 106= 1000000)
(0.000001)
Thus, pH 6 is ten times more acidic than pH 7. In a similar way, pH 5 is hundred times more
acidic than pH 7. The pH scale varies from 0 to 14. At pH 7, concentration of H + and OH- ions is
equal. If H+ ion concentration is 1 x 10-1, its pH will be 1. In this situation, the concentration of OH -
ions will be 1 x 10-13 (Since Kw (H+) (OH-) = 1 x 10-14)
1. Nitrogen: Plant absorbs most of their nitrogen in the form of nitrate whose availability depends on
the activity of nitrifying bacteria. The microorganisms responsible for nitrification are most active
when the pH is between 6.5 and 7.5. They are adversely affected if the pH falls below 5.5 and greater
than 9.0 (Fig. 7.3). The Nitrogen-fixing bacteria (like Azatobactor) also fails to function below pH
6.0. The decomposition of organic matter which is the primary source of nitrogen also slows down
under acidic condition.
2. Phosphorus: Its availability is at its highest when the reaction is between 6.5 and 7.5. When the
reaction is above or below this range, availability is reduced. In the strongly acidic soil (pH 5.0 or
less), iron, aluminium, manganese and other bases are present in a soluble state and in more quantity.
The phosphate ions react with these bases (iron, aluminium etc.) and insoluble phosphates of these
elements are formed and become unavailble.
Example : Al+++ + H2PO4-+ 2H20 = 2H+ + Al (0H)2.H2 PO4
(Soluble) (Insoluble)
The phosphates react with hydrated oxides of iron and aluminium and form insoluble hydroxy-
phosphates of iron and aluminium. Unavailability of phosphorus is called phosphorus- fixation.
Fixation of phosphate takes place even when the soil is alkaline (high pH). Phosphate ion
combines with calcium ion and calcium (or magnesium) carbonates and form insolulde calcium (or
magnesium) phosphate. The reaction is as follows :
(i) Ca (H2PO4 )2+ 2Ca++ = Ca3(PO4)2 + 4H+
(Soluble) (Absorbed) (Insoluble)
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i) Ca (H2PO4 )2 + 2CaCO3 = Ca3(PO4)2 + 2CO2 + 2H20
(Soluble) (insoluble)
The availability of phosphorous at, different pH is linked with the ionic form in which it is
present in soil solution. The monovalent H2PO4- ions Predominate in highly acid (at pH 4.0-5.0)
solutions. With decreasing acidity, the Divalent HPO 4- ions begin to appear. In alkaline soil the
trivalent PO4--- ions extremely small in quantity. At pH 9.0 and above, PO4--- is available to plants.
The ionic form has a large influence on the availability phosphorus to plants. The H 2PO4- and
HPO4 1 ions are considered to be more available than PO4--- ions. Thus, plants depend for their two
---
ionic tbrins to a much greater extent than PO4--- ions; H2PO4- at neutrality and below (acid range) and
HPO4- at neutrality and above (alkaline range),
In the pH range 6.0 to 7.0, phosphate-fixation is very slight. Consequently pH range
phosphorus availability is highest. At this pH range only 20-30% actually assimilated by the growing
plant, if soluble phosphorus is added in the soil as fertilizer. It is believed that the presence of calcium
hinders the absorption of phosphate by plant.
In acid soil (low pH), phosphorus becomes available by anion exchange. phosphate which has
reacted with iron and aluminium compounds is subject to replacement by other anions, such as the
hydroxyl ion (OH-) Such replacement is called anion exchange. It is the reverse reaction of phosphate
fixation is acid soil given above:
Al (0H)2.H2 PO4 + OH- = Al (0H)2 + H2 PO4-
(Insoluble) (Soluble)
One anion (OH ) has been exchanged for another (H2PO4-). The phosphorus (H2PO4-) becomes
-
3. Potassium: The availability of potassium does not influence by soil reaction to any great extent. In
acid soil, potassium is lost through leaching. The unavailability of is due to the conversion of
exchangeable to non-exchangeable potassium.
In alkaline soil, particularly if the alkalinity is due to CaCO 3 (or is brought about by over liming
in acid soil), the solubility of soil potassium is depressed (results in non. availability).
4. Calcium and Magnesium. Acid soils (base unsaturated) are poor in available calcium and
magnesium. In alkaline soil (pH not exceeding 8.5) availability of Ca and Mg nutrients is always high.
When the pH is above 8.5, the availability of these nutrients again decreases.
5. Iron, Aluminium and Manganese: When the pH is low the solubility of iron, aluminium and
manganese compounds is increased, and hence, they are readily available in acid soils. At the pH range
5.5 to 7.0, iron and manganese are present in the soluble ferrous (Fe++) and manganous (Mn++) forms.
At p1-1 below 5.5, the solubility of these compounds is considerably increased with the result that
they have a toxic influence on plant growth (Fig 7.3).
Under neutral and alkaline conditions, iron and manganese are usually present in ferric (Fe+++)
and magnaic (Mn++++) states. Hence, the soils with pli 7.5 and above, they become unavailable and
sometimes produce deficiency disease like chlorosis in plants.
6. Sulphur: The availability of sulphur is not affected by soil reaction as sulphur compounds are
soluble in the whole pH range. However, it is more soluble in acid soil and lost in leaching. Acid
77
conditions which retard the decomposition of organic matter, therefore, retard the release of available
sulphur. The availability of sulphur present in organic matter depends upon the decomposition of
organic matter.
7. Micronutrients: In general, the availability of boron, copper and zinc is reduced in alkaline soils
and that of molybdenum in acid soils.
The availability of boron, copper and zinc progressively decreases as the soil pH increases.
Their availability also decreases under highly acid condition when the pH is below 5.0.
Zinc availability in alkaline soils from insoluble zinc salts (calcium zincate) which reduces its
availability, zinc and copper adsorbed on the clay colloids and are not easily displaced and hence, not
available for plant growth. The availability of molybdenum is reduced under acid soils. It is more
available in neutral and alkaline soils.
Buffer Capacity: The colloidal complex thus acts as a powerful buffer in the soil and does not
allow rapid and sudden changes in soil reaction. Buffering depends upon the amount of colloidal
material present in soil. Clay soils and soils rich in organic matter are more highly buffered than sandy
soils. Buffer capacity of the soil varies with its cation exchange capacity (C.E.C.), the greater the
C.E.C. the greater will be its buffer capacity.
Thus, heavier the texture and the greater the organic matter content of' a soil, the greater the
amount of acid or alkaline material required to change its pH.
*****
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The colloidal system refers to a two-phase system in which one material in a very finely divided
state is dispersed through second. e.g. (i) Solid in gas: Clay in water (dispersion of clay in water) and
(ii) Liquid in gas: Fog or clouds in atmosphere. The clay fraction of the soil contains particles less
than 0.002 mm in size. Particles less than 0.001 mm size possess colloidal properties and are known
as soil colloids.
(2) Flocculation: The colloidal particles are coagulated by adding an oppositely charged ion.
Formation of flocs is known as flocculation. If the cations are held close to the negatively charged
particles, the negative charge would be neutralized and the colloidal particles flocculate and settle
down. Addition of any electrolyte brings all such dispersed particles together and get settle down. The
phenomenon of flocculation plays an important part in the soil aggregation, thereby soil structure.
When clay particles are flocculated, soil develops small clods of a crumby nature. Such a soil allows
free movement of air and water. If the particles are deflocculated, the aggregates get dispersed, the
soil gets water-logged, the movement of air and water is impeded. The cations like Ca+2 and Mg+2
enhance the flocculation process and maintain soil in good aggregated condition; while Na + is
deflocculating agent, which disperse the aggregated soil and result in poor aggregated soil condition.
(3) Electrical charge: Colloidal particles often have an electrical charge, some positive and some
negative. When clay colloids suspended in water, it carries a negative electric charge. Colloidal clay
develops negative electric charge due to dissociation of hydroxyl groups attached to silicon in silica
sheets of the clay mineral leaves residual oxygen (O--) carrying a negative charge.
(4) Adsorption: Colloidal particles possess the power of adsorbing gases, liquid and even solids from
their suspension. The phenomenon of adsorption is confined to the surface of colloidal particles.
Larger the surface area greater the adsorption for water, nutrients etc.
The adsorption of ions is governed by the type and nature of ion and the type of colloidal
particle. In the case of cations, the higher the valence of the ion, the more strongly it is adsorbed.
Exchange or replacement of cation would be difficult from colloidal particle. That is why divalent ions
(Ca+2 & Mg+2) are held more strongly than monovalent ion (Na+ & K+). A trivalent cation (Al+3) is
most readily adsorbed. Hydrogen ions (H+) behave as polyvalent ions so are adsorbed more strongly
than even Ca+2. Adsorption of anions (H2 PO4 -, HPO4-2 etc.) increases with the lowering or increasing
of pH. The adsorption of phosphate ions is the lowest when the medium is neutral; it increases when
the pH either falls or rises, due to fixation by iron and aluminum hydroxides in acid range and by
calcium in alkaline range. Among the clay minerals, kaolinitic clay has a greater anion adsorbing
capacity than montmorillonitic or illitic clay.
79
The property of adsorption plays an important role in soil fertility. Due to his property soil is
able to hold water and nutrients and keep them available to plant.
(5) Non-permeability: Colloids are unable to pass through a semi permeable membrane. The
membrane allows the passage of water and of the dissolved substance through its pores, but retains
the colloidal particle.
(6) Cohesion and adhesion: clay particles possess the properties of cohesion. While forming
aggregates, the colloidal clay particles unite with each other by virtue of the property of cohesion.
Clay particles envelop sand particles under the force of adhesion. The force of cohesion and adhesion
are developed in the presence of water. When colloidal substances are wetted, water adheres to the
particles and then brings about cohesion between two or more adjacent colloidal particles. Soil when
dried, the particles remain united because of the force of molecular cohesion. These two forces help
in the retention of water in the soil and thus used by plants and microorganism.
(7) Swelling: A soil colloid when brought in contact with water they imbibe a certain quantity of water
and swell and increase in volume.
(8) Plasticity: Soil colloidal particles may present in gel condition possess the property of plasticity.
Due to this property clay-colloids can be moulded in any shape.
The two together form the colloidal complex of the soil. In almost all soils the inorganic
colloids form a major portion of the colloidal complex. On the other hand, in peat soils, it consists
almost entirely of organic colloids. Colloidal particles float in a medium and do not tend to settle.
Colloids are referred as the dispersed systems. The substance in solution is termed as the dispersed
phase while the medium in which the particles are dispersed is called the dispersion medium. Soils
formed in tropical and semi-tropical regions almost the whole of the colloidal complex consists almost
entirely of inorganic colloids. Whereas soil formed in temperate regions usually contain more organic
colloids than those formed in tropical and sub-tropical regions. In a broad way, two groups of clay are
recognized - silicate clay so characteristic of temperate regions and the iron and aluminum hydrous
oxide clays found in tropical and semi-tropical.
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The silicate clay minerals are composed of two types of sheets, (1) silica sheet (tetrahedral)
and (2) alumina sheet (octahedral).
In a silica sheet one silicon cations is surrounded by four oxygen anions. The four-sided
configuration is called as a silica tetrahedron. An interlocking of a series of such silica tetrahedron
horizontally by shared oxygen anions gives a tetrahedral sheet. Similarly in alumina sheet aluminium
(or magnesium) ion is surrounded by six oxygen or hydroxyls gives an eight-sided configuration
termed as alumina octahedron. Numerous octahedrons linked together horizontally give an
octahedral sheet.
The tetrahedral and octahedral sheets are bound together in various combinations in different
silicate clay by shared oxygen anions. Such association is known as crystal units.
Fig. 17:
Fig. 18:
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bases are lost and part of silica is also lost as silicic acid. Due to the formation of insoluble Fe and Al-
phosphate, the availability of P decreases in these soils.
(1) Alteration: Alteration of mineral may be encouraged by chemical attacks with consequent removal
of certain soluble constituents and substitution of others within the crystal lattice. Muscovite mica is
altered to fine-grained mica is a good example of alteration. Muscovite is a 2:1-type primary mineral
with a nonexpanding crystal structure. As weathering occurs, the mineral is broken down and part of
the interlayer potassium is lost, and some silicon along with Ca2+or Mg2+ and water molecules are
added from weathering solutions. The net result is a less rigid crystal structure. Continued removal of
potassium and substitution of magnesium for some of aluminum in alumina sheet would result in the
formation of montmorillonite.
(2) Recrytallization: The crystallization of silicate clays from soluble weathering products of other
minerals is more important in clay genesis than is alteration. This process of recrystallization involves
the complete change from the structural makeup of original minerals and is the result of much more
intense weathering than that required by the alteration process. e.g. formation of kaolinite (1.1 type
clay mineral) from solutions containing soluble aluminum and silicon that came from the breakdown
of primary minerals having a 2:1 type structure.
In general, fine grained micas, chlorite and vermiculite are formed through minerals; whereas kaolinite
and oxides of Fe and Al are products of intense weathering, conditions of intermediate weathering
intensity encourages the formation of smectite. Silicate – clay genesis is accompanied by the removal
of soluble elements such as K, Na, Ca and Mg (fig. 19).
84
Fig 19: The formation of the soil colloids.
OR
85
One O valence is
satisfied within -O- ………..H+ Adsorbed Hydrogen
the crystal by Al or Si
Similarly, in minerals such as beidellite and illite the substitution of three-valent atom such as
Al for one of the four-valent silicon atoms in silica sheet leaves an unsatisfied negative valence. This
represents below.
Silica sheet Silica sheet
(no substitution) (Al substituted for Si)
O– -Si++++O-- O– -Al+++O--
86
The silicate clay minerals thus, posses negative charges due to isomorphous substitution and
dissociation of OH ion. They can attract positively charged cations like Ca +2, Mg 2+, K+, Na+, H+,
NH4+. These cations are adsorbed on the clay surface. They are in exchangeable and available to the
plants.
The cations and water are absorbed on the clay surfaces. In general the cations adsorbed on the
surface of silicate clays are in the order of:
H+ > Al3+> Ca+2 > = Mg 2+ > K+ = NH4+ > Na+
Certain cations are especially prominent under natural conditions.
(1) Humid region soils Al3+> H+ > Ca+2 = Mg 2+ > K+> Na+
(2) Well drained and semi arid soils Ca+2 > Mg 2+> Na+> K+> H+
(3) Sodic or Alkali soils Na+> Ca+2 > Mg 2+ > K+> H+
ION EXCHANGE
Soil colloids are the seat of reaction. Ion exchange (cation and anions) takes place in colloids.
The phenomenon of ion exchange is of great importance in agriculture. It has considerable influence
on the liberation of plant nutrients such as Ca, K, P etc. It controls soil structure and crumb formation.
It is also responsible for imparting a stable structure. It controls the processes plays an important role
in the reclamation of acid and alkali soils. It also influences the effect of fertilizers and fertilizer
practices. Ion exchange is two types: (1) cation exchange or base Exchange and (2) anion exchange or
acid exchange. Ion exchange is a reversible process in which cation and anion exchanged between
solid and liquid phase.
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(3) Water hull
Every cation carries some water with it. This water remains around the cation as a very thin
film, which is called water hull. As the valiancy of the cation decreases, it has higher water hull and
therefore, it remains away from the clay particles. Hence, it is loosely held and easily replaced. This
is because of decrease in the force of attraction between the two particles. In case of particles A, the
complementary ion of Ca+2 is hydrogen. H+ is more strongly bound than Ca+2. Therefore, in this case
Ca+2 will be replaced first. In case of B particles, the complementary ion Ca+2 are K+. Ca+2 are more
strongly bound than K+. Therefore, the K+ will be replaced first.
Anion exchange
The process of anion exchange is similar to that of cation exchange. Under certain conditions
hydrous oxides of iron and aluminium shows evidence of having positive charges on their crystal
surfaces. The positive charges of colloids are due to addition of hydrogen (H+) in hydroxyl group (OH-
) resulted in net positive charge (OH2+). This positive charge will attract anions. The capacity for
holding anions increases with increase in acidity. The lower the pH the grater is the adsorption. All
89
anions are not adsorbed equally readily. Some anions such as H 2PO4- are adsorbed very quickly at all
pH values in the acid as well as alkaline range. Cl- and SO4-- ions are adsorbed slightly at low pH but
none at neutral soil, while NO3- ions are not adsorbed at all. Hence, at the pH commonly prevailing in
cultivated soils- nitrate (NO3), chloride (Cl) and sulphate (SO4) ions are easily lost by leaching. In
general, the relative order of anion exchange is:
OH- > H2PO4- > SO4-- > NO3-
Accepting the proton as shown below develops the positive charge:
R.OH + HCl R.OH2 + Cl-
R.NH2 + HCl R.NH3 + Cl-
In this way there is fixation of phosphate ion, in which OH ion of silicate clay mineral is
substituted by H2PO4 ion by isomorphrous substitution. The H2PO4 ion becomes a part of the silicate
clay mineral and the phosphate fixed by this way is known as colloid bound phosphate. This
phosphate is not exchangeable and available to the plant. It has been observed that anion exchange
increases with the acidity. The anions like NO 3 and OH are not bound at pH above 7. The adsorbing
power of anions is in the order of:
Cl* = NO3* < SO4 < PO4.
* are not held by the soil colloids and are lost due to the leaching.
or OH > H2PO4 > SO4 > NO3 (relative order of anion exchange)
The adsorption of phosphate ions by clay particles from soil solution reduces its availability to
plants. This is also known as colloid bound phosphate fixation. The phosphate ion again becomes
available when lime is applied to increase the pH of acidic soil.
The OH ion originated not only from silicate clay minerals but also from hydrous oxides of
iron and aluminum present in the soil. The phosphate ions reacts with the hydrous oxides also get fixed
forming insoluble hydroxyl phosphate of iron and aluminum, which is called saloid bound phosphate.
_ _
Al (OH)3 + H2PO4 Al(OH)2 .H2PO4 + OH
Available Unavailable
90
If the reaction takes place at a low pH under strongly acid conditions, the phosphate ions are
irreversibly fixed and are totally unavailable for the use of plants.
values calculate (1) % base saturation (2) % base unsaturation or H- saturation , (3) % saturation of
individual exchangeable cation, (4) Mg/100 gm ppm, lb/acre kg/ha of individual cation and (5) amount
of lime (CaCO3) require to neutralize exchangeable H+.
(1) Total of Exchangeable Bases = 10+5+1+1 =17 me/100 gm soil
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10 me Exchangeable Ca+2 = me/100 gm x eq.wt. of Ca = 10 x 20
= 200 mg Ca/100 gm soil
ppm of Ca = mg Ca /100 gm x 10 = 200 x10 =2000 ppm Ca
lb/acre of Exch. Ca = ppm x 2 = 2000 x 2 =4000 lb/acre of soil
kg /ha= ppm x 2.24 = 2000 x 2.24 = 4480.00 kg Ca/ha
*****
92
CHAPTER 12: SOIL ORGANIC MATTER
Organic matter in the soil comes from the remains of plants and animals. As new organic
matter is formed in the soil, a part of the old becomes mineralized. The original source of the soil
organic matter is plant tissue. Under natural conditions, the tops and roots of trees, grasses and other
plants annually supply large quantities of organic residues. Thus, higher plant tissue is the primary
source of organic matter. Animals are usually considered secondary sources of organic matter.
Various organic manures, that are added to the soil time to time, further add to the store of soil
organic matter.
Composition of organic residues have un-decomposed soil organic matter (mainly plant
residues together with animal remains, i.e. animal excreta etc.) The moisture content of plant
residues varies from 60 to 90% (average 78%) and 25% dry matter (solid). Plant tissues (organic
residues) may be divided into 91) organic and (2) inorganic (elemental) composition. The
compounds constituting the plant residues or un-decomposed soil organic matter is shown in the
following diagram
Organic Residues
(Un-decomposed organic matter)
Organic Inorganic
(Mineral matter/ elemental composition or ash)
S, P, Cl, CO3 , Ca, Mg, Na,
K, Fe, Zn, Cu, Mn, etc.
Nitrogenous Non-nitrogenous
Insoluble: protein, peptides, Carbohydrates: Cellulose (insoluble);
Peptones etc starch, hemicelluloses, pectin, mucilage etc.
(Hydrolysable); sugars (soluble)
Water-soluble: Nitrates, Ether-soluble: Fats, oils, waxes, resins
ammonical compounds etc steroids etc.
93
Relative percentage of compound in plant material is shown in fig. 19.
The organic materials incorporated in the soil do not remain as such very long. They are at
once attacked by a great variety of microorganisms, worms and insects present in the soil especially if
the soil is moist. The microorganism for obtaining their food, break up the various constituents of
which the organic residues are composed, and convert them into new substances, some of which are
very simple in composition and others highly complex. The whole of the organic residues is not
decomposed all at once or as a whole. Some of the constituents are decomposed very rapidly, some
less readily, and others very slowly.
(A) Decomposition of soluble substances: Sugar and water-soluble nitrogenous compounds are the
first to be decomposed as they offer a very readily available source of carbon, nitrogen and energy for
the microorganisms. Thus, when glucose is decomposed under aerobic conditions the reaction is as
under:
aerobic
(1) Sugar + Oxygen Carbon dioxide + Water
condition
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(2) Ammonification: The transformation of organic nitrogenous compounds into ammonia is called
ammonification. During the course of action under aerobic conditions by heterotrophic organisms,
oxygen is taken up and carbon dioxide is released. Ammonification process involves a gradual
simplification of complex compounds. The ammonification occurs as a result of action of enzymes
produced by microorganisms. Their action is chiefly hydrolytic and oxidative (in presence of air).
(3) Nitrification: The process of conversion of ammonia to nitrite (NO2) and then to nitrate (NO3)
is known as nitrification. The production of nitrate is more rapid than that of nitrite, while the
formation of ammonia is the slowest process. That is why soil usually contains more nitrate nitrogen
than nitrite at any time. Nitrification is an aerobic process involving the production of nitrates from
ammonium salts. It is the work of autotrophic bacteria.
Nitrosomonas Nitrobacter
NH3 NO2 NO3
Ammonia Nitrite Nitrate
The process which involves conversion of soil nitrate into gaseous nitrogen or nitrous oxide is
called denitrification.
Pseudomonas /Bacillus
Nitrate Nitrogen gas
Water-logging (e.g., rice field) and high pH will increase nitrogen loss by denitrification.
Proteins are complex organic substances containing nitrogen, sulphur, and sometimes
phosphorus, in addition to carbon, hydrogen and oxygen. During the course of decomposition of plant
materials, the proteins are first hydrolyzed to a number of intermediate products, e.g., proteoses,
peptones, peptides, etc., collectively known as polypeptides.
95
Peptides Amides
Cellulose is the most abundant carbohydrate present in plant residues. The microorganisms
break up cellulose into cellobiose and glucose. Glucose is further attacked by organisms and converted
into organic acids.
hydrolysis hydrolysis
Cellulose Cellobiose Glucose
(Cellulase) (Cellobiase)
Oxidation Oxidation
Glucose Organic acids CO2 + H2O
The decomposition of cellulose in acid soils proceeds more slowly than in neutral and alkaline
soils. It is quite rapid in well-aerated soils and comparatively slow in those poorly-aerated.
Fats are first broken down by microorganisms through the agency of enzyme lipase into
glycerol and fatty acids. Glycerol is next oxidized to organic acids which along with the other fatty
acids are finally oxidized to carbon dioxide and water.
Lignin is deposited on the cell wall to impart strength to the skeleton framework of plant. Lignin
decomposes slowly, much slower than cellulose. Complete oxidation of lignin gives rise to carbon
dioxide and water.
96
(E) Simple decomposition products
As the enzymic changes of the soil organic matter proceed, simple products begin to manifest
themselves. Some of these, especially carbon dioxide and water, appear immediately. Others such as
nitrate-nitrogen accumulate only after the peak of the vigorous decomposition is over. The more
common simple products resulting from the activity of the soil microorganisms are as follows:
Enzymic
S + 2O2 SO4 --
Sulphur Oxidation Sulphate
Humus
Humus is a complex and rather resistant mixture of brown or dark brown amorphous and colloidal
substances that results from microbial decomposition and synthesis and has chemical and physical
properties of great significance to soils and plants. The humic and non-humic compounds collectively
make up humus.
(A) The humic substances comprise about 60-80 per cent of the soil organic matter. Humic substances
are dark in colour, amorphous and have high molecular weight. Humic substances can be classified
into three chemical groups. (i) Humin : The highest in molecular weight, dark in colour, insoluble in
both acid and alkali and more resistant to microorganism attack (ii) Humic acid: medium in molecular
weight, medium in colour, soluble in alkali but insoluble in acid, intermediate resistant to microbial
degradation and (iii) Fulvic acid: lowest in molecular weight, light in colour, soluble in both acid and
alkali and most susceptible to microorganism attack
97
(B) The non-humic group make up of about 20-30 per cent of the soil organic matter. Non- humic
substances consists polymers, polysaccharides, polyuronides. Polysaccharides are especially effective
in enhancing soil aggregate stability. Non-humic substances are less complex and less resistant to
microbial attack than humic sunstances.
Humus is more resistant to decay and may present in soils even hundreds of years. This
resistant property of humus is important in maintaining organic matter level in soils and protect
nitrogen and other essential nutrients that are founs in humus complex.
Humus fractions
The humified fractions are considered as the most active part of humus which consists of a
series of highly acidic, yellow to black coloured, high molecular weight polyelctrolytes known humic
acid, fulvic acid and so on.
Based on the solubilities, the group is generally divided into three classes: (i) fulvic acid, which
is thought to be of the lowest molecular weight and is alkali and acid soluble, (ii) humic acid which is
of medium molecular weight, alkali soluble and acid insoluble, (iii) humin which is apparently of the
highest molecular weight and is insoluble in both alkali and acid except under most drastic conditions.
The classical method of fractionation of humic substances is based on difference in solubility of the
constituents in aqueous acidic and alkaline solutions and ethanol. The steps in the separation and
fractionation of soil organic matter is shown hereunder:
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Carbon cycle
Carbon is a common constituent of all organic matter and is involved in essentially all life
processes. Transformation of carbon commonly spoken of as the carbon cycle, the biocycle, or life
cycle that makes possible the continuity of life on the earth. Plant assimilates CO 2 from the atmosphere
into organic compounds using energy from the sun. Man and other higher animals obtain energy and
body tissue from plant products and return wastes and residues to the soil. Macro- and microorganisms
digest these organic materials, releasing nutrients for plants and leaving CO2 and humus as relatively
stable products. Under optimum condition more than 100 kg/ha of carbon dioxide may be evolved per
day, 25-30 kg being more common. Carbonates and bicarbonates of Ca, Mg, K etc., are removed in
leaching, but eventually the carbon returns to the cycle in the form of CO 2. The total CO2 is released
to the atmosphere where it is again available for plant assimilation. These changes are shown in fig.20.
The ratio of the weight of organic carbon (C) to the weight of total nitrogen (N) in a soil (or
organic material), is known as C: N ratio. When fresh plant residues are added to the soil, they are rich
in carbon and poor in nitrogen. The content of carbohydrates is high. This results in wide carbon-
nitrogen ratio which may be 40 to 1. Upon decomposition the organic matter of soils changes to humus
and have an approximate C: N ratio of 10:1.
The ratio of carbon to nitrogen in the arable (cultivated) soils commonly ranges from 8:1 to
15:1. The carbon-nitrogen ratio in plant material is variable, ranging from 20:1 to 30:1. Low ratios of
carbon to nitrogen (10:1) in soil organic matter generally indicate an average stage of decomposition
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and resistance to further microbiological decomposition. A wide ratio of C: N (35:1) indicates little or
no decomposition, susceptibility to further and rapid decomposition and slow nitrification.
Significance of C:N Ratio
(1) Keen competition for available nitrogen when organic residues (with high C: N ratio) are added to
soils. When organic residues with a wide C/N ratio (50:1) are incorporated in the soil,
decomposition quickly occurs. Carbon dioxide is produced in large quantities. Under these
conditions, nitrate-nitrogen disappears from the soil because of the instant microbial demand for
this element to build up their tissues. And for the time being, little (or no) nitrogen is an available
to plants. As decomposition occurs, the C/N ratio of the plant material decrease since carbon is
being lost and nitrogen conserved. Nitrates-N again appear in quantity in the soil, thus, increases
plant growth (fig. 21).
(2) Consistency of C: N Ratio. As the decomposition processes continue, both carbon and nitrogen
are now subject to loss the carbon as carbon dioxide and the nitrogen as nitrates which are leached or
absorbed by plants. At a point carbon-nitrogen ratio, becomes more or less constant, generally
stabilizes at 10:1 or 12:1.
NO3 level
of soil
Fig.21:
It has always been considered profitable to mix the highly carbonaceous material like straw
with a small quantity of ammonium sulphate. With this available source of nitrogen from ammonium
sulphate, the decomposition of fresh organic material is hastened and thus, the release of available
nitrogen is affected in a shorter period of time.
Role of organic matter The effects of organic matter on fertility are as follows
(1) Organic matter binds soil particles into structural units called aggregates. These aggregates help
to maintain a loose, open, granular condition. The granular condition of soil increase water
infiltration and percolation.
100
(2) Water-holding capacity is increased by organic matter. The granular soil resulting from organic
matter additions, supplies more water than sticky and impervious soil.
(3) Surface run off and erosion are reduced by organic matter as there is good infiltration.
(4) Organic matter on the soil surface reduces losses of soil by wind erosion.
(5) Surface mulching with coarse organic matter, lower soil temperatures in the summer and keep
the soil warmer in winter.
(6)The organic matter serves as a source of energy for the growth of soil microorganisms.
(7) Organic matter serves as a reservoir of chemical elements that are essential for plant growth.
Upon decomposition, organic matter supplies the nutrients needed by growing plants, as well as
many hormones and antibiotics.
(8) Fresh organic matter has a special function in making soil phosphorus more readily available in
acid soils.
(9) Organic acids released from decomposing organic matter help to reduce alkalinity in soils.
(10) Fresh organic matter supplies food for such soil life as earthworms, ant and rodents. These
microorganisms improve drainage and aeration.
(11) Organic matter upon decomposition produces organic acids and carbon dioxide which helps to
dissolve minerals such as potassic and make them more available to growing plants.
(12) Humus (highly decomposed organic matter) provides a storehouse for the exchangeable and
available captions potassium and magnesium. Ammonium fertilizers are also prevented from
leaching because humus holds ammonium in an exchangeable and available form.
(13) It acts as a buffering agent. Buffering checks rapid chemical changes in pH.
*****
101
CHAPTER 13: SOIL ORGANISMS
Soil biology is the scientific study of life in and on the soil. Biomass – The amount of living
matter in a given area.
It is well to note that the major stress is placed, not on classification, but upon the biochemical
changes induced by the various organisms. Consequently, the grouping given below is very broad and
simple.
Bacteria
The bacteria are of two types. (a) Autotrophic bacteria: These bacteria manufacture their
food by using very simple inorganic substances. Autotrophic bacteria oxidise ammonia, nitrate,
sulphur, manganese, iron, carbon monoxide, hydrogen and methane, so that plant can utilize these
substances, and (b) Heterotrophic bacteria: These bacteria depend upon organic matter or living
body for their food. They bring about mineralization of organic matter through hydrolysis and
oxidation and release nitrogen, phosphorus and other nutrients in forms available to plants. They also
fix atmospheric nitrogen in the soil.
Fungi
These are non-green plant (no chlorophyll); thus, they cannot manufacture their own food and
so they depend on others for their food. They decompose the organic matter in form available to the
plants. The chemical substances such as streptomycin, a widely used antibiotic, are obtained from soil
fungus called Streptomyces and Penicillin from Penicillium fungus.
Algae
They contain chlorophyll and therefore, they manufacture their own food. The main groups of
algae are: 1. green 2. blue-green 3. yellow-green and 4. diatoms.
Actinomycetes
Actinomycetes are transitional between bacteria and fungi.
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are made smaller by grinding process and thus the water-holding capacity of the soil is also improved.
Nitrogen waste products of the worm are also added to the soil, thus increasing its nitrogen content.
(ii) Microorganisms
Protozoa. Soil protozoa feed either on soil organic matter or on bacteria, thus, regulating the number
of the bacteria in the soil.
Nematodes. They live in soil & water and cause diseases in many crops e.g. rice, tomato
(ii) Nitrification. The conversion of ammonia to nitrate (NO3) is known as nitrification. Autotrophic
bacteria performed this transformation. It is an aerobic process.
Nitrosomonas Nitrobacter
NH3 N02 NO
Ammonia Nitrite Nitrate
Decomposition of simple products. The more common simple products resulting from the activity of
the soil microorganisms are as follows:
Carbon: CO2, CO3--, HC03-, elemental carbon
Sulphur: S, H2S, SO3--, SO4+, CS2
Phosphorus: H2PO4-, HPO4--
Others: H2O,O2, H2, H+,OH-, K+,Ca++, Mg++ etc.
(iii) Mineralization of organic sulphur. Autotrophic bacteria (sulphur bacteria) oxidised sulphur into
sulphate form.
Enzymatic
S + 2O2 SO4--
Sulphur Oxygen Oxidation Sulphate
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(iv) Mineralization of organic phosphorus. Organic phosphorus compounds are mineralized by the
action of microorganism into inorganic phosphorus.
Microorganism
Organic phosphorus H2PO4- or HPO4--
available phosphorus
(b) Fixation of atmospheric nitrogen. The conversion of elemental nitrogen to readily available form
by nitrogen-fixing microorganisms is called biological nitrogen-fixation. The nitrogen fixed by
microorganisms in the soil is known as 'bio-fertilizers'. The nitrogen-fixing microorganisms are: (I)
Bacteria (II) Algae and (III) Mycorrhizae (fungus root).
(1) Bacteria. There are two main groups of bacteria which fix atmospheric nitrogen; symbiotic and
non-symbiotic. Addition of nitrogen in the soil by bacteria is called 'bacterial-fertilizers'.
(i) Symbiotic nitrogen-fixing bacteria. This group of bacteria (Rhizobium) fixes nitrogen in
association with leguminous plants, called symbiotic bacteria. The Rhizobium bacteria living in the
root of leguminous plants. They take their food from the leguminous plants and absorb nitrogen from
the atmosphere. They produce nitrogenous compound and supply to the leguminous host plants. Both
the legume (pulse) crop and bacteria are benefited by this association, known as 'symbiosis'. This
process also adds nitrogen to the soil.
(ii) Non-symbiotic nitrogen fixing bacteria. Azotobacter and Clostridium work independently of any
host crop. Azotobacter is an aerobic nitrogen-fixing bacteria and thrives well in neutral soil. It is
susceptible to a deficiency of phosphate. Clostridium is anaerobic and fixing less amount of nitrogen
than Azotobacter because of anaerobic fermentation releases only a small amount of energy. Whereas
aerobic changes produce large amount of energy which helps to fix more nitrogen.
(2) Algae. Most algae are chlorophyll bearing organisms. Blue-green algae are capable of fixing
atmospheric nitrogen to the soil. The main genus of algae which fix nitrogen are Anabaena, Nostoc
and Cylindrospermum. They prefer usually neutral or slightly alkaline soil. In water-logged rice field,
algae grow well and fix about 20-30 kg nitrogen per hectare. The algal material after decomposition
adds organic matter to the soil and improves the physical condition of the soils.
(3) Mycorrhizae (Myco =fungus; rhiza =root). Mutually beneficial association between certain
fungi and roots of higher plants is called mycorrhizae (fungus roots). By this symbiotic association,
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fungi get sugars and organic exudates from the roots of higher plant. In return, the fungi provide
several essential nutrients to the plant.
Mycorrhizae are divided into two types: ectomycorrhiza and endomycorrhiza. The
endomycorrhiza group are called vesicular-arbuscular mycorrhizae (VAM) increase the uptake of
phosphorus, enhance resistance against drought and certain root-infecting fungus.
(b) Development of plant diseases. The blight disease of rice, apple and pear is caused by bacteria.
Fungi cause more serious damage to crop plants. e.g. smuts and rusts of cereal crops and late blight of
potatoes.
(c) Formation of toxic compounds. Under anaerobic conditions toxic substances such as methane,
hydrogen sulphide are formed due to improper decomposition of organic matter.
(d) Competition for nutrients. Competition for plant nutrients between microorganisms and crop
plants is quite high. So plants get insufficient nutrients for their growth.
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