Soil Science

Scope of Soil Science

Soil Science has six well defined and developed disciplines. Scope of soil Science is reflected through these disciplines.

Soil Science : The science dealing with soil as a natural resource on the surface of the earth, including Pedology (soil genesis, classification and mapping) and the physical, chemical and biological and fertility properties of soil and these properties in relation to their management for crop production.

1. Soil fertility: Nutrient supplying properties of soil
2. Soil chemistry: Chemical constituents, chemical properties and the chemical reactions
3. Soil physics: Involves the study of physical properties
4. Soil microbiology: deals with micro organisms, its population, classification, its role in transformations
5. Soil conservation: Dealing with protection of soil against physical loss by erosion or against chemical deterioration i.e. excessive loss of nutrients either natural or artificial means.
6. Pedology: Dealing with the genesis, survey and classification

Soil can be compared to various systems of human body

Digestive    - matters decomposition
Respiratory - air circulation & exchange of gases
Circulatory - water movement with in the soil system
Excretory   - leaching out of excess salts
Brain         - soil clay
Colour       - soil colour
Height       - soil depth

Components of Soil (Volume basis)

Mineral matter – 45%
Organic matter – 5%
Soil water – 25%
Soil air – 25% 

Definition of Soil & Approaches of Soil Study

Definition of Soil

Whitney (1982) Hilgard (1892) Dokuchaiev (1900) Joffe (1936): Soil is a natural body of mineral and organic constituents differentiated into horizons usually unconsolidated, of variable depth which differs among themselves as well as from the underlying parent material in morphology, physical makeup, chemical properties and composition and biological characteristics.
SSSA (1970):
(i) The unconsolidated mineral matter on the surface of the earth that has been subjected to and influenced by genetic and environmental factors of parent material, climate (including moisture and temperature effects), macro and microorganisms and topography, all affecting over a period of time and producing a product, that is “SOIL” that differs from the material from which it is derived in many, physical, chemical, biological and morphological properties and characteristics.

(ii) The unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.

Approaches of Soil Study

Two Concepts: One treats soil as a natural body, weathered and synthesized product in nature while other treats soil as a medium for plant growth.
1) Pedological Approach: The origin of the soil, its classification and its description are examined in Pedology. (From Greek word pedon, means soil or earth). Pedology is the study of soil as a natural body and does not focus on the soil’s immediate practical use. A pedologist studies, examines and classifies soil as they occur in their natural environment.

2) Edaphological Approach: Edophology (from Greek word edaphos, means soil or ground) is the study of soil from the stand point of higher plants. Edaphologists consider the various properties of soil in relation to plant production. They are practical and have the production of food and fiber as their ultimate goal. They must determine the reasons for variation in the productivity of soils and find means for improvement.

Weathering of Rocks and Minerals

Rocks and minerals are formed under a very high temperature and pressure, exposed to atmospheric conditions of low pressure and low temperature and they become unstable and weather.

Soils are formed from rocks through the intermediate stage of formation of Regolith which is the resultant of weathering.

The sequence of processes in the formation of soils is:

Weathering of rocks and minerals -> formation of regolith or parent material ->formation of true soil from regolith

Rock ->Weathering ->Regolith ->Soil forming factors and processes ->True soil (otherwise)
Two processes involved in the formation of soil are:

1. Formation of regolith by breaking down (weathering) of the bed rock.
2. The addition of organic matter through the decomposition of plant and animal tissues, and reorganization of these components by soil forming processes to form soil.

Weathering: A process of disintegration and decomposition of rocks and minerals which are brought about by physical agents and chemical processes, leading to the formation of Regolith (unconsolidated residues of the weathering rock on the earth’s surface or above the solid rocks).

(OR)

The process by which the earth’s crust or lithosphere is broken down by the activities of the atmosphere, with the aid of the hydrosphere and biosphere

(OR)

The process of transformation of solid rocks into parent material or Regolith

Parent material: It is the regolith or at least its upper portion. May be defined as the unconsolidated and more or less chemically weathered mineral materials from which soil are developed

Two basic processes of Weathering:
Physical (or) mechanical - disintegration
Chemical – decomposition

In addition, another process: Biological and all these processes are work hand in hand
Depending up on the agents taking part in weathering processes, it is classified into three types.
Different agents of weathering
 

Physical/ Mechanical (disintegration)	Chemical (decomposition)	Biological (disint + decomp)
1.Physical condition of rock	1.Hydration	1.Man & animals
2.Change in temperature	2.Hydrolysis	2. higher plants & their roots
3.Action of H2O	3.Solution	3.Micro organisms
-fragment & transport	4.Carbonation
- action of freezing	5.Oxidation
- alter. Wet & drying	6.Reduction
- action of glaciers
4.Action of wind
5. Atmosp. electric pheno

Physical weathering of Rocks

The rocks are disintegrated and are broken down to comparatively smaller pieces, with out producing any new substances

1. Physical condition of rocks: The permeability of rocks is the most important single factor.

1. Coarse textured (porous) sand stone weather more readily than a fine textured (almost solid) basalt.
2. Unconsolidated volcanic ash weather quickly as compared to unconsolidated coarse deposits such as gravels

2. Action of Temperature: The variations in temperature exert great influence on the disintegration of rocks.

1. During day time, the rocks get heated up by the sun and expand. At night, the temperature falls and the rocks get cooled and contract.
2. This alternate expansion and contraction weakens the surface of the rock and crumbles it because the rocks do not conduct heat easily.
3. The minerals with in the rock also vary in their rate of expansion and contraction
4. The cubical expansion of quartz is twice as feldspar
5. Dark coloured rocks are subjected to fast changes in temperature as compared to light coloured rocks
6. The differential expansion of minerals in a rock surface generates stress between the heated surface and cooled unexpanded parts resulting in fragmentation of rocks.
7. This process causes the surface layer to peel off from the parent mass and the rock ultimately disintegrates. This process is called Exfoliation

3. Action of Water: Water acts as a disintegrating, transporting and depositing agent.

i) Fragmentation and transport: Water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean

1. Moving water has the great cutting and carrying force.
2. It forms gullies and ravines and carries with the suspended soil material of variable sizes.
3. Transporting power of water varies. It is estimated that the transporting power of stream varies as the sixth power of its velocity i.e the greater the speed of water, more is the transporting power and carrying capacity

Speed/Sec	Carrying capacity
15 cm	fine sand
30 cm	gravel
1.2 m	stones ( 1kg)
9.0 m	boulders ( several tons)

The disintegration is greater near the source of river than its mouth

ii) Action of freezing: Frost is much more effective than heat in producing physical weathering

1. In cold regions, the water in the cracks and crevices freezes into ice and the volume increases to one tenth
2. As the freezing starts from the top there is no possibility of its upward expansion. Hence, the increase in volume creates enormous out ward pressure which breaks apart the rocks

iii) Alternate wetting and Drying: Some natural substances increase considerably in volume on wetting and shrink on drying.(e.g.) smectite, montmorilonite

1. During dry summer/ dry weather – these clays shrink considerably forming deep cracks or wide cracks.
2. On subsequent wetting, it swells.
3. This alternate swelling and shrinking/ wetting or drying of clay enriched rocks make them loose and eventually breaks

iv) Action of glaciers:

1. In cold regions, when snow falls, it accumulates and changes into ice sheet.
2. These big glaciers start moving owing to the change in temperature and/or gradient.
3. On moving, these exert tremendous pressure over the rock on which they pass and carry the loose materials
4. These materials get deposited on reaching the warmer regions, where its movement stops with the melting of ice

4. Action of wind:

1. Wind has an erosive and transporting effect. Often when the wind is laden with fine material viz., fine sand, silt or clay particles, it has a serious abrasive effect and the sand laden winds itch the rocks and ultimately breaks down under its force
2. The dust storm may transport tons of material from one place to another. The shifting of soil causes serious wind erosion problem and may render cultivated land as degraded (e.g.) Rajasthan deserts

5. Atmospheric electrical phenomenon: It is an important factor causing break down during rainy season and lightning breaks up rocks and or widens cracks.

 

Chemical Weathering of Rocks

Decomposition of rocks and minerals by various chemical processes is called chemical weathering. It is the most important process for soil formation.

Chemical weathering takes place mainly at the surface of rocks and minerals with disappearance of certain minerals and the formation of secondary products (new materials). This is called chemical transformation.

Feldspar + water -> clay mineral + soluble cations and anions

Chemical weathering becomes more effective as the surface area of the rock increases.

Since the chemical reactions occur largely on the surface of the rocks, therefore the smaller the fragments, the greater the surface area per unit volume available for reaction.

The effectiveness of chemical weathering is closely related to the mineral composition of rocks. (e.g.) quartz responds far slowly to the chemical attack than olivine or pyroxene.
Average mineralogical composition (%)

Composition	Granite	Basalt	Shale	S. Stone	L. Stone
Feldspar	52.4	46.2	30.0	11.5	-
Quartz	31.3	-	2.3	66.8	-
Pyrox - amphi	-	44.5	-	-	-
FeO mineral	2.0	9.3	10.5	2.0	-
Clay mineral	14.3	-	25.0	6.6	24.0
Carbonates	-	-	5.7	11.1	76.0

Chemical Processes of weathering:

1. Hydration: Chemical combination of water molecules with a particular substance or mineral leading to a change in structure.

Soil forming minerals in rocks do not contain any water and they under go hydration when exposed to humid conditions. Up on hydration there is swelling and increase in volume of minerals. The minerals loose their luster and become soft.
It is one of the most common processes in nature and works with secondary minerals, such as aluminium oxide and iron oxide minerals and gypsum. (e.g.)

a) 2Fe2O3 + 3HOH    ->     2Fe2O3 .3H2O
   (Hematite) (Red)            (Limonite) (Yellow)

b) Al2O3 + 3HOH      ->     Al2O3 .3H2O
   (Bauxite)                      (Hyd. aluminium Oxide)

c) CaSO4 + 2H2O     ->      CaSO4 .2H2O
   (Anhydrite)                    (Gypsum)
d) 3(MgO.FeO.SiO2) + 2H2O  ->   3MgO.2SiO2.2H2O + SiO2 + 3H2O
     (Olivine)                                 (Serpentine)
2. Hydrolysis: Most important process in chemical weathering. It is due to the dissociation of H2O into H+ and OH- ions which chemically combine with minerals and bring about changes, such as exchange, decomposition of crystalline structure and formation of new compounds. Water acts as a weak acid on silicate minerals.
KAlSi3O8 + H2O       ->        HAlSi3O8 + KOH
(Orthoclase)                       (Acid silt clay)
HAlSi3O8 + 8 HOH   -> Al2O3 .3H2O        +      6 H2SiO3
(Recombination)         (Hyd. Alum. oxide)        (Silicic acid)
This reaction is important because of two reasons.

1. clay, bases and Silicic acid - the substances formed in these reactions - are available to plants
2. water often containing CO2 (absorbed from atmosphere), reacts with the minerals directly to produce insoluble clay minerals, positively charged metal ions (Ca++, Mg++, Na+, K+ ) and negatively charged ions (OH-, HCO3-) and some soluble silica – all these ions are made available for plant growth.

3. Solution: Some substances present in the rocks are directly soluble in water. The soluble substances are removed by the continuous action of water and the rock no longer remains solid and form holes, rills or rough surface and ultimately falls into pieces or decomposes. The action is considerably increased when the water is acidified by the dissolution of organic and inorganic acids. (e.g) halites, NaCl
NaCl + H2O -> Na+, Cl- , H2O (dissolved ions with water)

4. Carbonation: Carbon dioxide when dissolved in water it forms carbonic acid.
2H2O + CO2    ->   H2CO3
This carbonic acid attacks many rocks and minerals and brings them into solution. The carbonated water has an etching effect up on some rocks, especially lime stone. The removal of cement that holds sand particles together leads to their disintegration.

CaCO3       +       H2CO3   ->     Ca(HCO3)2
(Calcite) slightly soluble          (Ca bi carbonate) readily soluble
5. Oxidation: The process of addition and combination of oxygen to minerals. The absorption is usually from O2 dissolved in soil water and that present in atmosphere. The oxidation is more active in the presence of moisture and results in hydrated oxides.(e.g) minerals containing Fe and Mg.
4FeO (Ferrous oxide) + O2     ->   2Fe2O3 (Ferric oxide)
4Fe3O4 (Magnetite) + O2          ->   6Fe2O3 (Hematite)
2Fe2O3 (Hematite) + 3H2O    ->   2Fe2O3 .3H2O(Limonite)

6. Reduction: The process of removal of oxygen and is the reverse of oxidation and is equally important in changing soil colour to grey, blue or green as ferric iron is converted to ferrous iron compounds. Under the conditions of excess water or water logged condition (less or no oxygen), reduction takes place.
2Fe2O3 (Hematite) - O2 -> 4FeO( Ferrous oxide) - reduced form
In conclusion, during chemical weathering igneous and metamorphic rocks can be regarded as involving destruction of primary minerals and the production of secondary minerals.
In sedimentary rocks, which is made up of primary and secondary minerals, weathering acts initially to destroy any relatively weak bonding agents (FeO) and the particles are freed and can be individually subjected to weathering.

Biological Weathering of Rocks

Unlike physical and chemical weathering, the biological or living agents are responsible for both decomposition and disintegration of rocks and minerals. The biological life is mainly controlled largely by the prevailing environment.
1. Man and Animals:

1. The action of man in disintegration of rocks is well known as he cuts rocks to build dams, channels and construct roads and buildings. All these activities result in increasing the surface area of the rocks for attack of chemical agents and accelerate the process of rock decomposition.
2. A large number of animals, birds, insects and worms, by their activities they make holes in them and thus aids for weathering.
3. In tropical and sub tropical regions, ants and termites build galleries and passages and carry materials from lower to upper surface and excrete acids. The oxygen and water with many dissolved substances, reach every part of the rock through the cracks, holes and galleries, and thus brings about speedy disintegration.
4. Rabbits, by burrowing in to the ground, destroy soft rocks. Moles, ants and bodies of the dead animals, provides substances which react with minerals and aid in decaying process
5. The earthworms pass the soil through the alimentary canal and thus bring about physical and chemical changes in soil material.

2. Higher Plants and Roots: The roots of trees and other plants penetrates into the joints and crevices of the rocks. As they grew, they exert a great disruptive force and the hard rock may break apart. (e.g.) pipal tree growing on walls/ rocks

The grass root form a sponge like mass prevents erosion and conserve moisture and thus allowing moisture and air to enter in to the rock for further action.

Some roots penetrate deep into the soil and may open some sort of drainage channel. The roots running in crevices in lime stone and marble produces acids. These acids have a solvent action on carbonates.

The dead roots and plant residues decompose and produce carbon dioxide which is of great importance in weathering.
 
3. Micro- organisms: In early stages of mineral decomposition and soil formation, the lower forms of plants and animals like, mosses, bacteria and fungi and actinomycetes play an important role. They extract nutrients from the rock and N from air and live with a small quantity of water. In due course of time, the soil develops under the cluster of these micro-organisms.

This organism closely associated with the decay of plant and animal remains and thus liberates nutrients for the use of next generation plants and also produces CO2 and organic compounds which aid in mineral decomposition.

Soil Forming Factors

The soil formation is the process of two consecutive stages.

1. The weathering of rock (R) into Regolith
2. The formation of true soil from Regolith

The evolution of true soil from regolith takes place by the combined action of soil forming factors and processes.

1. The first step is accomplished by weathering (disintegration & decomposition)
2. The second step is associated with the action of Soil Forming Factors

Weathering Factors
 
Dokuchaiev (1889) established that the soils develop as a result of the action of soil forming factors
S = f ( P, Cl, O )

Further, Jenny (1941) formulated the following equation
S = f (Cl, O, R, P, T, …)
Where,
Cl – environmental climate
o – Organisms and vegetation (biosphere)
r – Relief or topography
p – Parent material
t- Time
… - additional unspecified factors
The five soil forming factors, acting simultaneously at any point on the surface of the earth, to produce soil
Two groups – Passive i) Parent material, ii) Relief, iii) Time
   Active IV) Climate, v) Vegetation & organism

Soil Texture

Definition of Soil Texture: Soil texture refers to the relative proportion of particles or it is the relative percentage by weight of the three soil separates viz., sand, silt and clay or simply refers to the size of soil particles.
The proportion of each size group in a given soil (the texture) can not be easily altered and it is considered as a basic property of a soil.
The soil separates are defined in terms of diameter in millimeters of the particles. Soil particles less than 2 mm in diameter are excluded from soil textural determinations.
Stones and gravels may influence the use and management of land because of tillage difficulties but these larger particles make little or no contribution to soil properties such as WHC and capacity to store plant nutrients and their supply.
Gravels: 2 - 4 mm
Pebbles: 4 - 64 mm
Cobbles: 64 - 256 mm
Boulders: > 256 mm

Particles less than 2 mm are called fine earth, normally considered in chemical and mechanical analysis.

The components of fine earth: Sand, Silt and Clay (Soil separates. The size limits of these fractions have been established by various organizations. There are a number of systems of naming soil separates.
 

1. (a) The American system developed by USDA
2. (b) The English system or British system ( BSI )
3. (c) The International system (ISSS)
4. (d) European system

i) USDA

Soil separates	Diameter in mm
Clay	< 0.002
Silt	0.002 - 0.05
Very Fine Sand	0.05 -  0.10
Fine Sand	0.10 - 0.25
Medium Sand	0.25 - 0.50
Coarse Sand	0.50 - 1
Very Coarse Sand	1 - 2 mm

ii) BSI

Soil separates	Diameter in mm
Clay	< 0.002
Fine Silt	0.002 - 0.01
Medium Silt	0.01 - 0.04
Coarse Silt	0.04 -  0.06
Fine Sand	0.06 - 0.20
Medium Sand	0.20 - 1
Coarse Sand	1 - 2 mm

iii) ISSS

Soil separates	Diameter in mm
Clay	< 0.002
Silt	0.002 - 0.02
Fine Sand	0.02 - 0.2
Coarse Sand	0.2 -  2

iv) European System 

Soil separates	Diameter (mm)
Fine clay	< 0.0002 mm
Medium clay	0.0002 – 0.0006
Coarse clay	0.0006 – 0.002
Fine silt	0.002 - 0.006
Medium silt	0.006 - 0.02
Coarse silt	0.02 - 0.06
Fine sand	0.06 - 0.20
Medium sand	0.20 - 0.60
Coarse sand	0. 60 - 2.00

Sand:

1. Usually consists of quartz but may also contain fragments of feldspar, mica and occasionally heavy minerals viz., zircon, Tourmaline and hornblende.
2. Has uniform dimensions
3. Can be represented as spherical
4. Not necessarily smooth and has jagged surface

Silt:

1. Particle size intermediate between sand and clay
2. Since the size is smaller, the surface area is more
3. Coated with clay
4. Has the physico- chemical properties as that of clay to a limited extent
5. Sand and Silt forms the SKELETON

Clay:

1. Particle size less than 0.002 mm
2. Plate like or needle like in shape
3. Belong to alumino silicate group of minerals
4. Some times considerable concentration of fine particles which does not belong to alumino silicates. (e.g.) iron oxide and CaCO3
5. These are secondary minerals derived from primary minerals in the rock
6. Flesh of the soil

Knowledge on Texture is important. It is a guide to the value of the land. Land use capability and methods of soil management depends on texture.
Particle size distribution/ determination
 
The determination of relative distribution of the ultimate or individual soil particles below 2 mm diameter is called as Particle size analysis or Mechanical analysis
Two steps are involved
i) Separation of all the particles from each other ie. Complete dispersion into ultimate particles

ii) Measuring the amount of each group

Separation

Sr. No.	Aggregating agents	Dispersion method
1	Lime and Oxides of Fe & Al	Dissolving in HCl
2	Organic matter	Oxidises with H2O2
3	High conc. of electrolytes ( soluble salts)	Precipitate and decant or filter with suction
4	Surface tension	Elimination of air by stirring with water or boiling

After removing the cementing agents, disperse by adding NaOH

Measurement
 
Once the soil particles are dispersed into ultimate particles, measurement can be done
i) Coarser fractions - sieving - sieves used in the mechanical analysis corresponds to the desired particle size separation for 2 mm, 1 mm and 0.5 mm – sieves with circular holes, for smaller sizes, wire mesh screens are used (screening)

ii) Finer fractions - by settling in a medium the settling or the velocity of the fall of particles is influenced by viscosity of the medium. Difference in density between the medium and falling particles, size and shape of object
Stokes' Law:

Particle size analysis is based on a simple principle i.e. "when soil particles are suspended in water they tend to sink. Because there is little variation in the density of most soil particles, their velocity (V) of settling is proportional to the square of the radius 'r' of each particle.
Thus V = kr2, where k is a constant. This equation is referred to as Stokes' law.
Stokes (1851) was the first to suggest the relationship between the radius of the particles and its rate of fall in a liquid. He stated that "the velocity of a falling particle is proportional to the square of the radius and not to its surface. The relation between the diameter of a particle and its settling velocity is governed by Stokes' Law:

V = 2/9 gr^2 (ds – dw) / n

Where,
V - Velocity of settling particle (cm/sec.)
g - Acceleration due to gravity cm/ sec2 (981)
ds - Density of soil particle (2.65)
dw - Density of water (1)
n - Coefficient of viscosity of water (0.0015 at 4oC)
r - Radius of spherical particles (cm).
Assumptions and Limitations of Stokes' Law

Particles are rigid and spherical / smooth. This requirement is very difficult to fulfill, because the particles are not completely smooth over the surface and spherical. It is established that the particles are not spherical and irregularly shaped such as plate and other shapes.
The particles are large in comparison with the molecules of the liquid so that in comparison with the particle the medium can be considered as homogenous i.e. the particles must be big enough to avoid Brownian movement. The particles less than 0.0002 mm exhibit this movement so that the rate of falling is varied.
The fall of the particles is not hindered or affected by the proximity (very near) of the wall of the vessel or of the adjacent particles. Many fast falling particles may drag finer particles down along with them.

The density of the particles and water and as well as the viscosity of the medium remain constant. But this is usually not so because of their different chemical and mineralogical composition.
The suspension must be still. Any movement in the suspension will alter the velocity of fall and such movement is brought by the sedimentation of larger particles (> 0.08 mm). They settle so fast and create turbulence in the medium.
The temperature should be kept constant so that convection currents are not set up.
 
Methods of Textural determination

Numerous methods for lab and field use have been developed
i) Elutriation method – Water & Air
ii)Pipette method
iii) Decantation/ beaker method
iv) Test tube shaking method
v) Feel method - Applicable to the field - quick method - by feeling the soil between thumb and fingers

Feel Method

Evaluated by attempting to squeeze the moistened soil into a thin ribbon as it is pressed with rolling motion between thumb and pre finger or alternately to roll the soil into a thin wire
η four aspects to be seen - i) Feel by fingers, ii) Ball formation, iii) Stickiness and iv) Ribbon formation

Soil Colloids

The colloidal state refers to a two-phase system in which one material in a very finely divided state is dispersed through second phase.
The examples are:

Solid in liquid - Clay in water (dispersion of clay in water)
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.

General Properties of Soil Colloids

1. Size: The most important common property of inorganic and organic colloids is their extremely small size. They are too small to be seen with an ordinary light microscope. Only with an electron microscope they can be seen. Most are smaller than 2 micrometers in diameter.

2. Surface area: Because of their small size, all soil colloids expose a large external surface per unit mass. The external surface area of 1 g of colloidal clay is at least 1000 times that of 1 g of coarse sand. Some colloids, especially certain silicate clays have extensive internal surfaces as well. These internal surfaces occur between plate like crystal units that make up each particle and often greatly exceed the external surface area. The total surface area of soil colloids ranges from 10 m2/g for clays with only external surfaces to more than 800 m2/g for clays with extensive internal surfaces. The colloid surface area in the upper 15 cm of a hectare of a clay soil could be as high 700,000 km2/g

3. Surface charges: Soil colloidal surfaces, both external and internal characteristically carry negative and/or positive charges. For most soil colloids, electro negative charges predominate. Soil colloids both organic and inorganic when suspended in water, carry a negative electric charge. When an electric current is passed through a suspension of soil colloidal particles they migrate to anode, the positive electrode indicating that they carry a negative charge. The magnitude of the charge is known as zeta potential. The presence and intensity of the particle charge influence the attraction and repulsion of the particles towards each other, there by influencing both physical and chemical properties.

The negative electrical charge on clays comes from
i) Ionizable hydrogen ions and
ii) Isomorphism substitution.
i) Ionizable hydrogen ions: Ionizable hydrogen ions are hydrogen from hydroxyl ions on clay surfaces. The -Al-OH or -Si-OH portion of the clay ionizes the H and leaves an unneutralized negative charge on the oxygen (-Al-O- or - Si-O). The extent of ionized hydrogen depends on solution pH; more ionization occurs in more alkaline (basic) solutions.
ii) Isomorphous substitution: The second source of charge on clay particles is due to the substitution of one ion for another of similar size and often with lower positive valence. In clay structures, certain ions fit into certain mineral lattice sites because of their convenient size and charge. Dominantly, clays have Si4+ in tetrahedral sites and A13+ in octahedral sites. Other ions present in large amounts during clay crystallization can replace some of the A13+ and Si4+ cations. Substitutions that are common are the Si4+ replaced by A13+, and even more extensive replacement of A13+ by one or more of these: Fe3+, Fe2+, Mg2+ or Zn2+ Since the total negative charge from the anions (the oxygen) remains unchanged, the lower positive charge because of substitution results in an excess negative charge at that location in the structure.

4. Adsorption of cations: As soil colloids possess negative charge they attract the ions of an opposite charge to the colloidal surfaces. They attract hundreds of positively charged ions or cation such as H+, A13+ Ca2+ , and Mg2+. This gives rise to an ionic double layer.

The process, called Isomorphous substitution and the colloidal particle constitutes the inner ionic layer, being essentially huge anions; with both, external and internal layers that are negative in charge. The outer layer is made up of a swarm of rather loosely held (adsorbed) cations attracted to the negatively charged surfaces. Thus a colloidal particle is accompanied by a swarm of cations that are adsorbed or held on the particle surfaces.

5. Adsorption of water: In addition to the adsorbed cations, a large number of water molecules are associated with soil colloidal particles. Some are attracted to the adsorbed cations, each of which is hydrated; others are held in the internal surfaces of the colloidal particles. These water molecules play a critical role in determining both the physical and chemical properties of soil.

5. Cohesion: Cohesion is the phenomenon of sticking together of colloidal particles that are of similar nature. Cohesion indicates the tendency of clay particles to stick together. This tendency is primarily due to the attraction of the clay particles for the water molecules held between them. When colloidal substances are wetted, water first adheres to the particles and then brings about cohesion between two or more adjacent colloidal particles.

6. Adhesion: Adhesion refers to the phenomenon of colloidal particles sticking to other substances. It is the sticking of colloida1 materials to the surface of any other body or substance with which it comes in contact.

7. Swelling and shrinkage: Some clay (soil colloids) such as smectites swell when wet and shrink when dry. After a prolonged dry spell, soils high in smectites (e.g. Vertisols) often are crises-crossed by wide, deep cracks, which at first allow rain to penetrate rapidly. Later, because of swelling, such soil is likely to close up and become much more impervious than one dominated by kaolinite, chlorite, or fine grained micas. Vermiculite is intermediate in its swelling and shrinking characteristics.

8. Dispersion and flocculation: As long as the colloidal particles remain charged, they repel each other and the suspension remains stable. If on any account they loose their charge, or if the magnitude of the charge is reduced, the particles coalesce, form flocs or loose aggregates, and settle out. This phenomenon of coalescence and formation of flocs is known as flocculation. The reverse process of the breaking up of flocs into individual particles is known as deflocculation or dispersion.

9. Brownian movement: When a suspension of colloidal particles is examined under a microscope the particles seem to oscillate. The oscillation is due to the collision of colloidal particles or molecules with those of the liquid in which they are suspended. Soil colloidal particles with those of water in which they are suspended are always in a constant state of motion. The smaller the particle, the more rapid is its movement.

10. Non permeability: Colloids, as opposed to crystalloid, are unable to pass through a semi-permeable membrane. Even though the colloidal particles are extremely small, they are bigger than molecules of crystalloid dissolved in water. The membrane allows the passage of water and of the dissolved substance through its pores, but retains the colloidal particles.

Soil Air

Soil air is a continuation of the atmospheric air. Unlike the other components, it is constant state of motion from the soil pores into the atmosphere and from the atmosphere into the pore space. This constant movement or circulation of air in the soil mass resulting in the renewal of its component gases is known as soil aeration.

Composition of Soil Air: The soil air contains a number of gases of which nitrogen, oxygen, carbon dioxide and water vapour are the most important. Soil air constantly moves from the soil pores into the atmosphere and from the atmosphere into the pore space. Soil air and atmospheric air differ in the compositions. Soil air contains a much greater proportion of carbon dioxide and a lesser amount of oxygen than atmospheric air. At the same time, soil air contains a far great amount of water vapour than atmospheric air. The amount of nitrogen in soil air is almost the same as in the atmosphere.

Composition of soil and atmospheric air

Percentage by volume
	Nitrogen	Oxygen	Carbon dioxide
Soil Air	79.2	20.6	0.3
Atmospheric Air	79.9	20.97	0.03

Factors Affecting the Composition of Soil Air:

1. Nature and condition of soil: The quantity of oxygen in soil air is less than that in atmospheric air. The amount of oxygen also depends upon the soil depth. The oxygen content of the air in lower layer is usually less than that of the surface soil. This is possibly due to more readily diffusion of the oxygen from the atmosphere into the surface soil than in the subsoil. Light texture soil or sandy soil contains much higher percentage than heavy soil. The concentration of CO2 is usually greater in subsoil probably due to more sluggish aeration in lower layer than in the surface soil.

2. Type of crop: Plant roots require oxygen, which they take from the soil air and deplete the concentration of oxygen in the soil air. Soils on which crops are grown contain more CO2 than fallow lands. The amount of CO2 is usually much greater near the roots of plants than further away. It may be due to respiration by roots.

3. Microbial activity: The microorganisms in soil require oxygen for respiration and they take it from the soil air and thus deplete its concentration in the soil air. Decomposition of organic matter produces CO2 because of increased microbial activity. Hence, soils rich in organic matter contain higher percentage of CO2.
4. Seasonal variation: The quantity of oxygen is usually higher in dry season than during the monsoon. Because soils are normally drier during the summer months, opportunity for gaseous exchange is greater during this period. This results in relatively high O2 and low CO2 levels. Temperature also influences the CO2 content in the soil air. High temperature during summer season encourages microorganism activity which results in higher production of CO2.

Soil reaction

Soil reaction is one of the most important physiological characteristics of the soil solution. The presence and development of micro- organisms and higher plants depend upon the chemical environment of soil. There fore study of soil reaction is important in soil science.

There are three types of soil reactions: 1. Acidic 2. Alkaline and 3. Neutral

1. Acidic: It is common in region where precipitation is high. The high precipitation leaches appreciable amounts of exchangeable bases from the surface layers of the soils so that the exchange complex is dominated by H ions. Acid soils, therefore, occur widely in humid regions and affect the growth of plants markedly.

2. Alkaline: Alkali soils occur when there is comparatively high degree of base saturation. Salts like carbonates of calcium, magnesium and sodium also give a preponderance of OH ions over H ions in the soil solution. When salts of strong base such as sodium carbonate go into soil solution and hydrolyze, consequently they give rise to alkalinity. The reaction is as follows:
Na2CO3 -----à 2Na + + CO3=
2Na+ + CO3= + 2HOH -----à 2Na+ + 2OH - + H2CO3
since sodium hydroxide dissociates to a greater degree than the carbonic acid, OH ions dominate and give rise to alkalinity. This may be as high as 9 or 10. These soils most commonly occur in arid and semi-arid regions.

3. Neutral: Neutral soils occur in regions where H ions just balance OH ions.

Soil pH: The reaction of a solution represents the degree of acidity or basicity caused by the relative concentration of H ions (acidity) or OH ions present in it. Acidity is due to the excess of H ions over OH ions, and alkalinity is due to the excess of OH ions over H ions. A neutral reaction is produced by an equal activity of H and OH ions. According to the theory of dissociation, the activity is due to the dissociation or ionization of compounds into ions.

Method of expressing acidity or alkalinity

Equivalent quantities of all acids or alkalies contain the same number of total H or OH ions, respectively. But, when they are dissolved in water they do not ionize to the same extent. The amount of acid or alkali ionized depends upon the content of free H and OH ions. When the dissociation is high e.g., hydrochloric acid (a strong acid), it dissociates to a larger extent than the weak acetic acid. Acetic acid dissociates only to about 10 % as compared to hydrochloric acid. In a 1N solution of hydrochloric acid there will be 1 gram of H ions per liter, while in a normal solution of acetic acid there will be 1/10 gram of H ions per liter. But in titration 1 ml of 1 N hydrochloric acid and 1 ml of 1 N acetic acid will require 1 ml of 1 N sodium hydroxide for neutralization separately, because the total acidity is the same and titration determines both the ionized and unionized H or OH ions.

Soil acidity: There are three kinds of acidity.
(i) Active acidity is due to the H+ ion in the soil solution.
(ii) Salt replaceable acidity represented by the hydrogen and aluminum that are easily exchangeable by other cations in a simple unbuffered salt solution such as KCl and
(iii) Residual acidity is that acidity which can be neutralized by limestone or other alkaline materials but cannot be detected by the salt-replaceable technique. Obviously, these types of acidity all add up to the total acidity of a soil.

i. Active acidity: The active acidity is a measure of the H+ ion activity in the soil solution at any given time. However, the quantity of H+ ions owing to active acidity is very small compared to the quantity in the exchange and residual acidity forms. For example, only about 2 kg of calcium carbonate would be required to neutralize the active acidity in a hectare-furrow slice of an average mineral soil at pH 4 and 200/0 moisture. Even though the concentration of hydrogen ions owing to active acidity is extremely small, it is important because this is the environment to which plants and microbes are exposed.

ii. Salt replaceable (exchangeable) acidity: This type of acidity is primarily associated with the exchangeable aluminum and hydrogen ions that are present in largest quantities in very acid soils. These ions can be released into the soil solution by an unbuffered salt such as KCl.
Al3+ + 4KCI +AlCI3+HCI' L~~~ H + L~~~~ ~ (Soil Solid) (Soil Solution) (Soil Solid) (Soil solution), in moderately acid soils, the quantity of easily exchangeable aluminum and hydrogen is quite limited. Even in these soils, however, the limestone needed to neutralize this type of acidity is commonly more than 100 times that needed for the soil solution (active acidity). At a given pH value, exchangeable acidity is generally highest for smectites, intermediate for vermiculites, and lowest for kaolinite. In any case, however, it accounts for only a small portion of the total soil acidity as the next section will verify.

iii. Residual acidity: Residual acidity is that which remains in the soil after active and exchange acidity has been neutralized. Residual acidity is generally associated with aluminum hydroxy ions and with hydrogen and aluminum atoms that are bound in non exchangeable forms by organic matter and silicate clays. If lime is added to a soil, the pH increases and the aluminum hydroxy ions are changed to uncharged gibbsite as follows. OH- OH- AI (OH) 2+ 7 AI (OH) 2+ 7 AI (OH) 3 In addition, as the pH increases bound hydrogen and aluminum can be released by calcium and magnesium In the lime materials [Ca (OH) 2 is used as an example of the reactive calcium liming material] The residual acidity is commonly far greater than either the active or salt replaceable acidity. Conservative estimates suggest that the residual acidity may be 1000 times greater than the soil solution or active acidity in a sandy soil and 50000 or even 100000 times greater in a clayey soil high in organic matter. The amount of ground limestone recommended to at least partly neutralize residual acidity is commonly 4-8 metric tons (Mg) per hectare furrow slice (1.8-3.6 tons/AFS).
It is obvious that the pH of the soil solution is only "the tip of the iceberg" in determining how much lime is needed. Buffering and Soil Reaction Buffer action: Buffering refers to resistance to a change in pH. If 1 ml HCI (of 0.1 N) is added to one liter of pure distilled water of pH 7.0, the resulting solution would have a pH of about 5.0. If on the other hand, the same amount of acid is added to a liter of soil suspension the resulting change in pH would be very small. There is, a distinct resistance to change in pH. This power to resist a change in pH is called buffer action. A buffer solution is one which contains reserve acidity and alkalinity and does not change pH with small additions of acids or alkalies. Buffer capacity: The colloidal complex 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 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 is the amount of acid or alkaline material required to change its pH. Importance of Buffering to Agriculture: Changes In soil reaction (pH) have a direct influence on the plants and it also affects the availability of plant nutrients. Deficiency of certain plant nutrients and excess availability of others in toxic amounts would seriously upset the nutritional balance in the soil. Buffering prevents sudden changes and fluctuation in soil pH, so it regulates the availability of nutrients and also checks direct toxic effect to plants.

Factors Controlling Soil Reactions

Soil reaction varies due to following factors

1. Nature of soil colloids: The colloidal particles of the soil influence soil reaction to a very greatest extent. When hydrogen (H+) ion forms the predominant adsorbed cations on clay colloids, the soil reaction becomes acid.

2. Soil solution: The soil solution carries a number of salts dissolved in capillary water. The cations of the salts intermingle with those of the diffuse double layer of the clay particle and increase the concentration. The concentration of cations in bulk of the solution is more or less (or nearly) the same as that near the particle surfaces. For an unsaturated soil (Clay), the more compact the layer the greater is the number of hydrogen ions dissociating into the solution. This increases the acidity of the soil solution or lowers its pH. Under field conditions, the concentration of salts varies with the moisture content of the soil. The more dilute the solution, the higher the pH value. Hence the pH tends to drop as the soil gets progressively dry. Soil reaction is also influenced by the presence of CO2 in soil air. As the CO2 concentration increases, the soil pH falls and increases the availability of the nutrients. Under field conditions, plant roots and micro-organism liberate enough CO2 , which results in lowering the pH appreciably. This principle of increasing the concentration of CO2 in soil air is also used in the reclamation of alkali soils.

3. Climate: Rainfall plays important role in determining the reaction of soil. In general, soils formed in regions of high rainfall are acidic (low pH value), while those formed in regions of low rainfall are alkaline (high pH value).

4. Soil management: Cultural operations in general tend to increase soil acidity. They make an acid soil more acidic, and an alkaline soil less alkaline. As a result of constant cultivation, basic elements are lost from the soil through leaching and crop removal. This leads to change the soil reaction to the acid side.

5. Parent materials: Soils developed from parent material of basic rocks generally have higher pH than those formed from acid rocks (e.g. granite). The influence of parent material is not very important as it is completely masked by the climatic conditions under which the soil is developed.
6. Precipitation: As water from rainfall passes through the soil, basic nutrients such as calcium (Ca) and magnesium (Mg) are leached. They are replaced by acidic elements including Al, H and manganese (Mn). Therefore, soils formed under high rainfall conditions are more acid than those formed under arid conditions.

7. Decomposition of organic matter: Soil organic matter is continuously being decomposed by micro-organisms into organic acids, carbon dioxide (CO2) and water, forming carbonic acid. Carbonic acid, in turn, reacts with the Ca and Mg carbonates in the soil to form more soluble bicarbonates, which are leached away, leaving the soil more acid.

8. Native vegetation: Soils often become more acid when crops are harvested because of removal of bases. Type of crop determines the relative amounts of removal. For example, legumes generally contain higher levels of bases than do grasses. Calcium and Mg contents also vary according to the portion (s) of the plant harvested. Many legumes release H ions into their rhizosphere when actively fixing atmospheric N2. The acidity generated can vary from 0.2 to 0.7 pH units per mole of fixed N.

9. Soil depth: Except in low rainfall areas, acidity generally increases with depth, so the loss of topsoil by erosion can lead to a more acid pH in the plough layer. The reason is that more subsoil is included in the plow layer as topsoil is lost. There are areas, however, where subsoil pH is higher than that of the topsoil.

10. Nitrogen fertilization: Nitrogen from fertilizer, organic matter, and manure and legume N fixation produces acidity. Nitrogen fertilization speeds up the rate at which acidity develops. At lower N rates, acidification rate is slow, but is accelerated as N fertilizer rates increase.

11. Flooding: The overall effect of submergence is an increase of pH in acid soils and a decrease in basic soils. Regardless of their original pH values, most soils reach pH of 6.5 to 7.2 within one month after flooding and remain at the level until dried. Consequently, liming is of little value in flooded rice production. Further, it can induce deficiencies of micronutrients such as zinc (Zn).

Influence of Soil Reaction on Availability of Nutrients

The unproductiveness of acid and alkali soils is very often due to the lack of available plant nutrients. In highly acid soils (low pH), the availability of some of the nutrients such as aluminum, iron, manganese etc., is increased to a point to become toxic to the plant. At the same time the supplies of available calcium, nitrogen, phosphorus etc., are reduced to starvation level (become unavailable). The same is the case at high pH (alkaline conditions), plant growth suffers due to the unavailability of nutrients like nitrogen, phosphorus and some minor elements (e.g., iron, manganese, boron etc). Another indirect effect occurs through the activity of microorganisms. Most microorganisms function at their best within a pH range 6.0 to 7.5. If soil reaction is changed beyond this range, the microorganisms become functionless. Consequently the supply of some of the essential plant nutrients like nitrogen is considerably reduced.

1. Nitrogen: Plant absorbs most of their nitrogen in the form of nitrate of which availability depends on the activity of nitrifying bacteria. The micro- organisms 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 rises above 9.0. Nitrogen fixing bacteria (like Azotobactor) also fail to function below pH 6.0. The decomposition of organic matter which is the primary source of nitrogen is also slowed 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, aluminum, manganese and other bases are present in a soluble state and in more quantity. The phosphates of these elements are formed and become unavailable.

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 K is due to the conversion of exchangeable to non-exchangeable potassium in alkaline soil. Particularly if the alkalinity is due to CaCO3 (brought about by over liming in acid soil), the solubility of soil potassium is depressed.

4. Calcium and magnesium: Acid soils (base unsaturated) are poor in / available calcium and magnesium. In alkaline soil (pH not exceeding 8.5) the availability of Ca and Mg nutrients are always high. When the pH is above 8.5, the availability of these nutrients again decreases.
5 Iron, aluminum and manganese: When the pH is low the solubility of "iron, aluminum and manganese compounds are 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 pH below 5.5 the solubility of these compounds considerably increased with the result that they have a toxic influence on plant growth. Under neutral and alkaline conditions, iron and manganese are usually present in ferric (Fe3+ ) and manganese (Mn++++) states. Hence in soils with pH 7.5 and above, they become unavailable and sometimes produce deficiency diseases like chlorosis in plants.

6. Sulphur: The availability of sulphur is not affected by soil reaction as sulphur compounds are soluble. in low pH range. However, it is more soluble in acid soil and lost in leaching. Acid 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 these nutrients 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 zincates) is reduced. Zinc and copper are adsorbed on the clay colloids and 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.

Definition, Objective and Philosophy of Soil Testing

Soil Testing:

Soil testing is a rapid chemical analysis to access available nutrient status of the soil and includes interpretation, evaluation and fertilizer recommendation based on the result of chemical analysis and other considerations.

Objectives of Soil Testing:

1. Grouping of soil into classes relative to the nutrient level.
2. Predicating the probability of getting a profitable response to the fertilizers.
3. To provide the basis for fertilizer recommendation.
Philosophy of Soil Testing Program:

1. To assess the adequacy of supply of each plant nutrients in the soil based on careful and thorough soil testing program.
2. To test the use of plant nutrients based on following consideration.
a. Application of plant nutrients only if the soil is inadequate or desired crop yield and Maintaince of soil supplies. Plant nutrient use is not recommended for crops on soils that able to supply adequate amount for the desired copy yields.
b. The level of production desired or yield goal. Higher level of production require more nutrient at a given soil supplied than lower level of productions.
3. The predicting capability of soil testing programme is based on
a. Calibration and correlation of soil analysis with crop yield research programme.
b. Thorough and careful collection of soils samples to represent the area being considered.
c. Accurate and reproducible laboratory measurement.

Need for Soil Testing

There are about 20 elements essential for plant growth out of which primary and secondary elements like N, P, K, Ca, Mg, S are involved in major metabolic functions of plants and their deficiency in soil affects crop yields. Soil PH is also one of the crop growth limiting factors which indicates acidic or alkaline soil condition under which growth of plants is restricted. Salt concentration is also one of the plant growth limiting factors.

Soil testing therefore involves analysis of available nutrients like N, P, K , Ca , Mg and S, micro nutrients and based on acidic and alkaline PH conditions lime and gypsum requirement of soil.

For efficient soil testing and better results, the soil testing programme can be divided into different phases viz.

1. Collection and preparation of soil sample.
2. Extraction of available nutrients and their determination.
3. Calibration and interpretation of results.
4. Recommendation of fertilizers based on soil testing results. 

Preparation of Soil Sample

Involve the farmer in collection of soil sample:

A. Division of Field to Collect the Representative Samples:
 
For routine soil fertility testing, first traverse the filed to be sampled. Note variation in slope, colour, texture, management and cropping pattern. Demarcate the filed into uniform patches, each of which may be sample separately.
 
i) Do not Sample Unusual Area:

Avoid areas recently fertilized, old bunds, marshy spots near trees, compost heaps and other non-representative locations.

ii) Use Proper Sampling Tools:

Satisfactory samples can be taken with a soil tube augar spade, trowl, khurpi or pickaxe. Tools should be clean and free from fertilizer contamination.

iii) Use Other Accessories:

Bucket, information sheet, plastic or tarpaulin piece, cloth bag lables.

B) Collection of Soil Samples:

Remove organic debris, rocks and trash from the surface of the soil sampling areas before the sample is collected the soil sample to plough layer. Select the sampling spot in a zigzag manner from about 10-20 places. Collect all the samples in a bucker. Depth of sampling is desired according to the purpose.

1. Soil fertility (15-25 cm)
2. Salinity and alkalinity (1m)
3. Establishment of gardens (2m)
4. Soil survey profile of (1-1.5m ) depth

Keep record of the areas sampled and simple sketch map for reference.

C) Handling and Dispatch of Sample to Laboratories:

After complete collection of soil samples from field the organic residues like tree leaves, twings, dung etc and gravels , stones and other unwanted material should be kept out and sample should be prepared for laboratory analysis by adopting following steps.

1. Drying
2. Grinding
3. Sieving
4. Mixing
5. Partitioning
6. Weighing
7. Storing
8. Labeling

The soil samples should be air dried first and then grinding should be followed. A wooden mortar and pastle should used for grinding to avoid contamination in the soil sample. After grinding soil samples should be sieved with 20-80 mesh sieve and all the samples are then mixed thoroughly by spreading over cloth or paper. From this bulk soil sample one representative soil sample should be collected following quartering method of portioning. About 250 to 500 g of soil sample is sufficient which should be stored in dry and clean poly bags, screw cap jars or card board boxes with proper labeling.

Test for Soil PH

The PH is the negative logarithm of the hydrogen ion activity or the logarithm of the reciprocal of the H ion concentration.

PH = long 10 (H+) = log 10 ( H)-1-

The PH of pure water at 25 0C is 7 while that of acid is below 7 and that of base is above 7.

Significance of Soil PH Measurement:

Plant can grow on different types of soils with wide varieties of properties and wide ranges of PH. However plants do have a preference for a specific range of soil reactions or soil PH. In above figure we have seen different ranges of soil PH like from slightly acidic to very strong acid from slightly alkaline to very strongly alkaline and intermediate of both, neutral.

This soil reaction does have its effects on availability of nutrients and thus on growth of plants. Soil PH has influence on various soil properties like chemical, physical and biological.

Rating Chart for Soil PH:

Less than 6.0: Acidic
7.0 – 8.5: Normal
8.6 – 9.0: Tendency to become alkaline.
Able 9.0: Alkali

Methods PH Measurement:

I. Glass Electrode PH Meter:

The glass electrode generally used for the PH measurement employes a glass membrane of special, chemically pure, soft glass paired with a calomel reference electrode are used for measuring the soil PH.

II. Hydrogen Electrode:

In order to measure hydrogen ion concentrations of pH of a solution use of hydrogen gas as a metal electrode and dip in a solution under test and measure the electrode potential.

III. Quinhydrone Electrode:

It consists of equimolecular mixture of quinone and hydroquinone added directly to the solution under test. This electrode does not give correct reading in alkaline solution for PH grater than 9.0.

Measurement of Electric Conductivity

Principle of Salt Bridge:

In salt bridge 2 fixed the resistance R1 and R2 and variable resistance Rv are connected in a branched circuit with the conductance cell having resistance Rx. An AC potential is applied at C and D. An AC potential is employed to prevent electrolysis of the solutions and polarization of electrode in the conductance at Rx. Ordinarily 1000 cycle source. As the frequency arises, capacitance effects become important and are compared for which the variable condenser in paralledl wih R2. the variable resistance by Rv is adjusted until there is no current pain the phone circuit from A to B as indicated by a minimum shadow or shift in the electric eyes.

There A and B are at the same potential and the voltage drop. IRX between BD must b equal to I’R2 the voltage drop between Ad.

IRx = I’R2

Also, Irv= I/R

IRx/Irv= I’R2/ IR1,

Hence,

Rx= I’R2/I’R1 x Irv.

Since R1 and R2 are fixed, the dial on Rv can be calibrated to read Rx the resistance of the test sample or the dial on Rv can be calibrated to read I/Rx that is directly in conductance of the solution.

% salts in soil= EC (mmhos/cm) X 0.064

Acidic Soils

Soils having PH less than 6 are categorised as acid soils. These soils can be defined as those having PH less than 5.5 in 1:1 soil water extract. More precisely a soil which is acid in reaction throughout the root zone are called as acid soils. Such types of soils have more H+ concentration than OH- in the soil solution.

A. Why Soils are Acidic:

There are various factors which influence formation of acid soils.

1. Panant Material:

Rocks like granite and rhyolite are acid rocks and on disintegration and decomposition of these rocks results in accumulation of acid soil material.

2. Climate:

In heavy rainfall areas continuous leaching process removes most of CaCO3 and gypsum from soil giving rise to increased soil acidity.

3. Organic Matter:

The carboxylic, enolic and phenolic groups present in organic matter debris dissociate releasing H+ thus contributes towards soil acidity.

4. Root Biomass and Soil Organisms:

Respiration by plant roots and soil micro organisms are major contributing factors to soil acidity . CO2 liberated during respiration reacts with water to produce carbolic acid which in turn breaks down to release H+.

CO2 + H2O----> H2CO3 + H+

On the other hand when plants exchange cations with external medium they give an equivalent number of H+ and thus contribute to soil acidity.

Soil Acidity Reactions:

Contribution of H+ in formation of acid soils.

1. CO2 from decomposition organic matter
CO2 + HOH à HCO3 + H+

2. Ammonia fertilizers are oxidized by the bacteria to form
NH4 + 2O2 à NO3 + H2O + 2H+

3. Aluminium ion in soil solution
Al ( H2O)6 +++ à Al ( H2O)5 OH++ + H+ ( PH bout 4.0)
Al ( H2O)5 OH)++ à Al ( H2O)4 ( OH) + H+ ( PH at 5.0)

4. Sulphur : as an ingredient in some fungicides and fertilizers.

Soil Test Summaries

The purpose of soil testing is to give the individual growers dependable information regarding the fertilizer and lime needs of their field. However, this may be not feasible to have soil test values fertilizer of each grower. Better fertilizer recommendation may be attained from the tested fields by making average recommendations.

To accomplish this purpose, summaries of the results of soil test may be made. A summary map will serve to alert agricultural workers and industry personal to the individual problems of the state. 

Objectives of Soil Test Crop Response Programme

1. To study the relationship between soil test values for available N, P, and K and yield response to importance crops.

2. To derive yield tat getting equations for important, crops for making fertilizer recommendations.

3. To evaluate various soil test method for their suitability under field conditions.

4. To evaluate the extent to which fertilizer needs of crop can be reduced in relation with conjunctive use of organic manure.

Irrigation Water

Chemical Composition of Irrigation Water:

1. Sources of irrigation water:
a. Rain water: Lowest salt content of all types of water.
                                                        
2. Surface water:
a. River and canles, (flowing water)
b. Tank (Stagnant water)
c. Ground Water:

The salt water content of ground water is dependent on the source of water and on the course overs which it flows.

d. Sea water:  

Sea water after its suitable dilution should be used for irrigation on sands to raise salt to tolerant crops.

2. The nature and amount of salts present in natural waters are effected by the following factors.

a. Physio-chemical characteristics of the rocks with which water is coming in contact.
b. Geological history of the area.
c. Climate of the area including duration and frequency of rainfall, humidity, intensity and duration of sunshine wind velocity, etc.
d. Microorganisms
e. Chemical, physical and mineralogical characteristics of the soil with which water comes into contact when it percolates into the underground reservoirs.
f. Topography of the area
g. Plant and vegetation cover
h. Ground water mixing.
i. Human interference in brining about micro changes in the hydrological cycles, by discharging industrial effluents, use of fertilizers and chemicals etc.

Collection of Irrigation Water Samples:

The minimum quantity of water required for ordinary chemically analysis is about 2L. care should be taken to obtain a representative sample. Satisfactory water samples can be obtained by mixing several portions collected at different times. Samples from wells should be collected after the pump has been running for some time and sample from stream should be taken from the running water.

In general shorter the elapsed time between collection and analysis of a sample, the more reliable will be the analytical data.

Description of location, village, taluka, district, etc will be noted while collecting the water sample.

Water Quality Criteria for Irrigation:

The quantity of irrigation, water is judged by the following characteristics.

A. Total salt concentration in terms of EC or salinity.

The salinity of water refers to the concentration of calcium ( Ca2+) , Magnesium ( mg) , sodium ( Na) and potassium ( K) salts of chloride (Cl) , Sulphate (SO4) , Bicarbonate ( HCO3) , carbonates ( CO3) and nitrates ( NO3). These are the major salts in surface and groundwater. It is expressed as dSm-1 or millimphos /cm.

B. Relative proportions of cations or anions:

a. Sodium Adsorption Ratio (SAR):

A high sodium in irrigation water leads to development of sodic soils. A simple method of evaluating danger of high sodium water is the sodium adsorption ratio. An increase in SAR of irrigation water increases the SAR of the soil solution and thus ultimately increases the Exchangeable Sodium Percentage ( ESP ) of the soil.

SAR= Na+
         _________
          >----------
             Ca+ Mg/2

The concentration are expressed in me/L.

b. Calcium / Magnesium Ratio:

In SAR equation, calcium and magnesium have been treated equal. Recent studies have shown that at the same level of salinity and SAR water with higher Mg/Ca ratio ( greater than one) cause more deteriorate in physical properties and induces more ESI in irrigated soils.

c. Residual Sodium Carbonate:
   
The presence of carbonates and bi-carbonates in irrigation water decreases the concentration of calcium and magnesium by precipitating them and thus ultimately concentration of the sodium increases in irrigation water.

It is calculated as: RSC( me/L) = CO3 Square + HCO3-) – (Ca2+ Mg2+)

Concentration in me/L.

C. Concentration of Certain Specific Elements:

Elements such as boron etc toxic to the plant growth beyond a concentration of few mg/L. Boron above 3 mg/ L and nitrate above 10 mg/L are harmful.

Use of Care of the Instrument in Soil Testing Lab

Care of Instrument:
   
Dust is harmful to electronic circuit, keep instrument tightly covered when not in use. Moisture is severe enemy of electric circuits. During period of high humidity, instruments should be turned daily, even if no analysis is performed. They must be enclosed in a plastic case with a 40 to 60 watt electric bulb burning to reduce humidity or enclosed with an open bottle of fused calcium chloride or similar dehydrating agent.

If chemical solutions are spilled on an instrument, when a test tube breaks in a calorimeter turn the instrument off and wash immediately and thoroughly with distilled water.

Equipment Maintenance:

An instrument that is not functioning properly will result in wither enormous results or no results at all. Proper equipments is essential to avoid or at lest mainmast such difficulties. Check your instrument regularly. Establish and follow a rigorous maintenance schedule.

Conductivity Bridge:

Conductivity bridge ordinarily do not require repairs. When difficulty is encouraged the defects often lies with the conductivity cells. Periodically inspection of the cells for the following and necessary corrections or replacement of the cell should normally take care of most of the troubles.

Photo Electro Colorimeter:

Problems:
    
The loose contact, burnt lamp, loose connection in photo cell, fluctuations in voltage defective photo cells are some of the cause of fluctuating readings.

Remedy:

To loose contact the burnt bulb or stabilizing voltage with the use voltage stabilizer can be done by following instructions. For measure internal defects, get instrument serviced.

Points to be remembered in making colourimeter measurements :

1. Use matched test tube
2. Observe cleanliness scrupulously
3. Do not handle the lower half portion of test solution before taking measurement
4. Always rinse the test tube with the portions of test solution before taking measurement.
5. Avoid scratching of the test tubes especially on the lower half of the test tube.
6. For accurate work a std, curve should be checked if necessary.

Flame Photometer:

For more accurate readings on flame photometer following may be kept in mind.

1. Std curve should be drawn at ease time
2. Run distilled water before and after testing each solution.
3. Do not use concentrated solution.
4. Do not light flame when the optical filter is not in position. Also do not change the filter when the flame is on.
5. Turbid extracts or solution should not be used.
6. Add few dop drops of isobutyl alcohol to improve the spray ability of the sample solution or extract.

Problems:

Unsteady flame, jumping flame, and flame blows off, blocking of suction capillary tube and atomize folts in electric controls, loose contact etc.

Remedies:

Use new gas cylinder with adequate pressure, suction capillaries be cleaned , loose contact be checked.