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The soil vs. a soil- If &quot;the soil&quot; is thought of like a forest, &quot;a soil&quot; would be an individual tree in that forest. The same applies to soil.
An average soil is composed of mineral matter, organic matter, and pore space, which may be occupied by air and/or water. The percentage of these four components can vary depending on how and where the soils were formed. 50% solids and 50% pore space; obviously, these are mixed in a natural environment and fluctuate greatly throughout the year.
Soil pH is a measure of the hydrogen ion activity and is expressed on a scale of 1 to 14. Soil pH can sometimes be confusing the way it is expressed. Many growers associate a one unit change in pH with very little change in soil acidity. They fail to recognize that soil pH is expressed on a logarithmic scale, and that a one unit change in pH is actually a 10X change in hydrogen ion concentration.
General relationship of plant health and soil pH. Some nutrients can reach toxic levels at low pHs, while deficiencies can occur at high pHs.
Organic matter affects the buffering capacity of the soil (more SOM-resistance to change pH) Soil parent materials such as basalt have higher pH levels than those of parent materials from granite. High temperature and moisture leach bases from soil (NW to SE tends to have more acid soils). Agricultural practices tend to lower pH crops removing nutrients leaching nutrients through soil decomposition of organic materials fertilization, particularly ammonium fertilizers Liming materials raise soil pH (refer to Agronomy Technical Note No. 8 on Liming).
Bulk density is simply dry weight of soil divided by total volume of soil.
Soils with a high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space. Sandy soils have bulk densities that are commonly higher than in the finer-textured soils such as clay loams and clays. This fact may seem counterintuitive at first because sandy soils are commonly referred to as ‘light’ soils, while clays are commonly referred to as ‘heavy’ soils. In this case, ‘light’ and ‘heavy’ do not refer to bulk density, but rather the amount of effort required for tillage.
There are 12 basic textural classes found on the texture triangle. Each class can determine fertility, ease of tillage, droughtiness, and general productivity. There are two widely acceptable ways to determine soil texture. The first is determining texture by feel. The second method requires some laboratory equipment, and is referred to the hydrometer method. The hydrometer method separates particles by their rate of settling in water. The texture of a soil can be found by knowing (or estimating) the percentage of two of the three components, usually clay and sand in the hydrometer method.
There are many different types of soil structure, but the most common are granular, platy, blocky, prismatic, and massive. Granular is normally found in the A horizon. Platy is sometimes found at the base of the A horizon, or near the top of the B horizon. Blocky and prismatic are most common in the B horizon and massive is usually associated with the C horizon. A common question to consider for agricultural interests is ‘How is structure formed?’ The largest component is microbial exudates. Therefore, additions of organic matter from residue or manure will in theory lead to formation of stable peds. Granular, platy, prismatic, columnar, and block are all generically referred to as peds. Single grain and massive are understood to be structureless.
Mollisols: *characterized by a thick, dark profile *predominant soils under grassland or prairie native vegetation *considered to be some of the most naturally fertile soils in the world *~25% of land in the U.S. is classified as mollisol (most in the world) *mollisols are dominant Illinois and Iowa; but they also dominate northern Argentina and southern Brazil. *the former USSR has vast amounts of soil classified as mollisols Alfisols: *characterized by an argillic (overlaying layer has deposited clays) Bt horizon *~14% of soils in the U.S. are classified as alfisols *dominates Ohio
Soils have chemical, biological, and physical properties that interact in a complex way to give a soil its quality or capacity to function. An indicator is an indirect measure of soil quality or soil function. They are not measures of soil quality, but collectively tell us whether the soil is functioning normally. Physical properties - infiltration, structure, aggregate stability, porosity, and bulk density. Chemical properties - salinity, soil pH, cation exchange capacity, plant nutrients, organic matter content, etc. Biological properties - earthworm numbers, and other animal activities that are important in soil development and fertility, soil respiration, pitfall trapping, microbial biomass, cotton strip assay, soil food web structure, etc. Organic Matter (OM) has physical, chemical, and biological properties. It is considered part of all three groups.
List of most commonly used physical soil quality indicators. Each will be covered in detail the slides to follow. All of these indicators are interrelated. What affects one, or by changing one, could affect one or more of the others. As individual indicators are discussed you will see this interaction. Last 4 specific to water movement. All important for root growth, aeration, erosion protection.
List of chemical indicators that will be covered in more detail in this section. Organized by general category: Optimize pH, nutrients Maximize CEC (OM) Minimize CaCO3, Na, contaminants
The importance of soil biology has long been recognized. Biological systems are highly sensitive and a change in the biological status of the soil may provide us with an early warning of potential environmental damage. The balance of the entire community perform many functions that affect agroecosystem health. But this web of complex interactions is very difficult to measure directly. Recently sophisticated methods of assessment been developed. However, in many cases their interpretation remains problematic. Other indicators: Cotton strip assay Arthropod Pitfall trapping Microbial quotient--microbial biomass divided by the soil respiration rate
Organic matter - Critical in soils since it contributes to infiltration, AWC, CEC, soil structure, and provides energy source for microorganisms, which are important in decomposition of OM and release of nutrients needed for crops and overall plant growth and aggregation of soils. Because OM is important to CEC as is clay; OM becomes very important in sandy soils since they are inherently low in clay. Hence, CEC in sandy soils may come almost entirely from the OM content.
Principles of soil science
Soil-111 : Principles of Soil Science
Dr. Pabitra Kumar Mani
Agril. Chemistry and Soil Science,
What is Soil?What is Soil?
Soil: Unconsolidated mineral or material on the
surface of the earth resulting from and influenced
by time, parent material, climate, organisms, and
Not all soil is created equal,Not all soil is created equal,
‘‘the soil’ vs. a soil.the soil’ vs. a soil.
The soil is the link between the rock core of the
earth and the living things on its surface. It is the
foothold for the plants we grow.
Soil refers to the weathered and fragmented outer
layer of the earth’s terrestrial surface. (Hillel, 1982)
• Natural Body that Occurs on the Land Surface
that are Characterized by One or More of the
– Consists of Distinct Horizons or Layers
– The Ability to Support Rooted Plants in a Natural
– Upper Limit is Air or Shallow Water
– Lower Limit is Bedrock or Limit of Biological
– Classification based on a typical depth of 2 m or
approximately 6.0 feet
The pedosphere: abiotic
and biotic entities
interacting in the soil
Better knowledge of the soils on which this road was built may have
allowed its engineers to develop a more stable design, thus avoiding
this costly and dangerous situation. (R. Weil)
Transcosna Grain Elevator,
Canada (Oct. 18, 1913)
Shear Failure under Foundation load
It deals with soil formation, soil genesis, soil classification
and description of soil properties.
A pedologist studies, examines, and classifies soils as they
occur in their natural environment.
Gr. Pedos, “ground,” and
original formed as Russian,
A pedon is the smallest volume of soil that
should be recognized as a soil individual
The range of properties observed in
several similar pedons, a polypedon are
aggregated to define a soil individual.
A soil profile is a vertical cross section
of a soil. It is divided into a number of
distinct layers, referred to as horizons.
The horizons are normally
designated by symbols and letters.
The presence or absence of
particular horizons allows pedologists
to classify the soil.
In addition, the organic or O horizon
can form above the mineral soil
commonly in forested areas,
resulting from the dead plant and
The basic unit of study: Soil Profiles
upper or A
middle or B
lower or C
important to different
soil functions and
is most likely to be
obtained by studying
different layers of the
The Regolith – What ?
• The Regolith is the Unconsolidated
material overlying Rock.
The Overburden !
• This material may be rather thin to
hundreds of feet thick and can
include material dislodged or
weathered from local rock,
transported to the area by wind,
water, ice, or gravity.
Image Source: http:///soils.ag.uidaho.edu
The Soil or Solum
• The Soil or Solum is the portion of
the Regolith that has been
influenced by the 5 Soil Formation
• The processes are controlled by
Time, Climate, Topography
(Landform and Position),
Organisms, and Parent Material.
• Formation Process are in 4 Broad
Categories (additions, losses,
transformations)- Genetic Soil
Forming Processes.Rock – R Horizon
Young Soil Old Soil
Well Defined Horizons
High Degree of
Poorly Defined Horizons
Low Degree of Weathering
Organic residues humus
ions & OM
Loss of gases
, N 2
Soil forming processes
Adapted from Stewart (1990)
refined the ideas of Russian
scientist V.V. Dokuchaev
into the following equation:
5 factors of soil formation
Soil = f (Cl, O, R, P, T…)
Study of soil in relation to growth, nutrition and
yield crops or plants.
Edaphologists consider the various properties of soils in
relation to plant production
Average Soil Composition
25% Air 5% Organic Matter
Soil pH - a “master variable”Soil pH - a “master variable”
• A measure of the hydrogen (H+
• One pH unit change = a ten fold
change in acidity or alkalinity
Why called “master variable”?Why called “master variable”?
• soil pH controls:
1) soil microbe activity
2) nutrient exchanges
3) nutrient availability
4) gaseous exchanges
5) chemical degradation
Possible pH Ranges Under Natural Soil Conditions
black walnut: 6.0-8.0
white pine: 4.5-6.0
Strong Moderate Slight Slight Moderate Strong
3 4 5 6 7 8 9 10 111 2 12 13 14
Most agricultural soils
Extreme pH range for most mineral soils
4.0 5.0 6.0 7.0 8.0
Relativeplantyield(%) (Weil and Kroontje,1984)
General relationship of plant health and soil pH. Some nutrients can reach toxic
levels at low pHs, while deficiencies can occur at high pHs.
Soren P.L. Sorensen invented
the pH scale in 1909.
[ ][ ]
[ ] [ ]
This form of ionisation constant equation is called the
Henderson- Hasselbalch equation.
pH of an weak acid
When one metal is brought in contact with another, a voltage
difference occurs due to their differences in electron mobility.
When a metal is brought in contact with a solution of salts or
acids, a similar electric potential is caused, which has led to the invention
Similarly, an electric potential develops when one liquid is
brought in contact with another one, but a membrane is needed to keep
such liquids apart.
A pH meter measures essentially the electro-chemical potential
between a known liquid inside the glass electrode (membrane) and an
unknown liquid outside. Because the thin glass bulb allows mainly the
agile and small hydrogen ions to interact with the glass, the glass electrode
measures the electro-chemical potential of hydrogen ions or the
potential of hydrogen.
To complete the electrical circuit, also a reference electrode is
needed. Note that the instrument does not measure a current but only an
electrical voltage, yet a small leakage of ions from the reference
electrode is needed, forming a conducting bridge to the glass electrode.
The majority of pH
combination electrodes that
both glass H+
placed in one housing.
In principle it should be possible to determine the H+
ion activity or concn. of a soln by
measuring the potential of a Hydrogen electrode inserted in the given soln. The
EMF of a cell, free from liquid junction potential, consisting of a Hydrogen
electrode and a reference electrode, should be given by,
E = E ref – RT/F ln aH+ F= Faraday constant ,96,485
E = E ref + 2.303 RT/F pH R= 8.314 J/mol/°K
∴pH = ( E- Eref )F/2.303 RT T= Kelvin scale
So, by measuring the EMF of the Cell E obtained by combining the H electrode with a
reference electrode of known potential, Eref , the pH of the soln. may be evaluated.
The electric potential at any point is defined as the work done in bringing a unit
charge from infinity to the particular point
Reduced state Oxidised state + n Electron⇋
M = Mn+
E(+) = E0 – (RT/F) ln aM
Principles of pH Meter
Factors Influencing pH
Soil Parent Material (carbonates, etc.)
Temperature and Precipitation
Organic matter affects the buffering capacity of the soil
(more SOM-resistance to change pH)
Soil parent materials such as basalt have higher pH levels than those of parent
materials from granite.
High temperature and moisture leach bases from soil .
Agricultural practices tend to lower pHAgricultural practices tend to lower pH
•crops removing nutrients
• leaching nutrients through soil;
•decomposition of organic materials;
• fertilization, particularly ammonium fertilizers;
•Liming materials raise soil pH
Soil Fertility: it is the potential of the earth or inherent
capacity of the soil to supply plant nutrients in quantity,
forms and proportion required for the growth and
development of the crop.
Fertility is measured by the amount of chemical
elements or compounds required for plant growth
Productivity of a soil is defined as its capacity to
produce plants under specified programme of management.
It is measured by the yield of the crop per unit area of
Fertility is one of the factors of soil productivity. Sometimes a
soil may be fertile but may not be productive.
This is the most wide spread
soil in india.This soil is formed
by the deposition of
materials by the rivers namely
Indus,the Ganga, the
Brahmputra. It is mainly found
in Northern Plains and
Eastern Coastal Plains. It is
the most fertile soil. Due to
high fertility, they are
intensively cultivated and are
This soil is black in colour and is
best for growing cotton and
sugarcane. This soil is formed by
the weathering up of igneous
rocks.It is rich in minerals like iron
and magnesium. It is mainly
confined to the north western part
of peninsular plateau, Deccan lava
plateau which includes the states of
Maharashtra, Saurashtra, Malwa,
Madhya Pradesh and Chhattisgarh.
These soils are formed by the
intense leaching in tropical
regions where the rainfall is
high. This soil is devoid of
nutrients and has low humus
content and is not suitable for
crop cultivation. It is mainly
found in Karnatka, Kerala,
TamilNadu,Madhya Pradesh and
hilly areas of Orissa and Assam.
Through soil conservation
techniques they are useful for
growing tea and coffee.
Desert Arid Soils
These are sandy,porous
and saline in nature. They
lack humus and moisture.
Hence they are not
suitable for cultivation
except in areas where
irrigation is available. This
soil is mainly found in
Rajasthan and some parts
Red soil develops on
crystalline igneous rocks
in areas of low rainfall in
the eastern and south
eastern parts of the
Deccan Plateau. This soil
is rich in iron compounds
and is reddish in colour
while it is yellowish in
colour when it occurs in
Pedon.—The pedon is presented in Soil Taxonomy (Soil Survey Staff, 1975) as a unit of
sampling within a soil. The limits on the area of a pedon establish rules for deciding whethe
consider one or two or more kinds of soil within a small-scale pattern of local lateral variabi
A pedon is regarded as the smallest body of one kind of soil large enough to represent the
nature and arrangement of horizons and variability in the other properties that are preserve
A pedon extends down to the lower limit of a soil. It extends through all genetic horizons an
the genetic horizons are thin, into the upper part of the underlying material. The pedon
includes the rooting zone of most native perennial plants. For purposes of most soil survey
practical lower limit of the pedon is bedrock or a depth of about 2 m, whichever is shallowe
depth of 2 m provides a good sample of major soil horizons, even in thick soil. It includes m
of the volume of soil penetrated by plant roots, and it permits reliable observations of soil
The surface of a pedon is roughly polygonal and ranges from 1 m2 to 10 m2 in area,
depending on the nature of the variability in the soil. Where the cycle of variations is less th
2 m and all horizons are continuous and nearly uniform in thickness, the pedon has an are
approximately 1 m2. Where horizons or other properties are intermittent or cyclic over an
interval of 2 to 7 m, the pedon includes one-half of the cycle (1 to 3 1/2 m). If horizons are
cyclic over an interval greater than 7 m, each cycle is considered to contain more than one
soil. The range in size, 1 to 10 m2, permits consistent classification by different observers
where important horizons are cyclic or repeatedly interrupted over short distances.
Polypedon.—The pedon is considered too small to exhibit more extensive features, such
Bulk Density DeterminationBulk Density Determination
For our example, let’s
assume we have 1 cubic
centimeter of soil that
weighs 1.33 grams
Soil is made of
To calculate Bulk Density:
Volume = 1 cm3
Weight = 1.33 grams
Bulk Density =
Weight of Soil
Volume of Soil
Bulk Density =
Bulk Density = 1.33 grams/cm3
Porosity value generally
ranges from 0.3 to 0.6 (30–
In clayey soils, the porosity is
highly variable because the soi
alternately swells, shrinks,
compacts, and cracks
it ranges between 0.3 and 2.
Comparison of Coarse Textured and Fine Textured Soils
Coarse Textured Soil
Less porespace but
Fine Textured Soil
More total porespace
Texture and Pore SpaceTexture and Pore Space
Soil StructureSoil Structure
• Soil structure is the combination or
arrangement of primary soil particles into
• The way soil particles are arranged to
form stable aggregates
• Compare this to clods, which are caused
by disturbance (plowing or digging)
• Compaction – results from implement
traffic, stable soil aggregates are broken
Common Types of SoilCommon Types of Soil
Common to Ohio soils just
below Ap (> 8”)
Deeper in profile (>3-4’)
. . . more on Soil Horizons. . . more on Soil Horizons
A black Leaching zone: roots here (topsoil)
B “red” Accumulating zone: nutrients (subsoil)
C rocky Weathered bedrock
D solid Bedrock
Black color is caused by organic particles Prevented by:
• Proper cultivation
• Perennial planting
Rotting material makes humus particles
• Improve drainage (air capacity)
• Increase water holding capacity
• Increase nutrient holding capacity leaves, grass clippings,
compost, green manure, horse
Typical electrode system for
measuring pH. (a) Glass
electrode (indicator) and
immersed in a solution of
(b) Combination probe
consisting of both an
indicator glass electrode
and a silver/silver chloride
reference. A second
electrode serves as the
internal reference for the
The two electrodes are
arranged concentrically with the internal reference in the center and the
external reference outside. The reference makes contact with the analyte
solution through the glass frit or other suitable porous medium.
Combination probes are the most common configuration of glass electrode
and reference for measuring pH.
Soil is a natural body consisting of layers (horizons) of mineral and/or organic
constituents of variable thicknesses, which differ from the parent materials in
their morphological, physical, chemical, and mineralogical properties and their
Built entirely from natural, locally occurring materials, this typical rural Tanzanian
house has soil walls made by daubing mud onto a framework of sticks. Both its soil
walls and grass thatch roof are shown in the process of being refurbished.
For scale, note man standing left of the house. (Photo courtesy of R. Weil)
Shear Strength of Soils
Dr. Attaullah Shah
Shear strength of a soil is the resistance to deformation by
continuous shear displacement of soil particles due to
tangential (shear) stress.
Climate and Soil Development
Image Source: University of Wisconsin, 2002
There are 5 main factors that influenceThere are 5 main factors that influence
soil formation processes.soil formation processes.
1. Type of Parent Material1. Type of Parent Material
2. Climate2. Climate
3. Topography3. Topography
4. Biotic Components4. Biotic Components
5. Time5. Time
Soil ProfileSoil Profile
What do we see?
• organic matter - surface soil isorganic matter - surface soil is
darker due to organic matterdarker due to organic matter
• iron oxides - subsoil hasiron oxides - subsoil has
brighter browns and tans duebrighter browns and tans due
to iron oxidesto iron oxides
• horizons - layers of differenthorizons - layers of different
color or texture; formed fromcolor or texture; formed from
the top downthe top down
Soil Quality Indicators
Biological Soil Quality Indicators
The “Big Picture:” the Soil Food
Soil Organic Matter
Considered the single most importantConsidered the single most important
indicator of soil qualityindicator of soil quality Organic matter - Critical in
soils since it contributes to
infiltration, AWC, CEC, soil
structure, and provides
energy source for
microorganisms, which are
important in decomposition
of OM and release of
nutrients needed for crops
and overall plant growth and
aggregation of soils.
Because OM is important to
CEC as is clay; OM becomes
very important in sandy soils
since they are inherently low
Pedon:: PedPed - Ground,- Ground, LogosLogos - Discourse/ Science- Discourse/ Science
Pedology-- The scienceThe science that deals with formation,that deals with formation,
morphology and classification of soilmorphology and classification of soil
bodies as land scape componentbodies as land scape component
Pedogenesis--The natural process of soil FormationThe natural process of soil Formation
Rock --------------Parent Material
Pedogenic processes are
Parent Material Soil
extremely complex involving
many physical, chemical &
Different processes or their combinations
Parent Material Soils
(Same type) depends on variation in natural environment
Parent Material Effect of same natural Soils
Different types Environment of different
This is the material from which the soil has developed and can vary
from solid rock to deposits like alluvium and boulder clay. It has been
defined as ‘the initial state of the soil system’.