Soil Physics 477
Manoj K. Shukla
Agronomy and Horticulture
 Introductory remarks on Soil Physics
 Soil Mechanics
 Soil properties, definitions, soil structure, surface tension, viscosity
 Soil Hydrology- soil water, soil water potential, Darcy's law
 Saturated/unsaturated flow through soil
 Water infiltration into soil
 Evaporation, evapotranspiration
Guest Lectures
 Soil aeration, gas exchange
Field Visit
 Heat flow and soil temperature
 Solute transport
Five laboratory practicals: soil bulk density, particle size distribution,
saturated hydraulic conductivity, soil-water characteristic and solute
transport
“Soil physics is just not an academic exercise. It
involves applications for understanding present
critical issues as food security, drinking water,
pollution of waters, contamination of soils, air
pollution, natural disasters as flooding and
landslides …..”
-Don Nielsen, Dean UCDavis
Precipitation
Evaporation
Soil-Air
Interface
Vadose Zone
Portion of aquifer where
pore spaces are occupied
with water and air
(unsaturated zone)
Applications of soil physics are crucial to sustainable use
of natural resources for agricultural and other land uses
Soil-Water
Interface
Ground water
Capillary fringe zone
Interaction of soil physics with basic and applied sciences
Applications of soil physics to environment quality
Soil physical properties
and processes
Greenhouse Effect:
- Gaseous efflux of
CO2, CH4, NOx
- C sequestration
aggregation
Quality of
Life
Air quality
Particulate
matter in air:
- Wind erosion
- Blowing salt
Environmental
Soil Physics
Soil Physics
and
Environment
Quality
Acid Rain:
- Water quality
- Vegetation
cover
- Biodiversity
Soil quality
Water quality
Fresh water
resources and
quality:
- Suspended and
dissolved loads
- Biological and
chemical O2 demand
- Pathogens
Soil buffers and
filters pollutants
out of environment
Soil properties are highly variable at
multiple scales
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Molecules
Particles or Pore
Aggregate
Column or Horizon
Field or Watershed
Regional
Pedosphere
Soil
(i) The unconsolidated mineral or organic material on the immediate
surface of the earth that serves as a natural medium for the growth
of plants.
(ii) The unconsolidated mineral or organic matter on the surface of the
earth that has been subjected to and shows effects of genetic and
environmental factors of: climate (including water and temperature
effects), and macro- and microorganisms, conditioned by relief,
acting on parent material over a period of time. A product-soil
differs from the material from which it is derived in many physical,
chemical, biological, and morphological properties and
characteristics.
Soil Genesis:
The mode of origin of the soil with special reference to the
processes or soil-forming factors responsible for development of
the solum, or true soil, from unconsolidated parent material.
According to Jenny (1941) soil is a
f (climate, organisms, relief, parent material, time)
Therefore, similar soil forming factors produce similar types of soils.
Soil Classification is generally done to provide people (e.g.,
scientists, growers, and resource managers) with the information
about the nature and properties of a soil found in a particular location.
The principles of Soil Taxonomy are: to classify soils on the basis of
properties, which are readily observable or measurable and should
either affect soil genesis or result from soil genesis.
Curtis F. Marbut (1930)
NRCS: 11 soil orders:
oxisols, aridsols, mollisols, alfisols, ultisols, spodsols, entisols,
inceptisols, vertisols, histosols, and andisols.
, water
OM
water
Mineral
Matter
Air
www.seafriends.org.nz/ enviro/soil/soil22.gif
Definitions
Soil Physics:
• study of soil physical properties and processes, their
interactions with one another and the environment,
spatial temporal variations in relation to the natural,
anthropogenic or management factors
• Application of principles of physics for understanding the
dynamic interactions between mass and energy status of
components (inorganic, organic) and phases (liquid,
solid, gas)
Soil Density:
ratio of mass and volume
•
•
•
•
Particle density (rs)
Bulk density (wet and dry) (rb)
Relative density or specific gravity (Gs)
Dry specific volume (Vb)
Soil Mapping: Cartographic representation of actually occurring soil
pedons or polypedons
Pedon: A three-dimensional soil matrix where horizons shape and
relations can be studied
Polypedons: A group of contiguous similar pedons
Map unit: A group of areas uniquely identified on a soil map. It
consists of a collection of polypedons
Soil map: A map showing the distribution and locations of a map
unit in relation to the prominent geographical, physical and cultural
features
Reconnaissance map: A map containing some areas or features
shown in greater detail than usual
Consociations: mapped areas consist of similar soils or are under a
single soil texon
Taxadjuncts: the properties are outside the range of a recognized soil
series
Soil taxonomy and Soil mapping units: Fundamentally different
Soil texa: grouping of soil properties for the purpose of classification
A soil mapping unit: pictorial representation of a pedon or polypedons
actually occurring in an area.
Soil Solids
(i) Inorganic (> 95%)
(ii) Organic
Soil is a storehouse of water and nutrients (N,P,K, Ca,
Mg, Zn, Cu etc)
Buffering -ability to withstand or adapt to sudden change
Filtering -ability to leach out pollutants
Inorganic Component
Primary Particles
Discrete units;
cannot be further
subdivided;
also known as soil
separates
sand, silt, clay
Secondary Particles
Consist of primary
particles; can be
further subdivided
into its separates
Particle size distribution
Texture
Quantitative
Qualitative – based
measure of particle
on feel method
size constituting
-coarse, gritty, fine,
the solid fraction
smooth
Particle size is important soil physical properties:
Total porosity, pore size, and surface area
Systems of Classification
1. United States Department of Agriculture (USDA)
2. International Society of Soil Science (ISSS)
3. American Society of testing materials (ASTM)
4. Massachusetts Institute of Technology (MIT)
5. US Public Road Administration (USPRA)
6. British Standard Institute (BSI)
7. German Standard (DIN)
USDA System
Soil separate
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Silt
Clay
Size range (mm)
2.00 - 1.00
1.00 - 0.50
0.50 - 0.25
0.25 - 0.10
0.10 - 0.05
0.05 - 0.002
< 0.002
ISSS System
Soil separate
Coarse sand
Fine sand
Silt
Clay
D > 2 mm is known as nonsoil or skeletal fraction
Size range (mm)
2.00 - 0.20
0.20 - 0.02
0.02 - 0.002
< 0.002
Sand – mostly quartz, feldspar and mica (fragments)
traces of heavy metal, low surface area
Silt – mineralogical composition is similar to sand,
intermediate surface area
Clay – reactive fraction of soil, colloidal, large surface
area, high charge density
Soil Separates
Property
Sand
Silt
Clay
Size
Shape
Feel
Plasticity
Cohesion
Surface area
Mineralogy
2-0.02 mm
jagged
gritty
not plastic
not cohesive
very low
primary
0.02-0.002 mm
slightly irregular
smooth, floury
slightly plastic
slightly cohesive
moderate
primary minerals
Heat of wetting
Secondary particles
Water holding
Capacity
Hardness
none
no
none/slight
minimal
few
moderate
<0.002 mm
platy/tube like
sticky
plastic
cohesive, gelatinous
very high
secondary clay
minerals
high
forms aggregates
high, hygroscopic
5.5-7 (on mhos
scale)
none
5.5-7.0
--
very low
high to very high
Ion exchange
capacity
Clay
Alumino-silicate
Secondary clay minerals
Also contain: Fine particles of
 Iron Oxide Fe2O3
 Aluminum Oxide Al2O3
 Calcium Carbonate CaCO3
 Other salts
Important properties of clay fraction
1. Easy hydration because of high affinity to water
2. High swell/shrink capacity because of expanding
nature of clay lattice
3. High plasticity as it can retain shape when moist
4. Develops cracks when shrinks
5. Forms a cake when swells (cohesive forces)
6. High density of negative charge, which leads to the
formation of electrostatic double layer when fully
hydrated
Process of determination of particle size fractions is
known mechanical analysis
Dispersion
Fractionation
Dispersion is removal of cementing materials to break
secondary particles into primary
Cementing Material
Dispersing Agent
Organic matter
Hydrogen peroxide (H2O2)
Oxides of Fe and Al
Oxalic acid, sodium sulfide
Electrolytes
Leaching with dilute acids
Cohesion/adhesion
Rehydration by boiling in H2O, shaking,
titration, ultrasound vibration
Fractionation is the process of physically separating the
particles into different size fractions
Methods of fractionation
Approximate size range (mm)
Sieving
Sedimentation
Optical Microscope
Gravity sedimentation
Permeability
Gas absorption
Electron microscope
Elutriation
Centrifugal sedimentation
Turbidimetry
100.0 - 0.05
2.0 - < 0.002
1.0 - 0.001
0.1 - 0.0005
0.1 - 0.0001
0.1 - 0.0001
0.005 - 0.00001
0.05 - 0.005
0.01 - 0.00005
0.005 - 0.00005
Sieving or Direct sieving:
Dispersed soil suspension is passed through a nest of
sieves of different seizes:
2 mm, 1mm, 0.5 mm, 0.25 mm, 0.10 mm
Primarily suited for coarse fraction
Sedimentation analysis:
Based on rate of fall of particles through liquid and
depends on particle size and properties of liquid
G.G. Stokes (1851) law –
“Resistance offered by a liquid to a falling rigid spherical
particle varies with the radius of the particle and not with
its surface”
Particle Size analysis:
1. Textural Classes
2. Frequency diagram
3. Summation Curve
4. Uniformity Coefficient
F (r)
r1
r2
Size distribution curve (schematics)
Uniformity Coefficient = D60/D10
% Finer
60
For uniform particle size
UC = 1
UC>1 for nonuniform
10
0.1 D10
D60
Diameter, mm
10
Particle Shape
Depends on :
(micrograph)
- Size of particle (coarser more irregular)
- Parent material
- Degree of weathering
Coarse fractions such as
sand and silt are often
angular or zigzag in
shape
Clay particles: plate or
tubular shape
Angularity (a shape having one
or more sharp angles) reflects
degree of weathering
- Inverse relationship
- Highly angular particles
are less weathered
- Become rounded with
progressive weathering
by water and wind
Indices for Particle Shape:
1. Roundness : measure of the sharpness of corners
2. Sphericity: how close to a sphere
n
ri
Roundness   R
i 1 n
ri – radius of corner
R- radius of maximum circle
Dd
Sphericity 
Dc
Dd – diameter of a circle with an
area equal to that of the particle
projection as it rests on flat surface
Dc- diameter of smallest
circumscribing circle
Dc
r1
Soil Shapes:
Well rounded
rounded
subrounded
subangular
angular
very angular
Specific Surface Area
Properties related to SSA are
CEC, retention and movement of chemicals, swell-shrink
capacity, plasticity, cohesion and strength
SSA is expressed as:
 Surface area per unit mass (am)
 Surface area per unit volume (av)
 Surface area per unit bulk volume (ab)
SSA is expressed as:
 Surface area per unit mass (am)
 Surface area per unit volume (av)
 Surface area per unit bulk volume (ab)
As
m2
am 

Ms
g
As – total surface area
As
m2
av   3
Vs
m
Ms – mass of soil
As
m2
ab   3
Vt
m
Vt – total volume
Vs – volume of soil solids
SSA can be determined by:
 For powdery substances such as clay
 Adsorption isotherms
 Using inert substances such as N2, water vapor
ethylene glycol
Amount adsorbed
Solution concentration
Methods of measuring SSA
By Ethylene Glycol
- Dry soil sample is saturated with ethylene glycol in a
vacuum desiccator
- Excess polar liquid is removed under vacuum
- Surface area is calculated from weight of ethylene
glycol retained
BET Method:
Brunauer, Emmett, Teller (1938)
Assumptions:
1. Nonpolar gas molecules are adsorbed in multilayer on
a solid surface
2. Amount of adsorbed gas in monolayer in contact with
the surface can be determined by constructing an
adsorption isotherm and analyzing it mathematically
Main assumption for BET equation
1. The molecules adsorbed on the first layer (directly on
surface) are more energetically adsorbed than
molecules on subsequent layers
2. Heat of adsorption of all layers after the first is equal
to the latent heat of condensation of gas
Linear form of BET equation
p
1
c 1 p


x ( po  p ) x m c x m c po
x = weight of gas adsorbed at equilibrium pressure
p = equilibrium gas pressure
po = saturation vapor pressure at temperature T
xm = weight of gas in a complete monolayer
c = exp(E1-L)/RTµ
E1 = heat of adsorption in the first layer
L = latent heat of condensation
R = gas constant/mole (1,336 calories/mole)
T = absolute temperature
Procedure
1. Conduct adsorption experiment by varying p and measuring x
(0.05 < p/po < 0.35)
2. Plot p/x(po-p) against p/p0
Intercept = 1/xmc = value
Slope =(c-1)/xmc = value
p/p0
Solve these two equations for xm
p/x(p0-p)
Total surface area of soil sample
xm
St 
N Am
M
St = total surface area
xm = experimentally determined weight of gas in an adsorbed monolayer
M = molecular weight of the adsorbate (28.01 for N2)
N = Avogadro’s Number (6.02 x 1023) (calculated value of the number of
atoms, molecules, etc. in a gram mole of any chemical substance)
Am = cross. sectional area of gas molecule in the monolayer (16.2 x 10-20 m2
for N2)
The specific surface area, am, is obtained by dividing the
total surface area by the sample weight.
Remember adsorption experiment must be conducted at or
below the temperature of condensation of gas in order for
significant adsorption to occur
Clay Minerals
Inorganic component consists of :
- crystalline and noncrystalline
- Primarily- Si, Al, Fe, H and O
- Also- Ti, Ca, Mg, Mn, K, Na, and P
- Colloidal
- Secondary minerals
Influences various soil properties: SA, CEC, Nutrient
and water holding capacities, buffering and filtering
capacities, water transport properties, soil structure etc.
Basic Structural Units in Clay Minerals
Tetrahedron (a pyramid
Octahedron
formed by four triangles )
(an eight-sided
geometric solid )
Silicon atom placed equidistant
from four oxygen or hydroxyls
Si4O6(OH)4
Closely packed oxygen or hydroxyl with
AL, Fe or Mg embedded
These two are joined in 1:1 or 2:1 to form clay minerals
Clay minerals are hydrous aluminum silicates
Mg+2 and Fe+3- proxy for AL+3
Commonly observed secondary minerals
Secondary Minerals
Geothite
Weatherability
Most resistant
Hematite
Gibbsite
Clay minerals
Dolomite
Calcite
Gypsum
Least resistant
Geothite is rich in iron and weathers slowly to form oxide clays
Hematite is an oxide mineral Fe2O3
Gibbsite is white crystalline mineral Al(OH)3
Dolomite is sedimentary rocks Ca or Mg(CO3)2
Calcite is mineral composed of CaCO3
Gypsum is natural crystalline mineral CaSO4.2H2O
Charge Properties of Clay minerals
Total charge on mineral surfaces is called intrinsic
charge density or permanent charge
Independent of soil reaction or pH
Variable charge is pH or proton dependent
Imbalance of complex proton and hydroxyl charges on surface
Most soils have a net negative charge
Some weathered soils may have net positive
Electric double Layer
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Negative charge on
clay particles is
balanced by the
cations in soil solution
(due to Coulomb
forces).
+
+
+
+
+
+
+
+
+
+
+
Dry
Fully hydrated
+
+
+
Force that acts in two
electrically charged
bodies is proportional
to the product of the
module of their
charges (q) divided by
the square of the
distance (d) between
them
F
q1 q 2
d2
+
+
+
+
+
+
+
+
+
+ +
Clay
Particle
+
+
+
+
+
+
+
+
+
Diffuse
layer
+
+
+
Soil Solution
Electric double layer is
due to the negative
charge on clay particles
and positive on
surrounding cations in
solution
There are three models for
explaining distribution of ion
in water layer adjacent to clay
Stern’s double layer
Potential
Helmholtz
layer (Fixed)
Gouy’s layer
(Diffuse)
Distance
Helmholtz Model: All balancing
cations are held in a fixed
layer between the clay
surface and soil solution
Gouy-Chapman Model: A
diffuse double layer due to
the thermal energy of cations
causing a concentration
gradient, which leads to a
condition of maximum
entropy or diffuse double
layer
Stern Model: Combines the two
concepts and proposes
condition of free energy.
Double layer comprises a
rigid region next to mineral
surface and a diffuse layer
joining the bulk solution
Stern double layer comprises of two parts:
 single ion thick layer fixed to solid surface
 diffused layer extending some distance into liquid phase
Nernst Potential or
Total Potential
Potential
Zeta Potential
Thickness of double layer is
the distance from the clay
surface at which cation
concentration reaches a
uniform or minimum value
Distance
Zeta Potential: is the potential difference between the fixed and freely
mobile diffuse double layer. It is also known as electrokinetic potential
Nernst Potential: is the difference in cross potentials at the interface
of two phases when there is no mutual relative motion. It is also called
thermodynamic or reversible potential
Stability of clay suspension
Clay lattice
High activity clays
Low activity clays
Greater distance between charged particles
kaolinite
montmorillonite, vermiculite
Fully hydrated clay particles are completely dispersed
Flocculation or Coagulation: sticking together in clusters
Deflocculation or Dispersion- opposite
Chemically
Sodium Hexametaphosphate
Mechanically
Stirring or Ultrasound vibration
Flocculation or Coagulation takes place once zeta
potential is below the critical level
Sodium hexametaphosphate increases the zeta
potential and suspension remains stable and does not
coagulate
Effectiveness of a cation in causing flocculation
depends on its valency
H+ > K+ > Na+ > Li+
Ba+2
>
Mg+2
Al+3 > Ca+2 > Mg+2
Dispersivity increases in
the opposite direction
Types of Flocculation
1. Incomplete
2. Random
3. Plate Condensation
- Presence of dilute solutionweak or incomplete flocculation
- Contact at the edges of clay plates
- Cations are aligned between two clay plates
1. Almost all particulate or macroscopic
materials in contact with a liquid
acquire an electronic charge on their
surfaces.
2. Zeta potential is an important and
useful indicator of this charge which
can be used to predict and control
the stability of colloidal suspensions
or emulsions.
3. The greater the zeta potential the
more likely the suspension is to be
stable because the charged particles
repel one another and thus
overcome the natural tendency to
aggregate.
4. The measurement of zeta potential
is often the key to understanding
dispersion and aggregation
processes
5. Zeta potential can also be a
controlling parameter in processes
such as adhesion, surface coating,
filtration, lubrication and corrosion.
A. The principal of determining zeta potential by microelectrophoresis is
that a controlled electric field is applied via electrodes immersed in the
sample suspension and this causes the charged particles to move
towards the electrode of opposite polarity.
B. Viscous forces acting upon the moving particle tend to oppose this
motion and an equilibrium is rapidly established between the effects of
the electrostatic attraction and the viscous drag. The particles therefore
reach a constant "terminal" velocity.
C. Terminal velocity dependents on electric field strength or voltage
gradient, dielectric constant, viscosity and the zeta potential.
D. It is usually expressed as the particle mobility or velocity under unit
field strength. For all practical purposes, the relationship between
mobility, µ, and zeta potential, z, in water at 25oC can be expressed as:
z = 12.85 µ
E. In practice, zeta potentials are usually negative, i.e. the surface is
negatively charged, but they can lie anywhere in the range from -100 to
+100 mV.
Dispersed Particles
Aggregated Particles
High Zeta Potential
Low Zeta Potential
Packing Arrangement
Influences several soil properties
Void
Solid
90o
r0
r = 0.73 r0
Cubic form
60o
Orthorhombic
45o
Rhombohedral
Porosity
Cubic form:
(8R3 – 4/3 pi R3)/8R3 = 0.48
Orthorhombic: (6.93 R3 – 4/3 pi R3)/6.9R3= 0.40
Rhombohedral:
(5.66 R3 – 4/3 pi R3)/5.66R3= 0.26
Soil Structure
Jacks (1963) “Union of mineral and organic matter to form
organomineral complexes is a synthesis as vital to the
continuance of life as, and less understood than,
photosynthesis”
- Arrangement of soil particles
- Dynamic varies spatially temporarily
- at multiple scales
- Complex and is not completely understood
- Most important soil physical properties
- Often called surrogate property
Soil Structure
Pedological Edaphological
Science dealing with influence
of soils on living things, plants
Ecological
- 3-D arrangement of particles (O + IO)
- Mechanistic with regard to components
- size, shape, arrangement, and packing
into identifiable units (aggregate, peds)
Engineering
1. Functional attributes
such as voids and pores
governing plant and root
growth
2. Soil-pore system
Ecological =
Pedological + Edaphological
1. Intraaggregate pore
2. Interaggregate pores
Macroaggregate
Intra-aggregate (within
aggregate) pore space
influences water
retention
> 0.25 mm
diameter
Differences in Inter-aggregate
(between aggregates) pore
space can influence water and
solute movement through soil
profile
Mechanisms of Aggregation
-
Russell’s theory of crumb formation
-
Calcium linkage theory
-
Clay water structure
-
Edge-surface proximity concept
-
Emerson’s model
-
Organic bond theory
-
Clay domain theory
-
Quasi crystal theory
-
Microaggregate theory
-
Aggregate hierarchy model
-
POM nucleus model
Russell’s (1934) Theory of Crumb Formation
(Clay particles bound together through inonic bond)
- Clay particles have charge when hydrated
- Charged particles are surrounded by electric double layer
- Every clay particle is surrounded by an envelop of water
- As moisture content decreases, thickness of water envelop decreases
- Each ion shares it’s envelop with two clay particles thus holding it tight
Criteria for Crumb Formation
- Particles must have high CEC and SSA
- Smaller than a particular size (sand and silt not essential)
- Liquid must have a dipole (property of water) moment
- Presence of polyvalent cations
Calcium Linkage Theory (Williams, 1935; Peterson, 1947)
- Negatively charged organic materials e.g., polysaccharides (long
chains of monosaccharide units bonded together; e.g., glycogen, starch,
and cellulose) are absorbed on clay by polyvalent cations
Clay – Mg – OH, Clay – Be - OH
(C6H10O5)n
Clay – Ca – OOC – R – Ca – OOC – R – Ca – Clay
+
+
-
+
+
-
C6H7O2 (OH)x (OC2H5)y
[O(CH2CH2O)mH]z]n
Clay- Water Structure (Rosenquist, 1959)
Adhesion (molecular attraction exerted between bodies in contact)
between clay particles is a function of the difference between the
surface energy of the adsorbed and pore water
Edge-Surface Proximity Concept (Schofield and Samson,
1954; Trollope and Chan, 1959)
 A card house structure based on establishment of equilibrium
between adjacent particles due to edge-surface proximity
 Flocculation occurs due to electrostatic attraction
 Much more stable than caused by lowering of zeta potential
Emerson’s model (1959)
- Extension of Russell’s model
- Positive edge and negative face
- Both clay and quartz (sand, silt)
- Structure disappears as soil dries if no polyvalent cation present
Following four types of bond were proposed
- Hydrogen bonding between carboxyl group and clay
- Ionic bonding between carboxyl group and clay
- Interaction of electric double layers leading to formation of domains
- Bonding between organic and inorganic colloids
Organic Bond Theory (Greenland, 1965)
Soil organic matter forms ionic bonds
Clay Domain Theory (Williams et al., 1967)
Soil macroaggregates
Sand or Silt
Particles
x
x
x
x
x
x
x
x
Microaggregates
x
x
x
- Exist in domains up to
about 5 mm in diameter
x
x
x
x
Organic molecule
x
Domain of clay
Crystals for
microaggregate
- Clusters of domains are
called microaggregates (51000 mm)
- Clusters of
microaggregates are
macroaggregates (1-5 mm)
Quasi Crystals Theory (Aylmore and Quirk, 1971)
- Modified Williams et al. 1967 theory
- Parallel clay crystals (5 mm in diameter) forms quasi crystals (0.011.3 mm)
- Quasi crystals are stable packets (Oades and Waters, 1991)
- the 3 stages of binding of clay particles are:
- into stable packets of < 20 mm
- into microaggregates of 20-250 mm
- stable macroaggregates >250 mm
Microaggregate Theory (Edwards and Bremner, 1967)
- soil consists of microaggregates (< 250 mm) bound on
macroaggregates (> 250 mm)
- bonds are stronger in micro than macroaggregates
- Microaggregate = [(Cl – P – OMx ]y
- Cl is clay, P- polyvalent cation, OM is organometallic complex)
Stages of Aggregation (Tisdall and Oades, 1982)
[Cl – P – OM]x
[Cl – P – OM]
< 0.2 mm
0.2
2
20
[(Cl – P – OM)x]y
250
2000 mm
Aggregate Hierarchy Model (Oades and Waters, 1991)
- For aggregates stabilized by organic materials- stages are:
< 0.2 mm
20- 90
90-250
>250 mm
POM Nucleus Model
Particulate organic matter form a nucleus –
around clay to form microaggregate and
around microaggregates to form macroaggregate
Factors Affecting Aggregation
- Drying and Wetting
- Freezing and Thawing
- Biotic Factors
- Soil Tillage
- Soil Amedments
Crusting or Surface Seal
Crusting: Hardening of the surface layers of soil
Aggregates at the soil-air interface are broken or dispersed by: rapid
wetting, drying, tillage or traffic
Reorientation of dispersed particles
Drying of the surface
Leads to the formation of soil crust or surface seal
Which has low porosity, high density, low permeability to air and water
Types of Crusts:
Physical Crusts
Chemical Crusts
Biological Crusts
Physical Crusts
Formed due to the alteration in structural properties
- Structural: due to the disruption of aggregates by rain
Upper surface (1-3 mm thick) has low permeability
- Depositional: Transport of fine particles by runoff
thicker than structural crusts
Chemical Crusts
- Formed due to salt incrustation on soil surface (arid/semi-arid)
Biological Crusts
- Are primarily formed by algal growth
- Such a crust is highly hydrophobic, low infiltration
Factors effecting Deflocculation
- Rainfall Factors
- Weather
- Soil properties
- Field Moisture Content
- Microrelief
Rainfall factor: Kinetic energy (0.5 m v2) of rainfall and momentum (M= mv)
Weather factor: wetting/freezing; freeze-thaw cycles
Soil Properties: texture, clay mineralogy, SOC, aggregates
Field moisture Content: Influences aggregate strength, slaking, dispersion
Microrelief: Rough soil bed decreases susceptibility to crust formation
Mechanism of Crust Formation
Dispersion
- Dispersion of aggregates
- Orientation and hardening by desiccation (dryness due to water removal)
Charge Distribution on Colloids
- Permanent charge (1:1 or 1:2 clay); Variable charge (oxides, SOC..)
- Low activity clays high dispersion
- Low SOC of soil high dispersion
Desiccation
Crust Development
- Ploughed field with clods
- Rainfall- clod breakdown, aggregate breakdown, particle
rearrangement
- Aggregate coalescence beneath crust, deposition of fine particles
- Maximum runoff, erosion of washed out layer
Rheology and Plasticity
Science dealing with the study of deformation-time propertiesof
material in response to applied stress
Soil consistance refers to the physical forces of cohesion and
adhesion acting with in the soil at a range of soil moisture content
Atterberg defined consistence as: Harsh-friable-soft-plastic-sticky and
viscous
Harsh- dry soil
Friable- easily crumbles into granules
Soft- visibly wet
Plastic- wet enough to be molded into different form
Sticky- adheres to other objects
Viscous- soil is near saturation and behaves like a viscous liquid
Soil Plasticity
It is soils ability to change shape without cracking
It depends on clay content of soil
Sandy/coarse textured soils are not plastic
Plasticity Theories
1. Water Film Theory: soil cohesion depends on van der waals
forces, electrostatic forces, cation bridging, surface tension
etc. water content increases soil cohesion decreases
2. Critical State Theory: Soil is deformed but does not change
volume . Soil is plastic and at critical state
Atterberg Constants
Shrinkage Limit: It is the lower limit of soil moisture content at
which no further change in soil volume occurs.
Lower Plastic Limit: Moisture content corresponding to lower limit
of plastic range (suction of 500 to 2000 cm of water)
Cohesion Limit: moisture content at which crumbs of soil cease
to adhere when placed in contact with one another
Sticky Limit: Lower limit of moisture content at which soil sticks to
a steel spatula
Upper Plastic Limit: this is known as liquid limit or lower limit of
viscous flow. Soil water mixture starts flowing at this stage.
Upper Limit of Viscous Flow: mixture of soil and water flows like
a liquid
Soil Indices
Plasticity Index: PI = UPL – LPL
Liquidity Index:LI = [w(%)- UPL]/PI
Activity Ratio: AR = PI/ Clay content (%)
Factors Affecting
Atterberg’s Limits
1. Clay Content
2. Clay Minerals
3. Exchangeable cation
4. Soil organic matter (no net
effect)
Methods of Measurement
1. Casagrande Test
2. Drop-Cone test
3. Indirect methods:
1. Proctor Test
2. pF Curve
3. Hydraulic Conductivity
4. Viscosity
5. Shear Strength
Soil and Water
evaporation
Created by Dr. Michael Pidwirny, Department of
Geography, Okanagan University College, BC, CA
Main Objectives:
 Comprehend characteristics and properties of water in soils
 Understand and capable of explaining terms and concepts
used in describing soil water
Key terms and Concepts:
 Cohesion and adhesion
 Surface tension
 Capillarity
 Soil water content
 Soil water energy (gravitational, matric, and osmotic)
 Maximum retentive capacity, field capacity, wilting point
References:
 Nature and Properties of Soil (Brady)
 Principles of Soil Physics (Lal and Shukla)
 Soil Hydrology (Kutilek and Nielsen)
 Environmental Soil Science (Hillel)
What is Soil?
 It is the interface between atmosphere and
lithosphere (the mantle of rocks making up
the Earth's crust)
 According to engineering definition it is all
unconsolidated material above bedrock
 According to soil science, it is naturally
occurring layers of mineral and (or) organic
constituents that differ from the underlying
parent material in their physical, chemical,
and mineralogical properties
Rock
What is Water?
 A binary compound (H2O) that occurs at room
temperature as a clear colorless, odorless, tasteless
liquid
 Freezes into ice below 0 degree centigrade and boils
above 100 degree centigrade
 Necessary for the life on earth (human, animals and
plants)
 Constitutes 60-70 % of a live stock animal’s body
 Constitute 55-60 % of young adults and ~75% of infants
www.atpm.com
Hydrogen
Electro positive
Hydrogen
H-O : 0.97 A
1050
H-H : 1.54 A
Polarity
Oxygen
Negative
Symmetrical
angstroms
Hydrogen bond
H+
H2O
=
O--
+
-
H+
Gives structural strength
Bond depends on temperature:
Higher is the temperature weaker is bond
Positive end attraction with -ve end of other water molecules
Polymer type of grouping
H+
H+
O-
Cations: Na+, K+, Ca2+ : become hydrated through their
attraction to the Oxygen
Anions or negatively charged clay surfaces: attract water
through hydrogen
Does water swell and shrink with Temperature?
1
0.998
40 C
0.996
Density (g cm-3)
0.994
0.992
0.990
Temperature (0C)
-10
0
10
20
30
40
50
Temperature range in liquid phase for H+ compounds
100
Boiling point
Freezing point
H2O (2+16=18)
Temperature (0C)
50
Hydrogen telluride
0
H2Te
Hydrogen sulfide
-50
H 2S
(130)
H2Se
(80)
Hydrogen selenide
(2+32=34)
-100
0
50
Molecular Weight
100
If water were an ordinary compound whose molecules are
subject to weak forces, its boiling and freezing point
would fall below hydrogen sulfide
Strong hydrogen bonding between water molecules
prevents this
Water occurs in all three states (solid, liquid, and gaseous)
at prevailing temperatures on the earth’s surface
Example: Ice cubes in a glass at room temperature
Why water wets clean glass?
 Surface of glass has O and unpaired electrons
 Water molecules form hydrogen bond
 Force stronger than gravity
Why water does not stick to glass surface coated
with grease?
 Surface of grease has no O and free electrons
 Water molecules cannot form hydrogen bond
 Therefore, water do not stick
Forces acting on a water molecules
Air-water
Interface
Air
At point A:
A
Attraction of air for water
molecules is much less
than that of water
molecules for each other.
B
At point B:
Forces acting on
water molecule
are equal in all
direction
Water
Consequently, there is a net downward force on the
surface molecules, and result is something like a
compressed film at the surface. This phenomenon is
called surface tension
Capillary Fundamentals and Soil Water
Cohesion: Attraction of molecules for each other
Adhesion: Attraction of water molecules for solid surfaces
By adhesion, solids hold water
molecules rigidly at their soil-water
surface
By cohesion water molecules
hold each other away from solid
surfaces
Together it is possible for soil solids to
retain water and control it’s movement
Gravity
Capillary
Water rises in the capillary against the force of gravity
!!!! What happens if there is no force of gravity !!!!!
Water
Water
Surface Tension
The cohesive forces between liquid molecules are responsible for
the phenomenon known as surface tension
The molecules at the surface do not have other like molecules on all
sides of them and consequently they cohere more strongly to those
directly associated with them on the surface. This forms a surface
"film" which makes it more difficult to move an object through the
surface than to move it when it is completely submersed.
 Surface tension is typically measured
in dynes/cm. The force in dynes
required to break a film of length 1 cm
 Equivalently, it can be stated as
surface energy in ergs/cm2
 Water at 20°C has a surface tension
of 72.8 dynes/cm compared to 22.3
for ethyl alcohol and 465 for mercury
The dipolar interaction between water
molecules represents a large amount of
internal energy (the energy associated
with the random, disordered motion of
molecules) and is a factor in water's
large specific heat (the amount of heat
per unit mass required to raise the
temperature by one degree Celsius).
The dipole moment of water provides a
"handle" for interaction with microwave
electric fields in a microwave oven.
Microwaves can add energy to the water
molecules, whereas molecules with no
dipole moment would be unaffected.
Dipolar Bonding in Water
Contact Angle
 Solid
 Liquid
 Gas
Young’s
equation
Liquid and gas (air) in contact with solid
Interface between air and water forms a definite angle
“contact angle”
g sa  g sw
cos a 
g wa
gsa > gsw; cos a = + or a <
900
Angle of contact is acute in a
liquid that wets the solid
L
Air
Solid
L
Angle of contact is obtuse
(between 90 and 180) in a liquid
that does not wet the solid
Air
Solid
Forces that affect movement of water into the soil
Gravity: a constant force that pulls the water downward
Cohesion: attraction of water molecules for each other. It is the force that
holds a droplet of water together
Adhesion: attraction of water molecules to other substances. This force
causes water molecules to adhere to other objects, such as soil particles
Placing a drop of water on a piece of newsprint paper
Force of adhesion between the water molecules and the paper
molecules is greater than the force of cohesion that holds the water
molecules together
The water droplet spreads out and soaks into the paper
Placing a drop of water on a piece of waxed paper
Force of adhesion between the water molecules and the paper
molecules is lower than the force of cohesion that holds the water
molecules together
The water droplet remains intact
Hydrophilic Versus Hydrophobic Soils
When the adhesive forces between water molecules and an object
are weaker than the cohesive forces between water molecules, the
surface repels water and is said to be hydrophobic. Hydrophobic soils
restrict the entry of water, which 'balls up' or sits on the soil in beads
rather than infiltrating the soil.
Hydrophobic soils exhibit an obtuse (greater than or equal to 90o)
wetting angle that causes capillary repulsion, so preventing water
from entering soil pores
Hydrophilic or normally wettable soils display an acute (less than 90o)
angle of contact with water, allowing infiltration. adhesive forces
between water molecules and an object are stronger than the
cohesive forces between water molecules
Capillary Mechanism
2 r1
Rise continues till:
Weight of water in the tube (force of
gravity) = Total cohesive and adhesive
forces
h1
2 r2
h2
Water
2r
Force of gravity = Mass of water column * Acceleration
= (volume of water * density) * g
= (p * r2* h) *dw * g …………(A)
Total cohesive and adhesive forces
h
= (perimeter) * surface tension
=2*p*r*g
At equilibrium:
…………(B)
Water
A=B
(p * r2* h) *dw * g = 2 * p * r * g
2 *g
h
r * dw * g
use
Show
0.15
h
r
g = 72.75 dynes/cm
dw= 0.9982 g/cm3
g = 980 cm/s2
If two principle radii r1 and r2
0.15
h
r
1 1
h  0.15  
 r1 r2 
This relationship tells us that:
Capillary Rise
 Capillary rise is higher in small pores
r = 0.1 cm; h = 1.5 cm
r = 1.0 cm; h = 0.15 cm
r = 10 cm; h = 0.015 cm
Radius
The inverse relationship between height of rise of water and radius
of soil pores may not be always valid:
 Soil pores are not straight uniform openings as a tube
 Some soil pores may entrap air and slow down the capillary rise
Soil solids
Tortuous flow
paths of water
Entrapped air
water
0.15
h
r
Height (cm)
Loam
Sand
Clay compacted
Time (days)
Brady,1984
Capillary water
Adsorbed water
Enlarged soil
particles or
aggregates
Two forms of water in soil
Soil solids tightly absorb water
Capillary forces hold water in capillary pores
Soil Water Content
Soil Moisture Content
Water that may be evaporated from soil by heating at 1050C to a
constant weight
mass of water evaporated (g)
Gravimetric moisture content (w) =
mass of dry soil (g)
volume of water evaporated (cm3)
Volumetric moisture content (q) =
volume of soil (cm3)
q= w*
bulk density of soil
density of water
3
cm
g

3
cm
g
g
3
cm 3  g g cm
g
g cm 3 g
cm 3
mass of dry soil (g)
Bulk density of soil (r) =
volume of soil (cm3)
Soil Moisture Content: Methods of Measurement
1. Difficulties encountered for accurate moisture measurement in the
field:
2. Soils are highly variable
3. Soil moisture is highly dynamic (spatial temporal variability)
4. Plant water uptake is highly variable depending upon the stage of
growth
5. State of growth is again dependent upon nutrient application, water
availability, pests etc.
6. Chemicals present in the soil can make measurements unreliable
7. Costs involved
Methods for soil water content
Direct method (Gravimetric;
Thermogravimetric)
Electrical
properties
Indirect methods
Radiation
technique
Acoustic
method
Thermal
properties
Chemical
methods
-Neutron scattering
g- ray attenuation
Electrical
Conductance
- Gypsum
blocks
- Nylon blocks
- Change in
conductance
Dielectric constant
TDR
Principles underlying different methods of
assessment of soil water content
DIRECT
Gravimetric: evaporating water at 1050C.
Thermogravimetric: Soil sample is weighted and saturated with alcohol
and burned several times until a constant dry weight is obtained
INDIRECT
Electrical Conductance
Methods of soil water
content determination
Methods for soil water content
Direct method (Gravimetric;
Thermogravimetric)
Electrical
properties
Hand-feel
method
Radiation
technique
Acoustic
method
Indirect methods
Thermal
properties
-Neutron scattering
g- ray attenuation
Electrical
Conductance
- Gypsum
blocks
- Nylon blocks
- Change in
conductance
Dielectric constant
TDR
FDR
ADR
Chemical
methods
DIRECT
Gravimetric: evaporating water at 1050C.
Feel Method:
Thermogravimetric: Soil sample is weighted and saturated with alcohol
and burned several times until a constant dry weight is obtained
Advantages: ensures accurate measurements, not dependent on
salinity and soil type, easy to calculate
Disadvantage: destructive test, time consuming, inapplicable to
automatic control, must know dry bulk density to transform data to
volume moisture content, inaccurate because of soil variability
There are many classifications for soil types and major differences
within each classification
Soil management can have a major impact upon these soil properties.
Compaction is the major cause of error in bulk density.
http://edis.ifas.ufl.edu/
INDIRECT
ELECTROMAGNETIC TECHNIQUES: Resistive Sensor (General)
Electromagnetic techniques include methods that depend upon the effect
of moisture on the electrical properties of soil.
Soil resistivity: depends on moisture content; hence it can serve as the
basis for a sensor. It is possible either to measure the resistivity between
electrodes in a soil or to measure the resistivity of a material in
equilibrium with the soil.
Advantage: can provide absolute soil water content, can determine water
content at any depth, sensor configuration can vary in size so sphere of
influence or measurement is adjustable, high level of precision when ionic
concentration of the soil does not change, can be read by remote methods
Disadvantage: difficulty with resistive sensors is that the absolute value
of soil resistivity depends on ion concentration as well as on moisture
concentration, calibration is required, calibration not stable with time , high
cost
o Porous blocks are made of: gypsum, ceramic, nylon, and fiberglass
o The blocks are buried in intimate contact with the soil at depths and
allowed to come to equilibrium with the surrounding soil
o Once equilibrium is reached, different properties of the block which
are affected by its water tension may be measured
One of the more common types of porous
blocks are electrical resistance blocks
Electrodes buried in the block are used to
measure the resistance to electrical current
flow between them.
Resistance is affected by the water content
of the block
Higher resistance readings mean lower
block water content and thus higher soil
water tension.
•
•
Electrical resistance blocks are best suited for finer-textured soils
•
For most coarse-textured soils readings of 100 cb and above are
well outside the available soil water range
They are generally not sensitive to changes in soil water tension
less than 100 centibars (cb)
 Thermal dissipation blocks are porous ceramic blocks in which a small
heater and temperature sensors are embedded
 This arrangement allows measurement of the thermal dissipation of
the block, or the rate at which heat is conducted away from the heater
 This property is directly related to the water content of the block
 Thermal dissipation blocks must be individually calibrated.
 Considerably more expensive than electrical resistance blocks.
Watermark Blocks. or granular matrix sensor: is a relatively new
 The electrodes are embedded in a granular matrix material which
approximates compressed fine sand.
 A gypsum wafer is embedded in the granular matrix near electrodes
 A synthetic porous membrane and a PVC casing with holes drilled in it
hold the block together
 The granular matrix material enhances the movement of water to and
from the surrounding soil, making the block more responsive to soil
water tensions in the 0 to 100 cb range
 Watermark blocks exhibit good sensitivity to soil water tension over a
range from 0 to 200 cb
 Are more adaptable to a wider range of soil textures and irrigation
regimes than gypsum blocks
Readings are taken by attaching a special electrical resistance meter to
the wire leads and setting the estimated soil temperature
Watermark blocks require little maintenance and can be left in the soil
under freezing conditions
The blocks are much more stable and have a longer life than gypsum
blocks
Soil salinity affects the electrical resistivity of the soil water solution and
may cause erroneous readings
The gypsum wafer in the Watermark blocks offers some buffering of this
effect.
Resistive Sensor (Gypsum, 1940): soil moisture tension,
response time: 2 to 3 hours
 One of the most common methods of estimating
matric potential is with gypsum or porous blocks
 The device consists of a porous block containing two electrodes
connected to a wire lead
 The porous block is made of gypsum or fiberglass
 When the device is buried in the soil, water will move in or out of the
block until the matric potential of the block and the soil are the same
 The EC of the block is then read with an alternating current bridge (0
as dry and 100 as wet)
 A calibration curve is made to relate EC to the h for any particular soil
Advantage: low cost , repeatability
Disadvantage: each block requires individual calibration, calibration
changes with time, life of device limited, provides inaccurate
measurement for soil salinity, prone to breakdown in alkaline soil
Dielectric Constant (K)

K
0
How an electric field affects and is affected by the medium (farads/m)
The dielectric constant is the relative permittivity of a dielectric material.
Dielectric constant for water is about 80 and for soil is 5 to 7 (Hz; cycle/s)
Dielectrics have the strange property of making space seem bigger or
smaller than it looks.
When you put some dielectric between
two electric charges it reduces the force
acting between them
Dielectric constant of a material affects
how electromagnetic signals (light, radio
waves, millimeter-waves, etc.) move
through the material
A high value of dielectric constant makes the distance inside the material
look bigger. This means that light travels more slowly
Dielectric constant determines the velocity of an electromagnetic wave
or pulse through the soil
In a composite material like the soil (i.e., made up of different
components like minerals, air and water), the value of the permittivity is
made up by the relative contribution of each of the components
Since dielectric constant of liquid water (K = 81) is much larger than that
of the other soil constituents (e.g. K = 2-5 for soil minerals and 1 for air)
The total permittivity of the soil or bulk permittivity is mainly governed by
the presence of liquid water
q = -5.3•10-2 + 2.29•10-2K1 - 5.5•10-4K2 + 4.3•10-6K3… Topp et al. (1980)
- Valid for most mineral soils and for moisture below 50%.
- For larger q, organic or volcanic soils, needs specific calibration
- At low frequencies (<100 MHz) it is more soil-specific
Capacitive Sensor- q, instantaneous
Q=CV
C- capacitance
Capacitor- a device that can store electric charge
Soil moisture content may be determined via its effect on dielectric
constant by measuring the capacitance between two electrodes
implanted in the soil
Where soil moisture is predominantly in the form of free water (e.g., in
sandy soils), the dielectric constant is directly proportional to the moisture
content
The probe is normally given a frequency excitation to permit
measurement of the dielectric constant
Disadvantages: The readout from the probe is not linear with water
content and is influenced by soil type and soil temperature, long-term
stability questionable, costly
Frequency Domain Reflectometry: radio
frequency (RF) capacitance techniques
Actually measures soil capacitance
A pair of electrodes is inserted into the soil
Soil acts as the dielectric completing a
capacitance circuit, which is part of a
feedback loop of a high frequency
transistor oscillator
As high frequency radio waves (about
150 MHz) are pulsed through the
capacitance circuitry, a natural resonant
frequency is established which is
dependent on the soil capacitance,
which is related to the dielectric constant
by the geometry of the electric field
established around the electrodes
Two commercially available instruments
using this technique: the Troxler Sentry
200-AP probe and the Aquaterr probe
Time Domain Reflectometry (TDR): q, 28 s
The soil bulk dielectric constant (K) is determined by measuring the
time it takes for an electromagnetic pulse (wave) to propagate along a
transmission line (L) that is surrounded by the soil
Since the propagation velocity (v) is a function of K, the latter is
therefore proportional to the square of the transit time (t, in seconds)
down and back along the L
K = (c/v)2 = ((c.t)/(2.L))2
where c is the velocity of electromagnetic waves in a vacuum (3•108 m/s
or 186,282 mile/s) and L is the length embedded in the soil (in m or ft)
 TDR determinations involve measuring the propagation of
electromagnetic (EM) waves or signals
 Propagation constants for EM waves in soil, such as velocity and
attenuation, depend on soil properties, especially q and EC
The propagation of electrical signals in soil is influenced by q and EC
The dielectric constant, measured by TDR, provides a good measurement
of this soil water content
Disadvantage: Costly, not really independent of salt content
Amplitude-Domain Reflectometry (ADR)
Impedance
When an electromagnetic wave
(energy) traveling along a transmission
line (L) reaches a section with different
impedance (which has two components:
EC and dielectric constant), part of the
energy transmitted is reflected back into
the transmitter.
Reflected wave interacts with the incident wave producing change of wave
amplitude along the length
If the soil/probe combination is the cause for impedance change in L,
measuring the amplitude difference gives the impedance of the probe
Influence of soil EC is minimized by choosing a signal frequency, so that
soil q can be estimated from the soil/probe impedance
Disadvantage: Measurement affected by air gaps, stones or channeling
water directly onto probe rods, and small sensing volume (0.27 in3)
Time Domain Transmission (TDT)
This method measures the one-way time
for an electromagnetic pulse to propagate
along a transmission line (L). Thus, it is
similar to TDR, but requires an electrical
connection at the beginning and ending
of the length.
Notwithstanding, the circuit is simple
compared with TDR instruments.
Disadvantages: Reduced precision, because the generated pulse is
distorted during transmission; soil disturbance during installation; needs
to be permanently installed in the field
NUCLEAR TECHNIQUES: Neutron Scattering, q, 1 to 2 min
With this method, fast neutrons emitted from a radioactive source are
thermalized or slowed down by hydrogen atoms in the soil
Since most hydrogen atoms in the soil are components of water
molecules, the proportion of thermalized neutrons is related to q
Advantages: can measure a large soil volume,
can scan at several depths to obtain a profile of
moisture distribution, nondestructive, water can
be measured in any phase
Disadvantages: high cost of the instrument,
salinity, must calibrate for different types of
soils, excess tube, radiation hazard,
insensitivity near the soil surface, insensitivity to
small variations in moisture content at different
points within a 30 to 40 cm radius, and variation
in readings due to soil density variations (error
rate of up to 15 percent)
Gamma Attenuation: volumetric water content, < 1 min
 This method assumes that the scattering and absorption of gamma
rays are related to the density of matter in their path
 The specific gravity of a soil remains relatively constant as the wet
density changes with increases or decreases in moisture
 Changes in wet density are measured by the gamma transmission
technique and the moisture content is determined from this density
change
Advantages: can determine mean water content with depth, can be
automated for automatic measurements and recording, can measure
temporal changes in soil water, nondestructive measurement
Disadvantages: restricted to soil thickness of 1 inch or less, but with
high resolution, affected by soil bulk density changes, costly and
difficult to use, large errors possible when used in highly stratified soils
Nuclear Magnetic Resonance: volumetric water content, < 1 min
 Water in the soil is subjected to both a static and an oscillating
magnetic field at right angles to each other
 A radio frequency detection coil, turning capacitor, and
electromagnet coil are used as sensors to measure the spin echo
and free induction decays
 Nuclear magnetic resonance imaging can discriminate between
bound and free water in the soil
Remote Sensing Techniques: Soil surface moisture, instantaneous
 This method includes satellite, radar (microwaves), and other noncontact techniques
 The remote sensing of soil moisture depends on the measurement of
electromagnetic energy that has been either reflected or emitted from
the soil surface
 The intensity of this radiation with soil moisture may vary depending on
dielectric properties, soil temperature, or some combination of both
 For active radar, the attenuation of microwave energy may be used to
indicate the moisture content of porous media because of the effect of
moisture content on the dielectric constant
 Thermal infrared wavelengths are commonly used for this measurement
Advantages: remote measurements, over large area
Disadvantages: system large and complex, costly, for surface soil
Ground Penetrating Radar (GPR). This technique is based on the same
principle as TDR, but does not require direct contact between the sensor
and the soil. When mounted on a vehicle close to the soil surface, it has
the potential of providing rapid, non-disturbing, soil moisture
measurements over relatively large areas (TDR is better for detailed
measurements over small areas)
Although it has been applied successfully to many field situations, GPR
has not been widely used because the methodology and instrumentation
are still only in the research and development phase
New remote sensing (non-contact) methods specially suited for soil
moisture monitoring over large areas and usually mounted on airplanes
or satellites: the active and passive microwave, and electromagnetic
induction (EMI)
Active and EMI methods (EC only) use two antennae to transmit and
receive electromagnetic signals that are reflected by the soil, whereas the
passive microwave (EC and q both) just receives signals naturally emitted
by the soil surface
Other: X-ray tomography and nuclear magnetic resonance (NMR)
Optical Methods: Soil water content, instantaneous
 Method relies on changes in the characteristics of light due to soil
characteristics
 These methods involve the use of polarized light, fiber optic sensors, and
near-infrared sensors
 Polarized light is based on the principle that the presence of moisture at
a surface of reflection tends to cause polarization in the reflected beam
 Using this device, an achromatic light source is directed at the soil
surface
 Fiber optic sensors are based on a section of unclad fiber embedded in
the soil
 Light attenuation in the fiber varies with the amount of soil water in
contact with the fiber because of its effect on the refractive index and
thus on the critical angle of internal reflection
 Near-infrared methods depend on molecular absorption at distinct
wavelengths by water in the surface layers; therefore, they are not
applicable where the moisture distribution is very nonhomogeneous
Neutron
Moderation
TDR
FD (Capacitance
and FDR)
ADR
Phase
Transmissi
on
TDT
Reading range
0-0.60 ft3ft-3
0.05-0.50 ft3ft-3
0-Saturation
0-Saturation
0.05-0.5
ft3ft-3
0.05-0.5 ft3ft-3
Accuracy (with
soil-specific
calibration)
±0.005 ft3ft-3
±0.01 ft3ft-3
±0.01 ft3ft-3
±0.01-0.05 ft3ft-3
±0.01 ft3ft-3
±0.05 ft3ft-3
Measurement
volume
Sphere (6-16
in. radius)
about 1.2 in. radius around
length of waveguides
Sphere (about 1.6 in.
effective radius)
Cylinder (about 1.2 in.)
Cylinder
(4-5
gallons)
Cylinder (0.2-1.6
gallons) of 2 in.
radius
Installation method
Access tube
Permanently buried in situ
or inserted for manual
readings
Permanently buried in
situ or PVC access
tube
Permanently buried in situ
or inserted for manual
readings
Permanentl
y buried in
situ
Permanently
buried in situ
Logging capability
No
Depending on instrument
Yes
Yes
Yes
Yes
Affected by
salinity
No
High levels
Minimal
No
>3 dS/m
At high levels
None
Organic, dense, salt or high
clay soils
None
None
None
Organic, dense,
salt or high clay
soils
Field maintenance
No
No
No
No
No
No
Safety hazard
Yes
No
No
No
No
No
Irrigation,
Research,
Consultants
Irrigation, Research,
Consultants
Irrigation, Research
Irrigation, Research
Irrigation
Irrigation
$10,00015,000
$400-23,000
$100-3,500
$500-700
$200-400
$400-1,300
Soil types not
recommended
Application
Cost