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

Soil aeration, gas exchange

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

Guest Lectures

Field Visit

“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

Ground water

Capillary fringe zone

Portion of aquifer where pore spaces are occupied with water and air (unsaturated zone)

Precipitation

Vadose Zone

Soil-Air Interface

Soil-Water Interface

Evaporation

Applications of soil physics are crucial to sustainable use of natural resources for agricultural and other land uses

Interaction of soil physics with basic and applied sciences

Greenhouse Effect:- Gaseous efflux of CO2, CH4, NOx - C sequestration aggregation

Particulate matter in air:- Wind erosion- Blowing salt

Fresh water resources and quality:- Suspended and dissolved loads- Biological and chemical O2 demand- Pathogens

Acid Rain:- Water quality- Vegetation cover- Biodiversity

Soil Physics and

Environment Quality

Applications of soil physics to environment quality

Environmental Soil Physics

Soil physical properties and processes

Air quality

Water quality

Soil buffers and filters pollutants out of environment

Soil quality

Quality of Life

Soil properties are highly variable at multiple scales

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

www.seafriends.org.nz/ enviro/soil/soil22.gif

water

Air

Mineral Matter

OM

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 (s)

• Bulk density (wet and dry) (b)

• 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 classificationA 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

Filtering

-ability to withstand or adapt to sudden change

-ability to leach out pollutants

Inorganic Component

Primary Particles Secondary Particles

Discrete units;

cannot be further

subdivided;

also known as soil

separates

sand, silt, clay

Consist of primary

particles; can be

further subdivided

into its separates

Particle size distribution Texture

Quantitative

measure of particle

size constituting

the solid fraction

Qualitative – based

on feel method

-coarse, gritty, fine,

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 ISSS System

Soil separate Size range (mm) Soil separate Size range (mm)

Very coarse sandCoarse sandMedium sandFine sandVery fine sand

SiltClay

2.00 - 1.001.00 - 0.500.50 - 0.250.25 - 0.100.10 - 0.05

0.05 - 0.002< 0.002

Coarse sandFine sandSiltClay

2.00 - 0.200.20 - 0.02

0.02 - 0.002< 0.002

D > 2 mm is known as nonsoil or skeletal fraction

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

SizeShapeFeelPlasticityCohesionSurface areaMineralogy

Heat of wettingSecondary particlesWater holding Capacity Hardness

Ion exchange capacity

2-0.02 mmjaggedgrittynot plasticnot cohesivevery lowprimary

nonenonone/slight

5.5-7 (on mhos scale)none

0.02-0.002 mmslightly irregularsmooth, flouryslightly plasticslightly cohesivemoderateprimary minerals

minimalfewmoderate

5.5-7.0

very low

<0.002 mmplaty/tube likestickyplasticcohesive, gelatinousvery highsecondary clay mineralshighforms aggregateshigh, hygroscopic

--

high to very high

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

Sieving 100.0 - 0.05Sedimentation 2.0 - < 0.002Optical Microscope 1.0 - 0.001Gravity sedimentation 0.1 - 0.0005Permeability 0.1 - 0.0001Gas absorption 0.1 - 0.0001Electron microscope 0.005 - 0.00001Elutriation 0.05 - 0.005Centrifugal sedimentation 0.01 - 0.00005Turbidimetry 0.005 - 0.00005

Methods of fractionation Approximate size range (mm)

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)

Size distribution curve (schematics)

r1 r2

Diameter, mm

% Finer

D60D10

60

10

Uniformity Coefficient = D60/D10

For uniform particle size

UC = 1

UC>1 for nonuniform

0.1 10

Particle Shape

(micrograph)

Depends on :

- 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

i

i

nR

r

Roundness1 c

d

D

DSphericity

ri – radius of corner

R- radius of maximum circle

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

r1

Dc

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)

3

2

3

2

2

m

m

V

Aa

m

m

V

Aa

g

m

M

Aa

t

sb

s

sv

s

sm

As – total surface area

Ms – mass of soil

Vs – volume of soil solids

Vt – total volume

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

ommo p

p

cx

c

cxppx

p 11

)(

Linear form of BET equation

x = weight of gas adsorbed at equilibrium pressurep = equilibrium gas pressurepo = saturation vapor pressure at temperature Txm = weight of gas in a complete monolayerc = exp(E1-L)/RTµE1 = heat of adsorption in the first layerL = latent heat of condensationR = 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

p/p0

p/x(p0-p)

Intercept = 1/xmc = valueSlope =(c-1)/xmc = value

Solve these two equations for xm

mm

t ANM

xS Total surface area of soil sample

St = total surface areaxm = experimentally determined weight of gas in an adsorbed monolayerM = 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 formed by four triangles )

Octahedron

(an eight-sided geometric solid )

Silicon atom placed equidistant from four oxygen or hydroxyls

Si4O6(OH)4Closely 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

Secondary Minerals Weatherability

Geothite Most resistant

Hematite

Gibbsite

Clay minerals

Dolomite

Calcite

Gypsum Least resistant

Commonly observed secondary minerals

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

++++

+

+

+

+

+

+

+

+

++

+

+

+++

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

++

+

+

+

+

+

+

+

++

+

+

+

+

+

Dry

Fully hydrated

Negative charge on clay particles is balanced by the cations in soil solution (due to Coulomb forces).

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

221

d

qqF

+++

+

+

+

+

+

+

+

+

+

++

+

+

Diffuse layer+

+

+

+Clay Particle

Soil Solution

++

+

Electric double layer is due to the negative charge on clay particles and positive on surrounding cations in solution

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

PotentialPotential

Distance

Stern’s double layer

Helmholtz layer (Fixed)

Gouy’s layer (Diffuse)

There are three models for explaining distribution of ion in water layer adjacent to clay

Potential

Distance

Zeta Potential

Nernst Potential or Total Potential

Stern double layer comprises of two parts: single ion thick layer fixed to solid surface diffused layer extending some distance into liquid phase

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

Thickness of double layer is the distance from the clay surface at which cation concentration reaches a uniform or minimum value

Stability of clay suspension

High activity clays Low activity clays

Clay lattice

Fully hydrated clay particles are completely dispersed

Greater distance between charged particles

montmorillonite, vermiculite

kaolinite

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 solution- weak 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, , in water at 25oC can be expressed as: = 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

- 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

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”

Soil Structure

Pedological Edaphological

- 3-D arrangement of particles (O + IO)

- Mechanistic with regard to components

- size, shape, arrangement, and packing into identifiable units (aggregate, peds)

Science dealing with influence of soils on living things, plants

EngineeringEcological

1. Functional attributes such as voids and pores governing plant and root growth

2. Soil-pore system

1. Intraaggregate pore

2. Interaggregate pores

Ecological =

Pedological + Edaphological

Intra-aggregate (within aggregate) pore space influences water retention

Differences in Inter-aggregate (between aggregates) pore space can influence water and solute movement through soil profile

Macroaggregate > 0.25 mm

diameter

- 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

Mechanisms of Aggregation

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

Clay – Ca – OOC – R – Ca – OOC – R – Ca – Clay

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

Clay- Water Structure (Rosenquist, 1959)

+-

+-

+-

+-

(C6H10O5)n

C6H7O2 (OH)x (OC2H5)y [O(CH2CH2O)mH]z]n

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)

- Exist in domains up to about 5 m in diameter

- Clusters of domains are called microaggregates (5-1000 m)

- Clusters of microaggregates are macroaggregates (1-5 mm)

Sand or Silt Particles

Soil macroaggregates

Microaggregates

x

xx

xx

x

x

xx

x

xx

x

xx

Organic molecule

x

Domain of clay Crystals for microaggregate

Quasi Crystals Theory (Aylmore and Quirk, 1971)

- Modified Williams et al. 1967 theory

- Parallel clay crystals (5 m in diameter) forms quasi crystals (0.01-1.3 m)

- Quasi crystals are stable packets (Oades and Waters, 1991)

- the 3 stages of binding of clay particles are:

- into stable packets of < 20 m

- into microaggregates of 20-250 m

- stable macroaggregates >250 m

Microaggregate Theory (Edwards and Bremner, 1967)

- soil consists of microaggregates (< 250 m) bound on macroaggregates (> 250 m)

- bonds are stronger in micro than macroaggregates

- Microaggregate = [(Cl – P – OMx ]y

- Cl is clay, P- polyvalent cation, OM is organometallic complex)

[Cl – P – OM] [Cl – P – OM]x [(Cl – P – OM)x]y

< 0.2 m 0.2 2 20 250 2000 m

Stages of Aggregation (Tisdall and Oades, 1982)

Aggregate Hierarchy Model (Oades and Waters, 1991)

- For aggregates stabilized by organic materials- stages are:

< 0.2 m 20- 90 90-250 >250 m

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

Types of Crusts:

Physical Crusts

Chemical Crusts

Biological Crusts

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

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

Created by Dr. Michael Pidwirny, Department of Geography, Okanagan University College, BC, CA

evaporation

Soil and Water

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

1050

Oxygen

Hydrogen Hydrogen Electro positive

Negative

Polarity

Symmetrical

H-O : 0.97 A

H-H : 1.54 A

angstroms

H+

H+

O--= + -H2O

Hydrogen bond

Gives structural strength

Bond depends on temperature:

Higher is the temperature weaker is bond

Positive end attraction with -ve end of other water molecules

O-

H+H+

Polymer type of grouping

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

0.996

0.994

0.992

0.990

-10 0 10 20 30 40 50

De

nsi

ty (

g c

m-3)

Temperature (0C)

40C

Temperature range in liquid phase for H+ compounds

100

50

0

-50

-100

0 50

Molecular Weight

Te

mp

era

ture

(0C

)

100

H2O

H2S

H2Se

H2Te

Boiling point

Freezing point

Hydrogen sulfide

Hydrogen selenide

Hydrogen telluride

(2+16=18)

(2+32=34)

(80)

(130)

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

Surface of grease has no O and free electrons

Water molecules cannot form hydrogen bond

Therefore, water do not stick

Why water does not stick to glass surface coated with grease?

Forces acting on a water molecules

A

B

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

Air

Water

Air-water Interface

At point B:

Forces acting on water molecule are equal in all direction

At point A:

Attraction of air for water molecules is much less than that of water molecules for each other.

By adhesion, solids hold water molecules rigidly at their soil-water surface

Gravity

Capillary

Capillary Fundamentals and Soil Water

Cohesion: Attraction of molecules for each other

Adhesion: Attraction of water molecules for solid surfaces

Together it is possible for soil solids to retain water and control it’s movement

By cohesion water molecules hold each other away from solid surfaces

Water rises in the capillary against the force of gravity

!!!! What happens if there is no force of gravity !!!!!

Water Water

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

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

Dipolar Bonding in WaterThe 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.

Solid Liquid Gas

Contact Angle

Liquid and gas (air) in contact with solidInterface between air and water forms a definite angle “contact angle”

Solid

AirL

sa > sw; cos = + or < 900

Angle of contact is acute in a liquid that wets the solid

Solid

Air

L

Angle of contact is obtuse (between 90 and 180) in a liquid that does not wet the solid

wa

swsa

cosYoung’s equation

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 paperForce 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

Water

Rise continues till:Weight of water in the tube (force of gravity) = Total cohesive and adhesive forces

2 r1

h1 h2

2 r2

Force of gravity = Mass of water column * Acceleration

= (volume of water * density) * g

= ( * r2* h) *dw * g …………(A)Total cohesive and adhesive forces

= (perimeter) * surface tension

= 2 * * r * …………(B)Water

2 r

h

At equilibrium: A = B

( * r2* h) *dw * g = 2 * * r *

gdrh

w **

*2

rh

15.0

use

= 72.75 dynes/cm

dw= 0.9982 g/cm3

g = 980 cm/s2

Show

rh

15.0

This relationship tells us that:

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

RadiusCa

pill

ary

Ris

e

If two principle radii r1 and r2

21

1115.0

rrh

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

Tortuous flow paths of water

Soil solids

Entrapped air

water

rh

15.0

He

igh

t (cm

)

Time (days)

Clay compacted

Loam

Sand

Brady,1984

Adsorbed water

Capillary 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

Gravimetric moisture content (w) =mass of water evaporated (g)

mass of dry soil (g)

Volumetric moisture content () =volume of water evaporated (cm3)

volume of soil (cm3)

= w *bulk density of soil

density of water g

cm

cm

g

g

g

cmg

cmg

g

g

cm

cm 3

3

3

3

3

3

Bulk density of soil () =mass of dry soil (g)

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)

Indirect methods

Electrical properties

Radiation technique

Acoustic method

Thermal properties

Chemical methods

Electrical Conductance

Dielectric constant

-Neutron scattering- ray attenuation

- Gypsum blocks- Nylon blocks- Change in conductance

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 for soil water content

Direct method (Gravimetric; Thermogravimetric)

Indirect methods

Electrical properties

Radiation technique

Acoustic method

Thermal properties

Chemical methods

Electrical Conductance

Dielectric constant

-Neutron scattering- ray attenuation

- Gypsum blocks- Nylon blocks- Change in conductance

TDR

Methods of soil water content determination

Hand-feel method

FDR ADR

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

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.

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

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.

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.

• Electrical resistance blocks are best suited for finer-textured soils

• They are generally not sensitive to changes in soil water tension less than 100 centibars (cb)

• For most coarse-textured soils readings of 100 cb and above are well outside the available soil water range

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 , repeatabilityDisadvantage: 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)

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

0

K

How an electric field affects and is affected by the medium (farads/m)

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

= -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 , organic or volcanic soils, needs specific calibration - At low frequencies (<100 MHz) it is more soil-specific

Capacitive Sensor- , instantaneous

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

Q = C V

C- capacitance

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

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

Time Domain Reflectometry (TDR): , 28 s

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 and EC

Disadvantage: Costly, not really independent of salt content

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

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 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, , 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

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 non-contact 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 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

TDRFD (Capacitance

and FDR)ADR

Phase Transmissi

onTDT

Reading range 0-0.60 ft3ft-3 0.05-0.50 ft3ft-3 0-Saturation 0-Saturation0.05-0.5

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 tubePermanently 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

Permanently 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

Soil types not recommended

NoneOrganic, dense, salt or high

clay soilsNone 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

Application Irrigation, Research,

Consultants

Irrigation, Research, Consultants

Irrigation, Research Irrigation, Research Irrigation Irrigation

Cost $10,000-15,000

$400-23,000 $100-3,500 $500-700 $200-400 $400-1,300