Introduction to soil water relationships

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Introduction to soil water relationships
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• Particle density (rs)
• Definitions
• Mass of soil particle divided by volume of
  soil particle
• Specific gravity, SG ratio of mass of soil
  particle to
• mass of equal volume of water of water
  at 4C
• Particle density in cgs or tonnes m-3
• numerically equal to SG
• mean particle density depends on
• ratio of OM to mineral matter
• constitution of soil minerals
• constitution of OM

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• Determination
• SG bottle
• boiled water to remove dissolved air
• de-aerate for several hours with vacuum pump
  to remove
• air trapped between particles
• problem of floating OM

• Typical values
• organic matter 1.3 g cm-3
• quartz 2.66 g cm-3
• average for clay 2.65 g cm-3
• orthoclase 2.5 to 2.6 g cm-3
• mica 2.8 to 3.2 g cm-3
• limonite 3.4 to 4.0 cm-3
• Fe (OH)3 3.75 cm-3
• normally taken as 2.65 cm-3

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Bulk density (rb) and related parameters
Bulk density rb mass of solids
total volume
Value is effected by particle density, degree of
compaction, organic matter content
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• Typical values
• 0.9 for organic soil (peaty) to 1.8 for
  compacted sand
• Sand generally has a higher density than clay -
  why?
• What do we mean by heavy light soils?
• Determination
• soil coring devices
• problems of compaction
• oven drying at 105C
• gamma ray transmission

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• Gamma ray transmission
• measures density
• 2 probes - transmitter detector

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Wet v dry bulk density
Ms Mw Vt
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Coefficient of linear extensibility (COLE) Bulk
density changes in swelling - shrinking
soils. COLE is a measure of this
Compares dry with saturated soil after it comes
to equilibrium. Cracks complicate the problem of
determining BD of swelling soils. Even allowing
for cracks the overall density may be higher on
shrinking as the surface becomes lower.
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Total pore space (T) volume of (air water)
vol. of (air soil water)
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Volume of (air water) total
volume (air soil water) - volume of soil
where Vt and Vs are the volumes of the total
sample and the soil particles respectively
Vs Ms /rs and Vt Ms /rb
where Ms is the mass of oven dry soil and rs and
rb are the particle density and bulk density
respectively. So
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and so
and
rb (1-T)/rs
Used in agricultural (soils) research especially
for compaction studies. Typical values 0.3 to
0.6. Often expressed as a .
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Packing density measure of compaction of
particular texture class
Void ratio (e) Used mainly in engineering
applications e volume of (air water)
volume of soil e T/(1-T) void ratio Typically
0.3 to 2.0
Air filled porosity volume of air volume
of total
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Moisture content and related parameters
(a) Volumetric basis volume of water
volume of total qv Vw/Vt
(b) Gravimetric basis mass of water mass
of soil qm Mw/Ms
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As Vw Mw/rw and Vt Ms/rb then
and so qv qmrb/rw
qmrb/1 (not dimensionally correct) in
metric measurements - density of water is 1
Often expressed as depth/depth for example mm/m
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Degree of saturation (s) degree of saturation
volume of water
volume of (water air) s qV/T Liquid
ratio Liquid ratio volume of water
volume of solid
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An example to try
A hole 30 cm X 30 cm x 30 cm is dug in a field.
The wet soil weighs 50.55 kg. The soil is taken
back to the laboratory and oven dried. The final
weight is 38.34 kg. (a) What is the bulk
density (b) What was the moisture content in the
field (i) by volume (ii) by weight (c) If the
mean particle density is 2.64, what is the
total pore space
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Graphical representation ....
Q. Why is the moisture content less at depth?)
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• Measurement of soil moisture
• Laboratory
• definitive
• weigh, oven dry at 105C for 24 hours, reweigh
• if volume of hole from which sample was taken is
  known, bulk density can be calculated and hence
  volumentric moisture content
• Field methods
• Include
• neutron scattering
• gamma ray transmission
• time domain reflectometry
• all need calibration against laboratory method

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Neutron scattering
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• H scatters and slows neutrons very effectively -
  elastic collisions with atomic nuclei
• called thermalisation of fast neutrons - come
  to same thermal (vibrational) energy as atoms at
  ambient temperature
• hydrogen, has nucleus of about same size mass
  as neutron and so has much greater thermalising
  effect on fast neutrons than any other element
• method detects mostly H atoms not water per se
• single probe containing radioactive source of
  high-energy neutrons such as radium-beryllium or
  americium-beryllium or caesium-137
• thermal neutron density easily measured
• thermal neutron density may be calibrated against
  water concentration on volume basis of other
  sources of H are constant

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• Time domain reflectometry
• measures dielectric constant - ability of soil to
  
• transmit electromagnetic (radar) waves -
• mostly but not entirely dependent on water

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Theta Probe
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• Simple parameters to characterise H2O O2
  availability
• Soil water potential
• matric potential
• gravitational potential
• pressure potential

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Note on units Soil water potential is the energy
density - usually per unit volume Since
dimensions of energy is ML2T-2 (force x
distance) dimensions of soil water potential has
dimensions of ML-1T-2 Pressure is force per
unit area so has units of MLT-2/L2
ML-1T-2 Soil water potential thus has same units
as pressure. It can this be expressed as bars, cm
H2O, cm Hg, atmospheres SI unit of Pressure,
and so energy density, is the Pascal 1 kPa 10
mb, 1 bar 100 kPa
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Capillarity and adsorbed water combine to
produce matric potential
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• Permanent wilting point
• Usually taken as 15 (1500 kPa) bars, but may be
  more,
• e.g. 20 bars (2000 kPa).
• Water held between 1500 and 2000 kPa negligible
• in virtually all soils.
• PWP strongly correlated with clay.
• In reality, a dynamic property which depends on
• potential evapotranspiration,
• unsaturated hydraulic conductivity of the
  soil,
• type of plant.

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• Field capacity
• the upper limit of available water
• traditionally defined as the moisture content
  of a soil
• 48 hours after saturation and subsequently
  being allowed
• to drain
• a high proportion of irrigation water added
  above
• field capacity is wasted
• FC has also been considered to be
• 0.33 bars 33 kPa in USA or
• 0.1 bars 10 kPa in the UK
• FC also sometimes considered as the mean soil
  moisture
• content in winter (cold climates) when the
  potential
• evapotranspiration is small (and so drainage
  is main factor
• governing equilibrium moisture content.

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The tension equivalent to FC will be at least
equal to the air entry potential - see below.
FC, PWP and AWC are strongly dependent on
texture, OM and BD
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Air capacity Defined as the air content () at
field capacity. Used in poaching studies. Low
air capacity usually means poor aeration.
Available water capacity Difference between FC
and PWP () often x soil depth to give mm
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Exercise
The moisture content of a soil at field capacity
was found to be 27.3 by weight. At wilting
point, the moisture content was 19.7. After
oven drying of a volumetric sample, it was found
that the bulk density was 1.42 g cm-3. What is
the available water capactiy as a percentage of
the volume? A crop has a rooting depth of 1.5 m.
How much water is potentially available to the
crop in mm equivalent. If irrigation is to take
place when the AWC is depleted by 40, how much
water would need to be added?
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Effect of bulk density on air capacity, wilting
point field capacity
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Dependence of compaction on moisture content
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Wet year
Any suggestions?
Dry year
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• Dynamic nature of FC, PWP, AWC
• It is important to realise that FC, PWP and AWC
  are
• commonly conceived as static soil properties
  but that
• in reality, the are used as proxies for
  characteristics of dynamic system.
• They do not take into account
• field conditions such as underlying horizons
• rainfall and or irrigation frequency and
  amount
• hydraulic conductivity of the soil
• run-off characteristics
• roots extension
• water infiltration and redistribution
• drainage from soil profile
• some water may drain at the same time as
• evapotranspiration takes place
• ground cover changes

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• crop height changes
• climate, especially evapotranspiration rate
  effect the
• values
• Beware of too simplistic a view.
• Even so, FC, PWP and AWC are very useful concepts.

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Measurement of soil potential Tensiometers
After Richards, 1965
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• Electrical resistance methods
• Gypsum blocks
• Granular Matrix Sensors
• e.g. WATERMARK sensor from Irrometer Co, USA

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Relationship of soil water potential to soil
vapour pressure
If vapour between soil particles is in
equilibrium with held water, the vapour pressure
is influenced by the pull of the soil water
...
where Yt is the sum of matric and osmotic
potential ? is the density of the water at the
prevailing temperature, R is the Universal Gas
Constant M is the molecular weight of water T is
the Temperature (K) e is the vapour pressure in
the soil pores e0 is the saturated vapour
pressure of free water at the particular
temperature
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The phenomenon is used as the basis of (a) the
determination of the potential of a soil in the
laboratory (often in order to determine the
moisture release characteristics) by
allowing a filter paper of known pore size
/ moisture release characteristics to come
into equilibrium with the moist air over the soil
which is also in equilibrium with the soil
water potential. (b) to determine the
soil water potential in the field by
determining the humidity of the soil air using a
thermocouple psychrometer
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• Moisture release characteristics
• Determination
• pressure plate apparatus
• sand sand/kaolin bath apparatus
• filter paper - allow to come into equilibrium
  and weigh
• paper
• solution - mixture so that vapour pressure is
  known and
• this can be equated to soil potential, allow
  soil to come
• into equilibrium with solution
• use of pF scale

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Filter paper method
top filter paper not in contact - measures sum
of matric and osmotic potential of soil bottom
filter paper is in pore contact so measures
matric potential
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Hysteresis
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Typical curves
Near air entry potential
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Air entry potential (fe) Also known as air entry
value or bubbling pressure pressure at
which largest pores begins to empty Related to
structure and field capacity. fe corresponds to
the largest pore size
where
and
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f es is the air entry potential when the bulk
density is 1.3 g cm-3 f is in J/kg dg
the geometric mean particle diameter, is in
mm, and sg is the geometric standard
deviation of the particle sizes in mm
(ranges from 1 to 30).
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• Example calculation of dg and sg (based on
  Campbell, p.9)
• It is assumed that
• clay has d lt 0.002 mm
• silt has 0.002 lt d lt 0.05 mm
• sand has 0.05 lt d lt 2 mm
• The predictor equations assumes that particle
  size
• distribution is log normal
• Logarithm of geometrical mean is given by
• ln dg S mi ln di
• where the di are the textural class sizes and mi
  are the
• amounts in each class
• The di for the size classes are calculated from
• (lower limit upper limit)/2

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thus dclay 0.001 mm ln(dclay) -
6.91 dsilt 0.026 mm ln(dsilt)
-3.65 dsand 1.025 mm ln(dsand) 0.025
If a soil is 0.6 clay, 0.25 silt and 0.15 sand,
then ln dg (0.6 x - 6.91) (0.25 x - 3.65)
(0.15 x 0.025) - 4.146 - 0.9125
0.00375 - 5.05475
0.00638 mm
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Substituting this in the above equation, the
standard air entry potential is - 12 J kg-1 To
make allowances for bulk density, we
need first to calculate sg. The normal standard
deviation is given by
In a similar way, the logarithmic standard
deviation is given by (ln sg )2 f1(ln d1)2
f2 (ln d2)2 f3 (ln d3)2 - (ln dg)2 The
geometric SD is the antilog of the SD of the log
transformed values. Thus ln sg 2.42 and
so sg e2.42 11.24 and b 2 x 12
0.2 x 11.24 24 2.25 26.25
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Thus for this soil,
For bulk densities of 1.1, 1.3 and 1.5, the air
entry potentials would be -
0.84, - 12
- 148.9 J kg -1 respectively
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Sand tension table (0 to 100 cm potential)
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Kaolin table (100 cm to 400 cm potential)
H should be added to difference between
atmospheric pressure and pressure in aspirator
bottle
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Pressure plate method for potentials from 1 bar
to 15 bars
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Prediction of matric potential Not reliable but
some workers use equations of the form
where qs saturation (vol) and Fe, the air
entry potential is calculated as before from
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Clays Treated here because flocculation in the
field is an essential part of reclamation of
sodic soils. Flocculation occurs when clay
particles stick together because of electrical
forces to form larger particles and hence
improves the hydraulic properties of the
clay Flocculation changes the hydraulic
conductivities and the moisture holding
properties of clay soils.
Clay 0.2 m -2m Colloidal clay lt 0.2m
m 10-6 m
Adsorption concentration of one material at
surface of another Absorption uptake of one
material into another
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Colloidal material is surrounded by thin layer of
solution which is different in composition from
the solution (relatively) far away from the
particles. Layer moves with the particle.
Micelle colloidal particle hydration
shell Intermicellar fluid solution between
micelles
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• Micelles usually negatively charged because of
• isomorphic substitution
• Si in the clay may be substituted by
  Fe or Al
• which makes the clay short of charge and
  so
• negatively charged - smectite or illite type
  materials
• Fe or Mg may replace Fe or Al in
  alumina or
• gibbsite sheets
• ionisation at the surface
• e.g. appearance of OH - at the surface and
  edges of
• micelle
• H2O adsorption and subsequent ionisation
  diffusion
• of H leaving a net negative charge because
  of the OH
• preferential adsorption of anions from
  solution for
• example the adsorption of CO3- onto calcium
  carbonate
• leaving an associated ion in solution

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Some mutual attraction occurs between particles
because of edge effects
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Double layer Double layer is name given to
accumulation of positively charged ions around
negatively charged micelles some attached, some
in solution

• controls flocculation and dispersion
• dependent on
• cation type
• cation concentration
• pH
• the thicker the double layer, the greater the net
  repulsion and the more dispersed a soil becomes
• important in structure and aggregation and
  reclamation of saline and sodic soils

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Negative ion concentration in solution increases
with distance positive ion concentration
decreases
negatively charged soil particle
a layer of cations directly satisfies some of
the negative charge
diffuse second layer eventually reaches
same concentration as surrounding bulk solution
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• Different models
• Helmholtz model - assumes the charge
  concentration
• decreases linearly with distance
• Gouy-Chapman model - assumes charge
  concentration
• decreases exponentially with distance
• Stern model - assumes linear decrease in the
  Stern
• layer near the surface and then an
  exponential
• decrease - thickness of Stern layer normally
  
• taken as equal to the ionic radius of the
  adsorbed
• species.

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The double layer is depressed by increasing the
valency of the ions in the intermicellar
solution and hence the packing of the charge near
the micelle surface. This is known as the
depression of the double layer.
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Effect of cation type
smaller cations, like Mg2, will decrease double
layer thickness
larger cations, like Na, will increase double
layer thickness
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If concentration of intermicellar solution is
increased, concentration of ions in double layer
reaches concentration of intermicellar
concentration nearer the micelle surface
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Effect of Cation Concentration
high ionic concentrations will decrease double
layer thickness
low ionic concentrations will increase double
layer thickness
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pH
Kaolinite edge
neutral pH
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London - van der Vaals forces In addition to
repulsive forces caused by accumulated positive
charges, there is also an attractive force
between clay particles caused by London - van
der Vaals forces. These forces, which occur
even between electrically neutral atoms, are due
to the fact that, although the average
electrical field of a neutral spherical atom is
zero, the instantaneous field is not zero but
fluctuates with the movements of the electrons
in the atom (or ion). When 2 atoms (or ions)
approach, they can synchronise their electronic
motions so that the electrical charge in one
surges towards the other when the fluctuations in
this second atom happen to leave its nuclear
field somewhat exposed in this particular
direction.
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Depending on relative strength of forces of
attraction and forces of repulsion, attractive
van der Waals forces may predominate in which
case flocculation takes place.
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The thickness of the double layer is altered by
both the concentration and the ratio of divalent
to monovalent ions (and the ratio of tri-valent
ions to mono-valent ions). If the constitution
and concentration of the soil solution is
changed, the constitution of the ions in the
double layer will change. Replacement of
monovalent ions by divalent ions in the
double layer makes it thinner and so more easy
for Van der Waals forces to take over and make
the particles stick together
Langmuir equation Relates the amount of
adsorption onto clay particles to the
concentration of the solution. Look it up.
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• Specific surface
• Important effect on
• cation exchange
• retention and release of various chemicals
• (nutrients and pollutants)
• swelling of clays
• retention of water
• engineering properties
• (e.g. plasticity, cohesion, strength )

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am As/Ms av As/Vs ab As/Vt where a is
the specific surface, As is the total surface
area in the sample, Ms is the mass of solids, Vs
is the volume of the solids, Vt is the total
volume of the sample. Suffixes m, v and b refer
to whether specific surface is on a mass basis,
volume of solids basis or volume of total soil
basis.
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Measured from amount of gas absorbed at certain T
and P. Can also be estimated from particle size
distribution distribution of minerals NB.
surface area/volume for sphere 6/d av
Typical values
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Texture and particle size distribution Definitions
of sand, silt, clay 1 ? 10-6 m 10-3 mm sand
50 - 2000 ? silt 2 - 50 ? clay lt 2?
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Texture triangles (using above definitions)
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• Mechanical analysis
• Separation of particles
• OM removed by H2O2
• sometimes CaCO3 cementing agent removed by HCl
• deflocculation by adding Calgon
• (sodium hexametaphosphate)
• mechanical agitation (shaking, stirring,
  ultrasound)

Sieving Use sieves down to 0.05 mm (very fine
sand)
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Sedimentation Theory Falling particle in a
fluid experiences a downward force and
resistance force (drag) in opposite direction.
Stokes (1851) found that the drag was given
by Fd 6phru u is terminal velocity, h is
the viscosity, r is the radius of the sphere
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When the two forces are in equilibrium, particle
reaches a terminal velocity. In that
condition, downward force on the particle
gravity - upthrust
due to fluid density Upthrust weight of
particle - weight of fluid displaced weight of
particle 4/3 p r3 rs g where rs is the
particle density weight of water displaced
4/3 p r3 rf g where rw is the density of fluid So
upthrust is 4/3 p r3 (rs - rf) g At terminal
velocity, 6phru 4/3 p r3 g(rs - rf)
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which can be rearranged as
where d is the diameter of the particle. Since u
h/t where h is height dropped and t is the
time elapsed t h/u and so
and
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Pipette method All particles gt d(h, t) will have
settled out by time t Proportion of original can
be determined by taking a sample After 8 hours
only clay is left in suspension
Hydrometer Measures density of remaining soil
suspension instead of taking a sample
X-ray transmission methods Transmission related
to density. Gives continuous distribution
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Soil structure see handout