Faculty of Environmental Science, Institute of Soil Sience and Site Ecology , Chair of Site Ecology and Plant Nutrition
Soils:
Soil Physics II
Dr. Stefan Julich
Tharandt, 15.12.2016
Part I:
Allready learned about…
•basic physical properties of soils
•
•
•
•
Bulk density dB
Porosity P
Soil texture
Soil structure / Soil Aggregates
• that control behaviour of soils with regard to
•
•
•
•
plant growth
water flow
agricultural management
engineering use
15.12.2016
Soil physics II
Slide 2
Water-Holding Capacity of Soil: Effect of Soil
Texture
Coarse Sand
Silty Clay Loam
Dry Soil
Gravitational Water
Water Holding Capacity
Available Water
Unavailable Water
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Soil physics II
Slide 3
Measurement of soil water content
destructive procedure
non-destructive procedure
Gravimetric method
Time-domain Reflectrometry (TDR)
(Neutron Scattering)
(Gamma-Ray-Absorption)
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Measurement of soil wetness
Gravimetric water
content ( ω )
Mw
ω
Ms
• Mw = mass of water
evaporated, g (24
hours @ 105oC)
• Ms = mass of dry soil,
g
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Soil physics II
Slide 5
Measurement of soil wetness
Gravimetric water
content ( ω )
Example:
•100cm³ soil sample
• wet  150g
• dry  110g
Mw
ω
Ms

• Mw = mass of water
evaporated, g (24
hours @ 105oC)
• Ms = mass of dry soil,
g
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Soil physics II
M 40 g
g

 0.3636
Ms 110 g
g
Slide 6
Measurement of soil wetness
Example:
Volumetric water
content (v)
•100cm³ soil sample
• wet  150g
• dry  110g
Vw
v 
Vb
•Density of water 1g/cm³
• Vw = volume of water
(cm³)
• Vb = volume of soil
sample (cm³)
15.12.2016
Vw 40cm ³
cm ³
v 

 0.4
Vb 100cm ³
cm ³
Soil physics II
Slide 7
Measurement of soil wetness
Time-Domain reflectrometry
Companies (e.g.):
• Soil moisture, Ca.
• Easy Test, Poland
• Imko, Germany
• Theta-Probe, U.K.
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Soil physics II
Slide 8
Time-domain reflectometry (TDR)
Permittivity: a measure of how an electric field
affects, and is affected by, a dielectric medium
water = 81
Soil = 4 to 8
v
air = 1
c

v…propagation velocity
c…velocity o light in vacuum (3*108 m/s)
…permittivity of the medium around
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Soil physics II
Slide 9
TDR – Operating principle
From Dirksen 1999
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Soil physics II
Slide 10
Time-domain reflectometry (TDR)
The propagation velocity of an electromagnetic
pulse travelling along a wave guide is:
2L
v
t
L…length of sensor
t…travel time between A and B
Equating this with the electrodynamic pulse
velocity:
 ct 
 

 2L 
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2
Soil physics II
Slide 11
Wave Form dependent on water content
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Soil physics II
Slide 12
Site spezific calibration: Organic soil
Water content [m · m-3]
0.9
Schwaerzel & Bohl 2003
0.6
0.3
valid for mineral soils
0.0
0
20
40
60
Dielectric number 
80
  .0393  .0284  4.19  104  2  2.77  106  3
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Soil physics II
Slide 13
Terms to know
Dielectric (Is a non-conductor of electricity):
• Is a dielectric placed between a capacitor, no net
flow of electric charge is allowed but only a
displacement of charge
Relative Permittivity (Dielectric number) :
• ratio of the capacitance of a capacitor with the
given substance as dielectric to the capacitance
of the same capacitor with air as a dielectric
water = 81 Soil = 4 to 8
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Soil physics II
air = 1
Slide 14
Pine stand, sandy soil:
Spatial variation of soil water content at the end of
the vegetation period
Greiffenhagen & Wolf 2002
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Soil physics II
Slide 15
Pine stand, sandy soil: Soil water dynamics in the field
Average soil water
content at several
soil depths and net
precipitation during
the experiment.
From Wessolek, 2008
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Soil physics II
Slide 16
Properties of Water - Polarity
• atoms in the water molecule not symmetrical
arranged
• hydrogen atoms attached to oxygen in an 105° angle
• charges are not evenly distributed in the molecule
• hydrogen end is positive; the oxygen end is negative
Source: Brady & Weil 2002
Soil physics II
Slide 17
Properties of Water – Hydrogen Bonding
•
•
•
•
•
Hydrogen atom of one molecule is attracted to an
oxygen atom of another molecule (low energy
bond) process called cohesion
Hydrogen bonding accounts for high boiling point,
specific heat and viscosity
Water molecules also could attach rigidly to soil
particles positive charged side of the molecule
attracted by negative charged surfaces  process
called adhesion
Attached water molecules  hold other water
molecules by cohesion
Through cohesion and adhesion soil particle retain
water or control its movement and use
Soil physics II
Source: Brady & Weil 2002
Slide 18
Bond Strength of adsorped water
dependends on water film thickness
10000 Å = 0.001 mm
From: Hartge & Horn
Soil physics II
Slide 19
Capillarity
Source: Flühler et al. 2005
Soil physics II
Slide 20
Capillarity
• capillary rise due to combined action of cohesion
and adhesion
• adhesion forces causes water molecules to
spread on glass surface  low contact angle
• cohesion forces among water molecules 
surface tension on the air-water interface in the
glass tube  meniscus
• because pressure on free water higher than
under the meniscus  water pushed up
• water is rising until weight of water (gravitation)
balances pressure difference
Soil physics II
Source: Brady & Weil 1996
Source: Brady & Weil 2002
Fup  Fdown
Slide 21
Capillary rise in a soil
• capillary rise in soil follow same principles as in glass
tube (discusse before)
• capillary rise is more irregular due to high variability
in shape and size of pores
• finer texture  more small sized pores  higher
rise  but slower due to higher friction
• movement in soil capillaries in all directions
Source: Brady & Weil 2002
Soil physics II
Slide 22
Soil Water & Energy
• retention and movement of water in soils; water uptake by plants; water loss
to atmosphere  energy related phenomenon
In general two kind of energy:
Kinetic energy: associated with motion
• of minor importance (can be neglected in soils)
Potential energy: associated with position
• of primary importance in determining the state and movement of water
in soil
Soil physics II
Slide 23
Potential Energy
Energy stored by the position of the arrow and
the bowstring.
No energy is stored.
Soil physics II
Slide 24
Total Soil Water Potential
= the work that is required for moving a quantity of
water from a reference state into the desired state
within the soil.
The performance is reversible and isotherm.
= describes the energy density of soil water.
Soil physics II
Slide 25
Total Soil Water Potential
Wet soils:
• most water in (large) pores or thick films around soil particles
• water molecules not very close to particle surfaces
• not tightly held by soil matrix
• Water molcules  high freedom of movement
Dry soils:
• small films around particles
•
water tightly bond (high adhesion)
•
water molecules  low freedom of movement
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Soil physics II
Slide 26
Total soil water potential - Reference or Standard State
In general states of soil water compared to that of pure water:
= pure water,
• at atmosphere pressure,
• at the same temperature as that of soil and,
• constant elevation.
• Soil water potential at the reference state is 0 (per
definition).
Soil water potenrial describes the difference of energy levels
between soil water and pure water!
Soil physics II
Slide 27
Components of Soil Water Potential Ψ
Total Soil Water Potential
w
Gravitational Potential
Tensiometer Pressure Potential
Osmotic Potential
g
tp
o
Pneumatic Potential Hydrostatic Potential
a
p > 0
Matric Potential
m < 0
Ψw  Ψg  Ψa  Ψp  Ψm  Ψo
• All act simultanously to influence water behavior in soils
Soil physics II
Slide 28
Soil Water Potential versus Soil Water Content
Soil water potential
Soil water content
Intensity
Capacity
„To what extent is the stored „How much water is stored?“
water available?“
„In which direction does the
water move?“
• To determine wheter
water will flow from one
point to another
• Useful for budgeting water,
planning drainage lines or
irrigation
Soil physics II
Slide 29
Direct Measurements of Components of Water
Potential
Gravitational potential
• Measure the vertical distance between the
reference elevation and the point of interest.
Osmotic potential
• Extract soil solution, measure its concentration.
Pneumatic potential
• Measure the soil air pressure with a barometer.
Hydrostatic potential
• Measure the vertical height of saturated water
above the point of interest.
Soil physics II
Slide 30
Water Retention of Soils
• due to electrostatic forces soils retain water against gravity
• energy of stored water less than free water
• this difference is called total soil water potential
• matric potential part of total soil water potential
• matric potential is defined as energy required to move water against adsorptive
and capillary forces
• relationship between matric potential and water content  water retention curve
Soil physics II
Slide 31
Water retention curve
http://thealmonddoctor.com/general/irrigati
on-scheduling-part-2-determining-waterholding-capacity/
• suction applied incrementally  first large pores emptied
• increase of suction  emptying smaller pores incrementally
• increase in suction  decrease of thickness around water
partcicles
• therefore increasing suction means decreasing soil water content
• the amount of water remaining in soil function of sizes and
volumes of water filled pores and of the amount of water
adsorbed to soil particles (matric suction)
suction
• function called water retention curve measured experimentally
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Soil physics II
Slide 32
Classical concept of soil-water availability to
plants
Soil physics II
Slide 33
Available Plant Water & Soil Texture
Soil physics II
Slide 34
WRC & Soil Texture
• Clay  few large pores and broad
distribution of particle sizes  gradual
decrease of water content with
increasing suction
• Sand  more larger pores drain at
modest suction  rapid decrease in
capillary region
Soil physics II
Slide 35
WRC & bulk density
Soil Structure & WRC
• compaction causes a shift in pore size distribution
• increasing bulk density  decreasing amount of coarse pores increasing amount of
medium and fine pores
• low suctions shape of WRC influenced by soil structure
• high suctions, the predominant mechanism of water retention is adsorptive shape of
the WRC depends on soil texture
Soil physics II
Slide 36
Progressive draining of water with increasing
suction
Soil physics II
Slide 37
Flow in saturated versus unsaturated soil
• all of the pores are water filled and conducting
• Flow over the whole cross-sectional area
Source:
http://www.terragis.bees.unsw.edu.au/terraGIS_soil/
sp_water-water_flow.html
• some of the pores air filled
• conductive portion of cross-sectional area decreases
• Large pores are emptied first  flow takes place in the
smaller pores
• larger pores must be circumvented (increase of tortuosity)
• progressive desaturation, the flow path length increases
• flow occur either as film creep
• tube flow through narrower and more tortuous channels
Source:
http://www.terragis.bees.unsw.edu.au/terraGIS_soil/
sp_water-water_flow.html
Soil physics II
Slide 38
Flow in saturated versus unsaturated soil
Source:
http://www.terragis.bees.unsw.edu.au/terr
aGIS_soil/sp_water-water_flow.html
Source:
http://www.terragis.bees.unsw.edu.au/terr
aGIS_soil/sp_water-water_flow.html
Saturated flow
• Driven by gravitation and by
hydrostatic pressure differences.
• All pores are water filled and
conducting.
• Water phase is continuous and the
conductivity maximal.
Soil physics II
Unsaturated flow
• Driven by matric forces.
• Pores are partially air filled.
• occur either as film creep along the
walls of wide pores, and as tube flow
through narrower channels.
• Conductivity is low.
Slide 39
Soil hydraulic properties - water retention and conductivity
4
log K(h) [cm d -1]
water content [m3 m-3]
0.45
0.3
0.15
Sand
Silt
0
-4
Sand
Silt
-8
0
-2
0
2
log |suction| [cm]
-2
4
0
2
log |suction| [cm]
4
• Water retention and soil hydraulic conductivity soil hydraulic properties.
• soil hydraulic properties are material characteristic
• every soil horizon has a own WRC and a own conductivity curve
• Water retention and conductivity are related to each other..
Unsaturated hydraulic conductivity decreases as water content decreases because:
1.
2.
3.
Cross sectional area of water flow decreases.
Tortuosity (flow path length) increases.
Increase of flow resistance in smaller pores.
Soil physics II
Slide 40
Infiltration
http://extension.oregonstate.edu/gardening/soiltexture-determines-how-much-and-how-often-water
... refers to the entry of water into a soil profile from the boundary.
The rate of infiltration affects
• the water economy of plant communities,
• the amount of overland flow and
• the soil erosion and stream discharge.
Knowledge of infiltration is prerequisite for efficient soil and water management.
Soil physics II
Slide 41
The process of Infiltration
• dry soil, the matrix forces draw water into the soil
• water pressure head of 0 cm dry soil negative pressure head 
a hydraulic gradient between soil surface and top soilwater will
move into soil
• First soil particles wetted
• more water enters soilsurface tension forces cause water to enter
most the small pores (forming of meniscii)
• more water penetrates into soil force of gravity takes over
water moves down
• early phase of the infiltration matrix forces are dominant, later,
the percolation is driven by the gravity
•rain stops and the supply of water decrease, water drains by gravity
until the menisci reappear and forces of surface tension again
dominate, holding water in pores.
Soil physics II
Slide 42
Daily Variation of Soil Temperature at
different depths
Lufttemp. in 2 m Höhe
Bodentemp in 5 cm
Bodentemp in 10 cm
Bodentemp in 15 cm
Bodentemp in 30 cm
Temperatur [°C]
25
20
15
10
0
4
8
12
Uhrzeit
Soil physics II
16
20
24
Slide 43
Course of Soil and Air Temperature over the year
30.0
Lufttemperatur in 2m Höhe
Temperatur [°C]
Bodentemperatur in 10 cm Tiefe
Schneebedeckung
15.0
0.0
-15.0
01.01. 02.03. 01.05. 30.06. 29.08. 28.10. 27.12.
Datum
Soil physics II
Slide 44
Course of soil and air temperature over the year
... Depends under equal
climate conditions on:
• Texture
• Water content
• Soil Cover
• (Soil Color)
• Topography /
Exposition
Soil physics II
Slide 45
Color
Soil Color
Color after A. Munsell
• Hue

• Value 
redness, yellowness
lightness, darkness
value of 0 = black
• Chroma  intensity, brightness
value of 0 = gray
Soil physics II
46
Slide 46
Color
Soil Color
Soil physics II
47
Slide 47
Soil Color
Stagnosol
Podzol
Soil physics II
Slide 48
Text Books
•Daniel Hillel (2004) : Introduction to Environmental Soil
Physics. Academic Press
•Klaus Bohne (2005) An Introduction into Applied Soil
Hydrology. Catena Verlag
•Don H. Scott (2000) Soil Physics – Agricultural and
Environmental Applications. Iowa State University Press
• William A. Jury & Robert Horton (2004) Soil Physics.
Wiley, Hoboken New Jersey.
Soil physics II
Slide 49