tillage

1. Course IWM 515: Soil−Water−Plant Relationship
Syllabus
1. Soil physical and chemical properties
2 Soil water characteristic function: Retention and availability
2. Soil water characteristic function: Retention and availability
3. Methods of soil-water measurement
4. Soil-water movement: Steady and unsteady state flow
5. Plant-water physiology
6. Root pressure theories and crop-water uptake functions
7. Stomatal response and physical models
8. Crop response to water
9 Stress tolerance and critical stress periods of crops
9. Stress tolerance and critical stress periods of crops
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2. Reference Books:
1. Plant and Soil Water Relationship: A Modern Synthesis – Paul J.
Kramer. McGraw-Hill Book Company, 1975.
2. Water and Plant Life: Problems and Modern Approaches – O.L.
Lange L Kappen and E D Schulze Berlin Springer-Verlag New
Lange, L. Kappen and E.D. Schulze. Berlin, Springer-Verlag, New
York, 1976.
3. Plant-Water Relationship – R.O. Slatyer. Academic Press, London,
New York, 1967.
S
4. Water, Soil and the Plant – E.J. Winter. The Macmillan Press Ltd.
London, 1978.
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3. Topic: Soil Physical and Chemical Properties
Soil
¾ weathered and fragmented outer layer of the earth’s surface
¾ formed from disintegration and decomposition of rocks
¾ formed from disintegration and decomposition of rocks
(by physical and chemical processes)
¾ later influenced by the activity of residues of biological species
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4. Soil physics
¾ a branch of soil science
¾ deals with
9 h i l ti f th il
9 physical properties of the soil
9 description, management, and control of physical processes
9 state and movement of matter (water, air) in soil
9 fluxes and transformations of energy in soil
fluxes and transformations of energy in soil
¾ provides tools for management of a soil by
9 irrigation
9 drainage
9 soil and water conservation
9 soil and water conservation
9 soil tillage
9 soil structure improvement
9 soil aeration
9 heat regulation
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5. Soil fertility
¾ chemical fertility
9adequate amount of substances required for plant nutrition
¾ physical fertility
9loose, soft and friable to permit root development
9pore volume and size distribution to ensure movement and
retention of water and air
¾ overall productivity depends on physical and chemical fertility
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6. S il i di d th h t
Soil is a dispersed three-phase system
Phase
¾ a region inside a system with uniform physical properties
¾ a region inside a system with uniform physical properties
¾ e.g., mixture of ice and water: chemically uniform,
physically heterogeneous
¾ so water – ice mixture has two phases
¾ so, water – ice mixture has two phases
¾ homogeneous system
¾ consists of a single substance
¾ properties in all parts similar
¾ properties in all parts similar
¾ heterogeneous system
¾ properties differ between the phases and between internal
parts of each phase
parts of each phase
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7. Dispersed system
Dispersed system
¾ at least one of the phases is subdivided into small particles
¾ they give large surface area
¾ soil is a heterogeneous polyphasic particulate disperse and
¾ soil is a heterogeneous, polyphasic, particulate, disperse, and
porous system
¾ dispersed nature of a soil and its interfacial activity cause:
9swelling shrinkage dispersion
9swelling, shrinkage, dispersion
9aggregation, adhesion, adsorption
9ion exchange, etc.
¾ A soil has three phases: solid liquid and gaseous phases
¾ A soil has three phases: solid, liquid and gaseous phases
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8. Soil particles
¾ clay (<2 mm) is chemically and physically reactive
¾ formed as secondary products from weathering of rocks
y p g
¾ non-clay fraction is inert mineral and rock fragments
¾ non-clay divided into silt, sand and gravel
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9. Pore space
¾ soil occurs as a collection of single individual grains (sands)
¾ also linked into clusters or aggregates of varying stability
gg g y g y
¾ properties of the particles are masked by clustering
¾ between soil particles there is a complex system of pore space
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10. Soil density
¾ particle density
¾ bulk density: dry and wet bulk density
¾ bulk density: dry and wet bulk density
¾ dry specific volume: volume of a unit mass of dry soil
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11. Soil wetness/soil-water content
¾ mass wetness/gravimetric or thermo-gravimetric water content
¾ volume wetness/volumetric water content
¾ volume wetness/volumetric water content
¾ degree of saturation: volume of water in a soil relative to the
volume of pores
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12. Soil texture
¾ relative proportions of sand, silt and clay
¾ expression of the predominant size
¾ i f th ti l
¾ or size range of the particles
Soil structure
¾ mutual arrangement, orientation, and organization of particles in a soil
¾ determines soil productivity by affecting water, air, heat
¾ influences soil mechanical properties
¾ affects seed germination, seedling establishment, root growth
¾ affects tillage, irrigation, drainage, planting
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13. Soil profile
¾ vertical section through a soil mass
g
¾ a soil column has a series of distinct layers
¾ the layers are approx. parallel with soil surface (soil horizons)
¾ 3 distinct horizons: A, B, C
, ,
¾ sub-groups for A and B-horizons
¾ A-horizon: zone of maximum biological activity
¾ B-horizon: influenced strongly by soil forming factors
g y y g
¾ B-horizon has a blocky or prismatic structure
¾ organic matter is low in B-horizon
¾ C-horizon: layer of unconsolidated material,
C o o aye o u co so dated ate a ,
less affected by organisms
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14. Specific surface
¾ total surface area of particles per unit mass of a soil
¾ depends upon sizes of soil particles
¾ sand: <1 m2/g
¾ clay: 800 m2/g (montmorillonite)
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15. Behaviour of clay
¾ clay exhibits large specific surface area
¾ most active in physicochemical processes
¾ adsorbs water and cause swelling and shrinkage
¾ negatively charged
¾ negatively charged
¾ form an electrostatic double layer with exchangeable cations
¾ sand and silt is soil skeleton, clay is flesh
¾ clay minerals are layered aluminosilicates
¾ clays are silicates: negatively charged O2- are co-ordinated around
positively charged silicon Si4+, aluminium Al3+ or magnesium Mg2+
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16. ¾ isomorphous replacements:
9 Al3+ for Si4+
9 Mg2+ for Al3+
9 Mg for Al
¾ internally unbalanced negative charges occur
¾ incomplete charge neutralization of terminal atoms
¾ -ive charges balanced by exchangeable cations
¾ -ive charges balanced by exchangeable cations
¾ cations can be replaced/exchanged by other cations
¾ cation exchange phenomenon is important
¾ affects retention and release of nutrients and salts
¾ affects retention and release of nutrients and salts
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17. Exchangeable cations
¾ ability of a soil to exchange cations with those in solutions
y g
¾ Cation exchange capacity (CEC)
¾ CEC: adsorbed cations on particle surfaces per unit
mass of a soil under chemically neutral conditions
ass o a so u de c e ca y eut a co d t o s
¾ constant and independent of the species of cation
¾ soils vary in CEC from nil to 0.60 meq/g or more
¾ montmorillonite has a CEC of 0.95 meq/g
¾ montmorillonite has a CEC of 0.95 meq/g
¾ kaolinite has a CEC of 0.04-0.09 meq/g
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18. Diffuse-Double Layer (DDL)
¾ clay surface has negative charges
y g g
¾ cations of opposite charges are in solution
¾ constitute a diffuse electrical double layer (EDL)
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19. Swelling and shrinking
¾ swelling on wetting
¾ shrinking on drying
¾ have marked effects on structure and water movement
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20. Hysteresis
y
¾ soil wetness vs. matric potential relation not unique
¾ the relation obtained in:
9 in desorption
p
9 in sorption
¾ soil wetness at a given suction is greater in desorption than in
sorption
¾ dependence of wetness and its state upon direction of the
processes is hysteresis
¾ Reasons:
9 geometric non-uniformity of pores
9 contact-angle effect
9 entrapped air
9 swelling, shrinkage
9 aging phenomena
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21. Topic: Soil
Topic: Soil-
-water characteristic function: retention and availability
water characteristic function: retention and availability
Energy state of soil water
¾ soil water contains kinetic and potential energy
¾ soil water contains kinetic and potential energy
¾ it is under the influence of several forces:
9weight of water standing above a submerged position
9interaction with soil matrix in unsaturated soil
9 l t i il t
9solutes in soil water
9action of external gas pressure
9elevation (gravitation)
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22. ¾ potential gradient gives water-moving force
p g g g
¾ total soil-water potential is the amount of work that must
be done per unit quantity of pure water in order to
transport reversibly and isothermally an infinitesimal
transport reversibly and isothermally an infinitesimal
quantity of water from a pool of pure water at a specified
elevation at atmospheric pressure to the soil water
(ISSS).
¾ total potential consists of pressure/matric potential,
gravitational potential, and osmotic potential.
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23. Retention of water by soil matrix
Retention of water by soil matrix
¾ water is held within soil matrix
9by adsorption at particle surface
9by capillarity in the pores
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24. S il t t ti f ti
Soil-water retention function
¾relates energy state of soil water to its water content
¾needed for characterizing soils and vadoze zone
g
¾needed for simulating fluid flow and mass transport
¾related to unsaturated hydraulic conductivity function
¾parametric models needed for this function.
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25. Characteristics of water retention curves
¾ θ = volumetric soil-water content
¾ θ = saturated water content
¾ θs = saturated water content
¾ (θs ≈ 0.85 − 0.90φ; φ is porosity)
¾ hm = matric pressure head
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26. A typical soil-water retention curve with various features
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27. ¾ θ remains at θs for hm slightly < 0
¾ h at which a soil starts to desaturating is air entry value/
¾ hm at which a soil starts to desaturating is air entry value/
air entry pressure/bubbling pressure, hm.a
¾ as hm decreases below hm.a, θ decreases according to
a S-shaped curve with an inflection point, hm.i
¾ with further decrease in hm, θ decreases asymptotically
towards a residual water content, θr
¾ most models describe soil-water retention curves for θr ≤ θ ≤θs
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28. ¾ an effective saturation, Se, used in some retention models
( )
( )
r
s
r
e
S
θ
θ
θ
θ
−
−
=
¾ Se ranges between 0 and 1
¾ θr a fitting parameter
¾ hm.a, hm.i, θs and θr are major parameters of a retention curve
¾ often a pore size distribution index λ is included in the models
¾ often a pore size distribution index, λ, is included in the models
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29. Models for soil-water retention curves
Brooks and Corey model (1964)
¾ power function model
¾ power function model
¾ effective saturation, Se, as a power function of hm
¾ for hm < hm.a
λ
⎟
⎠
⎞
⎜
⎜
⎝
⎛
= a
m
e
h
h
S .
¾ Se = 1 for hm ≥ hm.a
¾ λ is a pore size distribution index (0.3−10.0)
⎠
⎝ m
h
¾ λ is a pore size distribution index (0.3 10.0)
¾ solution for hm.a & λ by plotting Se against (-hm) on a log-log paper
¾ solution by model fitting
¾ good for soils with well-defined air-entry values and J-shaped
¾ good for soils with well defined air entry values and J shaped
retention curves
¾ poor for S-shaped retention curves (finer-textured soils and
undisturbed field soils)
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30. C b ll d l (1974)
Campbell model (1974)
identical to Brooks and Corey model
dependent variable is degree of saturation (θ/θs), not Se
depe de t a ab e s deg ee o satu at o (θ/θs), ot Se
the model is
for h < h
λ
θ ⎞
⎜
⎜
⎛
= a
m
h .
for hm < hm.a
for hm ≥ hm.a
θ ⎠
⎜
⎝ m
s
h
λ
θ
1
= m m.a
θs
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31. van Genuchten model (1978, 1980)
( )
[ ]m
n
h
S
−
1
¾ α = parameter (>0) used to scale the matric head
( )
[ ]
n
m
e h
S −
+
= α
1
¾ α parameter ( 0) used to scale the matric head
¾ m & n = dimensionless parameters (n>1)
¾ ( >1 0 < < 1)
m
1
1
=
¾ (n >1, 0 < m < 1)
¾ solution by fitting algorithm (RETC or UNSODA)
¾ increases parameters from 2 to 3
n
m 1−
=
p
¾ more flexible in fitting retention curve
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32. Exponential model
¾ Tami (1982), Russo (1988), and Ross and Smettem (1993)
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=
i
m
m
i
m
m
e
h
h
h
h
S
.
.
exp
1
¾ gives less accurate results than 2 & 3 parameter models
Log normal distribution model
Log normal distribution model
¾ Gardner (1956) introduced such a model
¾ rarely used
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33. Multi-modal retention function
¾ A soil is unimodal when characterized by a single pore-size
distribution function
¾ undisturbed soils may exhibit retention curves with more than one
inflection point
¾ fitting of a single, sigmoidal retention curve model is then
¾ fitting of a single, sigmoidal retention curve model is then
unsatisfactory
¾ a multi-modal function is needed
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34. Water capacity function
¾ t l t t d t fl (Ri h d ) ti
¾ to solve unsaturated water flow (Richards) equation:
9 water capacity function (C) required
9 C function computed from slope of retention curve
( ) ⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
=
=
m
e
r
s
m
m
dh
dS
dh
d
h
f
C θ
θ
θ
)
(
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35. Unsaturated hydraulic conductivity function
¾ commonly used hydraulic conductivity function is
( )
[ ]2
/
1
1
1
m
m
e
l
e
r S
S
K −
−
=
Kr = relative hydraulic conductivity (= Kunsat/Ksat)
Se = effective saturation
l = tortuosity parameter
m = van Genuchten parameter
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36. Model fitting
¾ model of soil-water retention
¾ model of unsaturated hydraulic conductivity
¾ models fitted through measured data
¾ nonlinear least-square optimization procedures
¾ parameters optimized to minimize objective function
¾ objective function contains residual sum of squares of observed
and fitted values
and fitted values
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37. Soil water availability
Soil moisture tension
¾ measure of firmness with which water is retained in soil
¾ force per unit area required to remove water from soil
¾ measured in terms of potential energy of water
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38. Soil-water constants
¾ S
¾ Saturation capacity
¾ Field capacity
¾ Permanent wilting point
¾ Ultimate wilting
¾ Ultimate wilting
¾ Available water
¾ Readily available water
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39. Topic: Methods of Soil-Water Measurement
¾ Destructive methods:
9 gravimetric/thermo gravimetric method
g g
¾ Non-destructive methods:
9 TDR, capacitance probe
¾ Direct methods:
9 gravimetric method, neutron moderation
¾ Indirect methods:
9 tensiometer, gypsum block
, gyp
¾ Electrical methods:
9 TDR, capacitance probe
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40. Topic: Soil-Water Movement
¾ Early theories of fluid dynamics based on ideal/perfect fluid
¾ Id l fl id t ti l hibit
Topic: Soil-Water Movement
¾ Ideal fluid: contacting layers exhibit
no tangential forces
but only normal forces
Flux/flux density/Darcy velocity
¾ volume of water passing per unit X-sectional area
perpendicular to flow direction per unit time
¾ dimension is length per unit time
¾ dimension is length per unit time
¾ flux is different from flow velocity
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41. Flow velocity
¾ flow velocity in a soil is highly variable
¾ wider pores conduct water more rapidly
¾ water in the centre of a pore moves faster
¾ no a single velocity of water flow in soils
¾ but average velocity
¾ but average velocity
¾ no flow through entire cross-sectional area
¾ real flow area smaller than total area
¾ actual avg. flow velocity > flux
¾ mean flow velocity: flux /soil-water content
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42. Tortuosity
¾ actual flow path is greater than length of soil column
¾ tortuosity: actual flow path / apparent or straight flow path
¾ a dimensionless factor, range 1 − 2
Hydraulic conductivity
¾ flux per unit hydraulic gradient
¾ flux per unit hydraulic gradient
¾ or slope of the flux vs. gradient curve
¾ constant for saturated soils of stable structure
¾ 10-2 − 10-3 cm/s for sandy soil, 10-4−10-7 cm/s for clayey soil
¾ ff t d b il t t d t t h i l
¾ affected by soil texture and structure, chemical,
physical and biological processes
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43. Permeability and fluidity
¾ hyd. conductivity depends upon attribute of soil and fluid
y y p p
¾ soil characteristics that affect K is pore geometry
¾ fluid attributes that affect K are fluid density and viscosity
¾hydraulic conductivity kf
K =
¾hydraulic conductivity
¾ intrinsic permeability of soil
kf
K =
g
Kv
k
ρ
=
¾ fluidity of fluid
v
g
f
ρ
=
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44. Hydraulic conductivity−permeability−pore geometry relation
y y p y p g y
¾ Permeability: a characteristic physical property of soil
¾ relates to certain measurable properties of pore geometry
¾ Kozeny theory its modification by Carman (1939) are accepted
¾ Kozeny theory, its modification by Carman (1939) are accepted
¾ Kozeny-Carman equation is:
3
η
η = the porosity
2
2
)
1
( η
η
−
=
ca
k
η = the porosity
a = specific surface exposed to fluid
c = constant representing a particle shape factor
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45. Principle of water movement in soil
¾ total potential = gravitational + pressure + osmotic potentials
¾ soil water moves from greater to lesser energy
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46. Poiseuille’s equation
¾ soil is a bundle of straight, smooth tubes of uniform radius
g ,
¾ flow rate = sum of flow rates through individual tubes
¾ for laminar flow equation:
p
R4
∆
π
vL
p
R
Q
8
∆
=
π
Q = flow rate through a section of soil of length L
R = radius of flow tube
v = viscosity of water
∆p = pressure difference between two ends of flow tube
Limitations
¾ soil pores do not resemble uniform, smooth tubes
¾ highly irregular, tortuous and interconnected
¾ constrictions, necks and dead-end spaces limit flow through soil pores
¾ actual geometry and flow pattern of soil is complicated
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46

47. Darcy’s Law
¾ flow through soil is described in terms of a macroscopic flow vector
¾ it is overall avg. of microscopic velocities over total soil volume
¾ it is overall avg. of microscopic velocities over total soil volume
¾ detailed flow pattern ignored
¾ soil is treated as a uniform medium
¾ flow is assumed to spread out over entire cross section
¾ D ’ l i i il t li t t ti f h i
¾ Darcy’s law is similar to linear transport equations of physics
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48. Limitations
¾ not always valid for all conditions of water flow in soil
¾ at high flow velocities, inertial forces are not negligible compared to
viscous forces
¾ linearity of flux vs. hydraulic gradient relationship then fails
¾ applies only for laminar flow
¾ applies only for laminar flow
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49. Flow through saturated soils
¾ a driving force due to potential gradient causes soil-water flow
¾ flow takes place in the direction of decreasing potential
¾ flow takes place in the direction of decreasing potential
¾ rate of flow is proportional to potential gradient
¾ equation for flow through a saturated anisotropic soil:
2
2
2
H = hydraulic head
0
2
2
2
2
2
2
=
∂
∂
+
∂
∂
+
∂
∂
z
H
K
y
H
K
x
H
K z
y
x
H = hydraulic head
Kx, Ky, Kz = hydraulic conductivity in x, y, z axe
¾ isotropic soil: Kx = Ky = Kz
¾ the equation reduces to the Laplace equation:
0
2
2
2
2
2
2
=
∂
∂
+
∂
∂
+
∂
∂
z
H
y
H
x
H
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49
∂
∂
∂ z
y
x

50. Flow through unsaturated soils
¾ water in an unsaturated soil is subject to a sub-
t h i / ti
atmospheric pressure/suction
¾ gradient of suction constitutes a moving force
¾ water tends to flow from low to high suction
¾ moving force is the greatest at wetting front
g g g
¾ field soil is unsaturated most of the time
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51. ¾ equation for flow through unsaturated isotropic soil under non-steady-
state condition:
K ∂
∂
∂
∂
∂
∂
∂
∂ θ
t
z
K
z
K
z
y
K
y
x
K
x ∂
∂
=
∂
∂
+
∂
∂
∂
∂
+
∂
∂
∂
∂
+
∂
∂
∂
∂ θ
ϕ
ϕ
ϕ
)
(
)
(
)
(
K = unsaturated hydraulic conductivity
ϕ = total potential
θ = soil-water content at time t
t
x
K
x ∂
∂
=
∂
∂
∂
∂ θ
ϕ
)
(
¾ for flow in horizontal direction:
¾ for flow in vertical direction:
t
z
K
z
K
z ∂
∂
=
∂
∂
+
∂
∂
∂
∂ θ
ϕ
)
(
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52. Diffusivity = hydraulic conductivity / water capacity
¾ flow equation is changed into the form of diffusion equation
¾ solutions are available for such equation
¾ by chain rule: ∂
∂
=
∂ θ
ϕ
ϕ
= wetness gradient
x
x ∂
∂
∂ θ
.
x
∂
∂ θ
∂
= reciprocal of the specific water capacity c(θ)
= slope of soil moisture characteristic curve
θ
ϕ
∂
∂
ϕ
θ
θ
∂
∂
=
)
(
c
ϕ
∂
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52

53. ¾ flow equation for horizontal flow
t
x
D
x ∂
∂
=
∂
∂
∂
∂ θ
θ
)
(
¾ flow equation for vertical flow
t
z
K
z
D
z ∂
∂
=
∂
∂
+
∂
∂
∂
∂ θ
θ
)
(
¾ D is soil-water diffusivity
)
/(
)
(
θ
θ
∂
∂
= K
D = hydraulic conductivity / water capacity
)
(
)
(
ϕ
∂ = hydraulic conductivity / water capacity
∂
∂ θ
θ
2
t
x
D
∂
∂
=
∂
∂ θ
θ
2
¾ for constant D, is diffusion equation.
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54. Topic: Plant-Water Physiology
Soil-plant-atmosphere continuum (SPAC)
¾ soil, plant and atmosphere form a physically integrated dynamic system
¾ various flow processes occur interdependently like links in a chain
¾ the unified system is the soil-plant-atmosphere continuum (J.R. Philip)
¾ flow rate through each segment of SPAC is proportional
¾ flow rate through each segment of SPAC is proportional
9directly to the potential gradient
9inversely to the segment’s resistance
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54

55. The flow path in SPAC includes:
¾ liquid water movement in soil toward root
¾ liquid and vapour movement across soil to root contact zone
¾ liquid and vapour movement across soil-to-root contact zone
¾ absorption into roots and across their membranes to vascular tubes of
xylem
¾ transfer through xylem up the stem to leaves
¾ evaporation in intercellular spaces within leaves
¾ vapour diffusion through substomatal cavities and out the stomatal
perforations to adjacent boundary air layer in contact with leaf surface, and
through it
through it
¾ finally, to the turbulent atmosphere that carries away water extracted from
soil
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56. ¾ <1% of ater absorbed b plants is sed in photos nthesis
¾ <1% of water absorbed by plants is used in photosynthesis
¾ >98% is lost as vapour through transpiration
¾ the process is made inevitable by moist cell surfaces
¾ necessary to facilitate absorption of CO2 and O2
y p
¾ transpiration is a necessary evil (Sutcliffe, 1968)
¾ transpiration induces greater uptake of nutrients from soil
(once believed)
¾ but water and nutrient uptakes are independent
¾ but water and nutrient uptakes are independent
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57. Plant-water content
¾ expressed by cell turgidity
¾ full turgidity: at equilibrium with water at its standard state
¾ full turgidity: at equilibrium with water at its standard state
(pure free water at 1 bar)
¾ relative turgidity: water content of a cell / water content at full turgidity
¾ relative turgidity > indicative of deviation of actual situation from maximum
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58. Transpiration rate
¾ a plant of vigorous growth transpires heavily
¾ velocity of solute transport increases/decreases with transpiration rate
¾ compensation of solute concentration and velocity in xylem
¾ l l it hi h t ti d i
¾ low velocity, high concentrations and vice versa
¾ low transpiration rate-induced nutrient deficiency is rare
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59. Energy relations
¾ water potential is important in plant water relations
¾ determines whether a process is at equilibrium or not
¾ determines direction of a spontaneous reactions
¾ potential differences are the driving forces
¾ water of potential <-15 bars is unavailable to plants
¾ water of potential < 15 bars is unavailable to plants
¾ plants wilt under such condition
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60. Pressure relations
Pressure relations
¾ cell swelling governed by pressure difference
¾ turgor pressure essential for mechanical support of leaves
¾ turgor pressure needed for cell elongation
¾ guard cells regulated by turgor pressure
¾ outside pressure on a leaf is atmospheric
¾ transmitted to interior via cell wall system
¾ transmitted to interior via cell wall system
¾ leaf structure withstands internal pressure through elastic extension
¾ cannot balance external pressure through compression of cell walls
¾ cell walls in mesophytic tissues tend to collapse and cells shrivel
¾ cytorrhysis a major mechanism of injury under severe drought
¾ water under tension (turgor pressure <-1 bar) in metastable state
¾ When bubble forms, cell water transfers to neighbouring cells
¾ transfer involves some cytoplasma along plasmadesmata
¾ transfer involves some cytoplasma along plasmadesmata
¾ the injury is permanent
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61. Metabolism and water relations
¾ water a metabolic agent in plant life
¾ water a metabolic agent in plant life
¾ source of H2 atoms for CO2 reduction in photosynthesis
¾ solvent and conveyor of transportable ions and compounds
¾ major structural component of plants
j p p
¾ remains in cell vacuoles under positive pressure
¾ keeps cells turgid and gives rigidity to plant
¾ cell wall consists of cellulose/polysaccharides/protoplasmic
material >cytoplasm
material >cytoplasm
¾ cell wall permeable to water molecules
¾ water molecules diffuse or flow into and out of cells
¾ cell wall contains large interstices and allows solute movement
¾ inside cell wall and surrounding cytoplasm is a lipoprotein cell
membrane (plasmalemma)
¾ it is selectively permeable
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62. ¾ solutes contained in a central cavity (vacuole)
¾ l d d b b (t l t)
¾ vacuole surrounded by a membrane (tonoplast)
¾ dissolved sugars and salts cause negative osmotic
potential in cell water
¾ Potential: range from -5 to -50 bars
g
¾ transpiration rate controls stomata opening/closing
¾ light intensity, CO2 conc. and temperature control stomata closure
¾ stomata affected by tissue hydration and turgidity of guard cells
¾ in absence of transpiration plants absorb water from soil by osmosis
¾ in absence of transpiration, plants absorb water from soil by osmosis
¾ plant water exhibits a positive hydrostatic pressure
¾ when transpiration takes place, plant-water pressure falls below
atmospheric pressure
¾ resulting suction induces upward mass flow from roots to leaves
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