Soil water potential and its components.pptx

1. SOIL WATER POTENTIAL
AND ITS COMPONENTS
Presented By
Syed Farhan Haider Bukhari
Mohammad Ashfaq

2. Water Potential
 Water potential is the energy required, per quantity of water,
to transport an infinitesimal quantity of water from the
sample to a reference pool of pure free water.
 To understand what that means, compare the water in a
soil sample to water in a drinking glass.
 The water in the glass is relatively free and available; the
water in the soil is bound to surfaces diluted by solutes and
under pressure or tension.
 Soil water has a different energy state from “free” water.

3. Soil-Water Potential
 The energy with which the water is held by the soil is as
important as the amount of water in a soil.
 This energy at any given temperature usually is
measured with reference to a flat surface of pure water at
some specified elevation and at a particular pressure.
 Pure water in a saturated soil sample at the same
elevation, pressure and temperature as the reference has
a total water potential of zero.

4.  As defined by the International Society of Soil Science
(1963), the total potential of soil-water is “the amount of
work that must be done per unit quantity of pure water in
order to 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 (at the point under consideration).
 This total water potential (ψt) can be divided into parts to
distinguish between the actions of different force fields.
The algebraic sum of these parts or component potentials
must always equal to the total water potential.

5. Component Potentials
The matric or capillary potential (ψm) which results from
the interaction of soil particles surfaces with water
The osmotic potential (ψ0) which results from the solutes
dissolved in the soil- water
The gravitational (ψg) which results from elevation with
respect to reference level
The pressure potential (ψp) which results from external
pressure on the soil-water.

7. Osmotic Potential
 Measurement of ions (dissolved salt) which exert positive
attraction for water.
 Negative value.
 Indirect measurement of attraction that ions have in soil water
potential equation.
 Assoil salt content increases, more free ions available, larger
negative number.
 Ψo = π where, π = Osmotic pressure due dissolved salts or
solutes.

8. Gravitational Potential (Ψg)
 The gravitational potential, (ψg), is that portion of the total
water potential that is due to the gravitational force field of the
earth and is dependent on the vertical location of the water
relative to the reference level.
 When the water is above the reference level, its gravitational
potential is positive, because it will tend to flow toward the
reference level due to the force of gravity.
 Water below the reference level has a negative gravitational
potential because water at the reference level would tend to
flow toward it.

9. Gravitational potential may be expressed
 ψg (mass) per unit mass = Eg/M = MgZ/M = gZ
 ψg (volume) per unit volume = Eg/v = ρwvgZ/v = ρwgZ

10. Pressure Potential (ψp)
 In a soil-water system, the pressure is usually the result
of overlying water or submergence depth (h) and
atmospheric pressure is the reference.
 Thus, in a soil-water system, the pressure potential will
be positive in a saturated soil and zero in an unsaturated
soil.
 In a plant-water system, the pressure potential is the
result of the resistance to expansion of the cell walls.

11.  The pressure potential in a plant-water system normally
will be positive, but under dry conditions or when the soil-
water has a solute potential lower than the solute potential
of the plant sap, the plant-water potential may become
negative and cause plasmolysis, a separation of the cell
membranes from the cell walls.
 In plants, the pressure potential is sometimes called the
turgor pressure (TP).

12. Pressure potential may be expressed
 ψp (mass) per unit mass = pdv/ρwdv = ρwgh/ρw = gh
 ψp (volume) per unit volume = pdv/dv = p
 ψp (weight) per unit weight = pdv/ρwgdv = p/ρwg
= ρwgh/ρwg = h

13. Matric Potential (ψm)
 The matric potential (ψm) is that portion of the total water
potential associated with the more or less solid colloidal
matrix of the system.
 It has been defined in the literature as both a negative
pressure and a positive suction head.
The matric potential includes the forces of adsorption at
the soil-water interfaces and the forces caused by surface
tension at the air-water interfaces.

14.  Free water has zero matric potential and will move into a
dry soil because of these forces, so the matric potential is
negative for an unsaturated soil and zero for a saturated
soil.
 Thus, the removal of water from a soil-water system
decreases the matric potential of the water remaining in
the system.
 ψm (volume) per unit volume = pdv/dv = p = ρwgh
 ψm (mass) per unit mass = pdv/ρwdv = p/ρw = ρwgh/ρw =
gh
 ψm (weight) per unit weight = pdv/ρwgdv = p/ρwg
= ρwgh/ρwg = h

16. Conclusion of suitable value of
soil water potential for plants
For all the practical purposes, the level of water being
held between -0.1 to -10 bars/atmospheres is really
usable water that being stored in the soil plant.
At -10 to -100 bars/atmospheres is very little water left in
the soil profile. Rarely plants are able survive and utilize
water from soils down to this level.