Soil water potential and its components.pptx

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 (ψ t ) 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. C omponent P otentials The matric or capillary potential ( ψ m ) which results from the interaction of soil particles surfaces with water The osmotic potential (ψ ) which results from the solutes dissolved in the soil- water The gravitational ( ψ g ) which results from elevation with respect to reference level The pressure potential ( ψ p ) which results from external pressure on the soil-water.

6. 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. Ψ o  = π where, π = Osmotic pressure due dissolved salts or solutes.

7. Gravitational Potential ( Ψ g ) The gravitational potential, ( ψ g ), 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 .

8. Gravitational potential may be expressed ψ g  (mass) per unit mass = Eg /M = MgZ /M = gZ ψ g  (volume) per unit volume = Eg /v =  ρ w vgZ /v =  ρ w gZ

9. Pressure Potential ( ψ p ) 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.

10. 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 ).

11. Pressure potential may be expressed ψ p  (mass) per unit mass = pdv / ρ w dv =  ρ w gh / ρ w  = gh ψ p  (volume) per unit volume = pdv /dv = p ψ p  (weight) per unit weight = pdv / ρ w gdv = p/ ρ w g =  ρ w gh / ρ w g = h

12. Matric Potential ( ψ m ) The matric potential ( ψ m ) 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.

13. 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 . ψ m  (volume) per unit volume = pdv /dv = p =  ρ w gh ψ m  (mass) per unit mass = pdv / ρ w dv = p/ ρ w  =  ρ w gh / ρ w  = gh ψ m  (weight) per unit weight = pdv / ρ w gdv = p/ ρ w g =  ρ w gh / ρ w g = h

14. 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 .