Soil-Water Potential: Meaning and Types | Soil Management

In this article we will discuss about:- 1. Meaning of Soil-Water Potential 2. Types of Soil-Water Potential 3. Measurement.

Meaning of 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.

As the water content of the soil decreases, the force with which the remaining water is held by the soil particles (adhesion) increases. Since energy must be added to this water to restore to the reference state, its potential energy is said to be negative. Similarly, water potential of a soil at a lower elevation than the reference is negative.

If it is higher than the reference level, its water potential can be positive. The same holds good for samples at different pressures than the reference (Fig 4.31). Solutes in the soil-water also reduce its potential energy.

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.

The four component potentials distinguished are:

1. The matric or capillary potential (ψ_m) which results from the interaction of soil particles surfaces with water

2. The osmotic potential (ψ₀) which results from the solutes dissolved in the soil- water

3. The gravitational (ψ_g) which results from elevation with respect to reference level

4. The pressure potential (ψ_p) which results from external pressure on the soil-water.

In unsaturated soils, the pressure potential is usually considered zero and in saturated soils the matric potential is usually considered zero. Osmotic potential is of considerable importance with respect to plant growth but of less consequence where water movement is considered.

If the quantity of water involved in the above definition of water potential is a mass, water potential has units as joules kg^-1 or ergs g^-1. The quantity of water can be expressed in terms of a volume, in which case the water potential has units such as ergs cm^-3, which is dimensionally the same as pressure. Since 106 cm^-3 is numerically equal to a bar pressure, water potential can be expressed in bars.

To avoid the use of negative quantities for expression of the energy state of soil-water, alternate systems of terminology are often used. One of these, the suction system, defines total suction as the negative gauge pressure, relative to the external gas pressure, on the soil- water, to which a pool of pure water must be subjected in order to be in equilibrium through a semi-permeable membrane with the soil-water at the same elevation.

Total suction is the sum of osmotic suction and matric suction. Except for algebraic sign, these component suctions are identical to the same component water potentials defined above where unit volume of water is chosen as a basis for computation. Suction can be expressed in any pressure units. However, atm. or bars or equivalent height of a hanging water column, are the most popular.

Types of Soil-Water Potential:

Soil-water potential (soil-water tension) is a measure of the tenacity with which water is retained in the soil and shows the force per unit area that must be exerted to remove water from the soil. It is measured in terms of potential energy of water in the soil measured, usually, with respect to free water. It is expressed in atmospheres, the average air pressure at sea level. Other pressure units such as cm or mm of water or mercury are also used.

1 atmosphere = 1036 cm of water = 76.39 mm of mercury.

1 bar = 10⁶ dynes cm^-2. = 1036 cm of water.

1 milli bar = 1/1000 bar.

The term capillary potential or pF is also used to define the energy with which water is held by the soil. The pF function, analogous to acidity-alkalinity scale pH, is defined as the numerical value of the negative pressure of the soil moisture expressed in cm of water.

We need not need to compute the soil-water potential directly by computing the amount of work needed, but measure the soil-water potential indirectly from pressure or water height measurements.

i. Total Water Potential (ψ_t):

The total water potential is the sum of the gravitational potential, the matric potential, the pressure potential and the osmotic/solute potential and any other external potential which may be working on the system.

ψ_t = ψ_g + ψ_m + ψ_p + ψ_o

where, ψ_g = Gravitational potential – Only matters when the soil is saturated

ψ_m = Matric potential – Water potential of soils

ψ_p = Pressure potential – Negligible in soils

ψ_o = Osmotic potential/solute potential (ψ_S). It matters when the soil is salty.

ii. 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.

Assuming a point at a height Z above a reference level, the gravitational potential energy, Eg is:

Eg = MgZ = ρ_wgvZ

where, M = Mass of water, g

g = Acceleration due to gravity, cm s^-2

ρ_w = Density of water, g cm^-3

v = Volume of water, cm³.

Gravitational potential may be expressed as;

ψ_g (mass) per unit mass = Eg/M = MgZ/M = gZ

ψ_g (volume) per unit volume = Eg/v = ρ_wvgZ/v = ρ_wgZ

iii. 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.

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).

The pressure of water, p with reference to atmospheric pressure is:

P = ρ_wgh

The pressure potential energy of water, ψ_p is:

ψ_p = pdv = p

where, dv = Infinitesimal volume of water.

Pressure potential may be expressed as:

ψ_p (mass) per unit mass = pdv/ρ_wdv = ρ_wgh/ρ_w = gh

ψ_p (volume) per unit volume = pdv/dv = p

ψ_p (weight) per unit weight = pdv/ρ_wgdv = p/ρ_wg = ρ_wgh/ρ_wg = h

iv. 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.

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.

Assuming as infinitesimal volume of water, dv with pressure deficit p, the matric potential will be:

ψ_m = pdv

Matric potential can be expressed as:

ψ_m (volume) per unit volume = pdv/dv = p = ρ_wgh

ψ_m (mass) per unit mass = pdv/ρ_wdv = p/ρ_w = ρ_wgh/ρ_w = gh

ψ_m (weight) per unit weight = pdv/ρ_wgdv = p/ρ_wg = ρ_wgh/ρ_wg = h

v. Osmotic Potential (Ψ_o)/ Solute Potential (Ψ_s):

The solute potential (ψ_S) is that portion of the total water potential associated with the combined effects of all solute species present in the system. It would exist in a soil system between water at the surface and water deeper in the soil when evaporation is occurring at the surface because the concentration of solutes would gradually increase at the surface as only pure water would leave by evaporation.

Generally, however, the solute potential is important only when plants are included in the system because the plant roots act as a semi­-permeable membrane separating systems of different solute concentrations. Pure water can move through a semi-permeable membrane much easier than the solutes can, and pure free water tends to move through the membrane into the solution, thus, the solute potential is negative in the solution, and is reduced by the addition of more solutes.

Osmotic potential is expressed as:

Ψ_o = π

where,

π = Osmotic pressure due dissolved salts or solutes.

Methods for Measuring Soil-Water Potential:

Soil-water potential can be measured in the field with tensiometers, electrical resistance blocks, heat dissipation sensors and psychrometers.

1. Tensiometers: 0 to -80 kPa (-0.8 to ~ -800 cm)

2. Electrical resistance blocks: 0 to -890 or -1,500 kPa (0 to -10 or -15 bars)

3. Thermocouple psychrometers: -98 to -3,000 kPa (-1 to -30 bars)

4. Heat dissipation sensors: -9.8 to -100,000 kPa (-0.1 to -1,000 bars).

The above methods are discussed elaborately under measurement of soil moisture (Table 4.5). These instruments measure the energy potential of soil-water either in negative units of pressure or with positive units of tension, which is the opposite of pressure. More energy is required to extract water from soil at lower or more negative water potentials. Bars, atmospheres (atm), pounds per square inch (psi) and kilo Pascals (kPa) are several examples of common pressure units (Table 4.5).

Values in the above Table 4.5 are approximations due to differences in physical or chemical conditions in the soil, plants or atmosphere.