Soil Science

Water Potential

The movement of water through the soil-plant-atmosphere continuum involves energy-related phenomena. Water can be held in soil at different levels of energy. The energy status of soil water (the soil water potential) can be characterized in terms of free energy. Dynamically, water will have a tendency to move from a higher to a lower free energy level. A saturated zone, where a substantial amount of water can move freely through larger macropores, would represent a high free energy level (high water potential), and a dry soil zone, where most water is held tightly in micropores or as adsorption water, would represent a low free energy level (low water potential).

The total soil water potential is in effect the sum of component potentials that represent the various forces acting on soil water. Component potentials include gravitational, matric, pressure, and osmotic potential.

Water potential gradients, which would give rise to water movement in unsaturated soil, are calculated on the basis of gravitational and matric potentials. The sum of these two potentials is called hydraulic potential. The Greek letter psi (Ψ) is often used to signify water potentials.

Water potential formula diagram (Ψ=Ψg+Ψm)

Gravitational Potential

Gravity acts on soil water just as it does on other bodies, with the attraction being toward the earth's center. The free energy of soil water at any reference plane in the profile is thus higher than that of pure water at some lower elevation. The difference in free energy level gives rise to water movement.

Groundwater is pumped to a reservoir. This is work done against the gravitational field, and the reservoir water now has the positive potential of the height "z" above the groundwater. Water entering the soil still has a higher free energy level than groundwater. The gravitational potential has the value of the height "z."

Matric Potential

Matric potential is the result of two forces: adsorption and capillarity. The net effect of these forces is to reduce the free energy of soil water compared to unadsorbed or pure water. Because these two forces are attractive, matric potential is always negative (soil moisture tension refers to the same property but takes the opposite sign).

Diagram of a tensiometer

Saturation

At saturation, all soil pores are filled with water. Water is at a high free energy level, and soil moisture tension is near zero. This can be measured with a tensiometer.

 

Field Capacity

Field capacity is defined as the amount of water remaining in a well-drained soil when the rate of downward movement of water has become small (usually after 1-2 days).

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Further Drying

Water loss due to evaporation will further dry the soil. Water movement is now restricted to the micropores and can be very slow.

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Units of Measurement

Potentials can be expressed in various units, depending on which unit quantity (volume, mass, or weight) is used.

Units of measurement diagram

The following table compares these and other units used to measure soil water potentials. The logarithm to the base 10 of the moisture tension expressed in cm of water column is defined as pF, in analogy to pH. The bar (equal to 100 kPa) is a pressure unit that was much used in soil physics, but it is not part of the SI system, and its use is declining.

 

Soil Moisture TensionEquivalent toSoil Water Condition
(m H2O)(bar)(pF)Matric Potential (kPa)Cylindrical Pore Diameter 
0.010.0010-0.13 mmNear Saturation
0.10.011-1300 μm
10.12-1030 μmField Capacity
1013-1003 μm
100104-1000300 nm
150154.2-150096 nmPermanent Wilting Point

 

 

 

 

 

 

 

 

 

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