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. In fact, the soil water has a different energy state from “free” water. The free water can be accessed without exerting any energy. The soil water can only be extracted by expending energy. Water potential expresses how much energy you would need to expend to pull that water out of the soil sample.
Water potential is a differential property. For the measurement to have meaning, a reference must be specified. The reference typically specified is pure, free water at the soil surface. The water potential of this reference is zero. Water potential in the environment is almost always less than zero, because you have to add energy to get the water out.
Intensive vs. extensive variables
Water movement in the environment is really a physics problem, and to understand it, we have to distinguish between intensive and extensive variables. The extensive variable describes the extent or amount of matter or energy. The intensive variable describes the intensity or quality of matter or energy. For example, the thermal state of a substance can be described in terms of both heat content and temperature.
The two variables are related, but they are not the same. Heat content depends on mass, specific heat, and temperature. You can’t tell by measuring heat content whether or not heat will be transferred to another object if the two touch each other. So you also don’t know if the object is hot or cold or whether it will be safe to touch.
These questions are much easier to answer if you know the intensive variable—temperature. In fact, though it can be important to measure both intensive and extensive variables, often the intensive variable gives you more useful information. In terms of water, the extensive variable is water content, and it tells you the extent, or amount, of water in plant tissue or soil. The intensive variable is water potential, and it describes the intensity or quality of water in plant tissue or soil. Many questions about water availability and movement are best answered by measuring water potential.
Water potential answers two key questions
1. Water movement
Water will always flow from high potential to low potential. This is the second law of thermodynamics—energy flows along the gradient of the intensive variable.
2. Plant availability
Liquid water moves from soil to and through roots, through the xylem of plants, to the leaves, and eventually evaporates in the substomatal cavities of the leaf. The driving force for this flow is a water potential gradient. Thus, in order for water to flow, the leaf water potential must be lower than the soil water potential.
Water potential names and units
Water potential is frequently called water tension, soil suction, and soil pore water pressure. We typically use units of pressure to describe water potential, including megapascals (MPa), kilopascals (kPa), bars, and meters (mH2O), centimeters (cmH2O), or millimeters of water (mmH2O).
Water potential components
The total water potential is the sum of four different components.
The binding of water to surfaces
Binding to solutes in the water
The position of water in a gravitational field
Hydrostatic or pneumatic pressure on the water
Water potential is actually measured in energy per unit of mass, so the official units should be joules per kilogram, but if you take into account the density of water, the units become kilopascals, therefore we typically describe it using units of pressure.
Matric potential arises because water is attracted to most surfaces through hydrogen bonding and van der Waals forces. Soil is made up of small particles, providing lots of surfaces that will bind water. This binding is highly dependent on soil type. For example, sandy soil has large particles which provide less surface-binding sites, while a silt loam has smaller particles and more surface-binding sites.
The following figure, showing moisture release curves for three different types of soil, demonstrates the effect of surface area. Sand, containing 10% water, has a high matric potential, and the water is readily available to organisms and plants. Silt loam, containing 10% water, will have a much lower matric potential, and the water will be significantly less available.
Matric potential is always negative or zero and is the most significant component of soil water potential in unsaturated conditions.