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Secrets of Water Potential: Learn the Science Behind the Measurement

This month in a 3 part series, we will explore water potential —the science behind it and how to measure it effectively.

water potential

To understand water potential, compare the water in a soil sample to water in a drinking glass.

Definition of 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. 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.

water potential

You can’t tell by measuring heat content whether or not heat will be transferred to another object if the two touch each other.

Extensive vs. Intensive 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.  Water content can only tell you how much water you have. If you want to know how fast it can move, you need to measure hydraulic conductivity. If you want to know whether it will move and where it’s going to go, you need water potential.

water potential

If you want to know whether water will move and where it’s going to go, you need water potential.

Two Key Water Potential Questions:

1. Where will water move? 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. What is the availability of water to plants? 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. In order for water to flow, therefore, the leaf water potential must be lower than the soil water potential.

Next week learn about the four components of water potential—osmotic potential, gravitational potential, matric potential, and pressure potential.

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