Accurate, inexpensive soil moisture sensors make soil volumetric water content (VWC) a justifiably popular measurement, but it may not be the right measurement for every application. Water potential (or soil suction), which measures the energy state of water in soil, explains more about water availability, whether water will move, and where it’s going to go.
Intensive variables don’t change with size or situation
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. Water potential is often compared to temperature. Both are considered “intensive” variables that describe 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. Temperature is an obvious way to define the intensity of comfort in a human being.
Heat content doesn’t define comfort because heat content at a specific comfort level will be higher in a large room and lower in a small room. The temperature measurement defines comfort without any other variables entering into the equation.
Water content doesn’t predict how water moves
Similar to heat content, water content is an amount. It’s an extensive variable. It changes with size and situation. Consider the following paradoxes:
- A soil with fairly low volumetric water content can have plenty of plant-available water, and a soil with high water content can have almost none
- Gravity pulls water down through the profile, but water moves up into the soil from a water table
- Two adjacent patches of soil at equilibrium can have significantly different water content
In these and many other cases, water content data are confusing because they don’t predict how water moves. Water potential measures the energy state of water and thus explains realities of water movement that otherwise defy intuition. Like temperature, water potential defines the comfort level of a plant. Similar to the room-size analogy for temperature, if the water potential is known, it’s possible to predict whether plants will grow well or be stressed in any environment.
Water potential defines the comfort level of a plant
Water content is not an indicator of plant “comfort” because soil, clay, sand, potting soil, and other media, all hold water differently. Imagine a sand with 30% water content. Due to its low surface area, the sand will be too wet for optimal plant growth, threatening a lack of aeration to the roots, and flirting with saturation. Now consider a fine-textured clay at that same 30% water content. The clay may appear only moist and be well below optimum “comfort” for a plant due to the surface of the clay binding the water and making it less available to the plant.
Water potential measurements clearly indicate plant available water, and, unlike water content, there is an easy reference scale–plant optimal runs from about -2-5 kPa which is on the very wet side, to approximately -100 kPa, at the drier end of optimal. Below that, plants will be in deficit, and past -1000 kPa they start to suffer. Depending on the plant, water potentials below -1000 to -2000 kPa cause permanent wilting.
Most applications require both water potential and water content
Though water potential is a better indicator of plant available water than water content, in most situations, it’s useful to use both water potential sensors and soil moisture sensors. The intensity measurement of water potential doesn’t translate directly into the quantity of water stored or needed. Water content information is also required in applications such as irrigation management and water balance studies. For more information, read: “When to water–Dual measurements solve the mystery”.