Skip to content

Water Potential Versus Water Content

Dr. Colin Campbell, soil physicist, shares why he thinks measuring soil water potential can be more useful than measuring soil water content.

A horsetail plant showing possible signs of guttation where the water potential in the soil overnight is high enough to force water out of the stomates in the leaves.

A horsetail plant showing possible signs of guttation where the water potential in the soil overnight is high enough to force water out of the stomates in the leaves.

I know an ecologist who installed an extensive soil water content (VWC) sensor network to study the effect of slope orientation on plant available water.  He collected good VWC data, but ultimately he was frustrated because he couldn’t tell how much of the water was available to plants.

He’s not alone in his frustration. Accurate, inexpensive soil moisture sensors have made soil VWC a justifiably popular measurement, but as many people have discovered, a good hammer doesn’t make every soil water problem a nail. I like to compare water potential to temperature because both are considered “intensive” variables that define the intensity of something.

People often try to quantify their own environment, because those measurements define comfort and happiness.  Long ago, they discovered they could make an enclosed glass tube, put mercury inside, and infer this intensive variable called temperature from the changes in the mercury’s volume. This was an obvious way to define the comfort level of a human being.

Thermometer laying on top of wood

People discovered they could make an enclosed glass tube, put mercury inside, and infer an intensive variable called temperature.

They could have measured the heat content of their surroundings.  But they would have discovered that while heat content would be higher in a larger room and lower in a smaller room, you would feel the same comfort level in both rooms.  The temperature measurement helps you know whether or not you’d be comfortable without any other variables entering into the equation.

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 can be 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 we know the water potential, we can know whether plants will grow well or be stressed in any environment.

sand with plants poking out and a blue sky in the background

Soil, clay, sand, potting soil, and other media, all hold water differently.

Plants don’t understand the concept of a content in terms of “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. We know that plant optimal runs from about -2-5 kPa which is on the very wet side, to about -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.

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

Learn more

Soil moisture is more than just knowing the amount of water in soil. Learn basic principles you need to know before deciding how to measure it. In this 20-minute webinar, discover:

  • Water content: what it is, how it’s measured, and why you need it
  • Water potential: what it is, how it’s different from water content, and why you need it
  • Whether you should measure water content, water potential, or both
  • Which sensors measure each type of parameter

Many questions about water availability and movement are best answered by measuring water potential.  To find out more, watch any of the virtual seminars below, or visit our new water potential website.

Download the “Researcher’s complete guide to water potential”—>

Water Potential 101: Making Use of an Important Tool

Water Potential 201:  Choosing the Right Instrument

Water Potential 301: How to Push Your Instruments Past their Specifications

Water Potential 401: Advances in Field Water Potential

Find out when you should measure both water potential and water content.

Take our Soil Moisture Master Class

Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together.  Plus, master the basics of soil hydraulic conductivity.

Watch it now—>

Download the “Researcher’s complete guide to soil moisture”—>

34 Comments Post a comment

Trackbacks & Pingbacks

  1. Do the Standards for Field Capacity and Permanent Wilting Point Need to Be Reexamined?
  2. What does SMAP mean for in situ soil water content measurement?
  3. The History and Future of Water Potential
  4. Water Content Innovation Involves Growing Pains
  5. Learn to Measure Water Potential at a Bodentag
  6. Measuring Osmotic Sap Water Potential
  7. Despite Drawbacks scientific collaboration pays off
  8. New Applications for TDR Probes Measuring Water Content
  9. Volumetric Water Content: Keeping your Eye on the Goal
  10. Spectral Reflectance and Water Content Wasatch Plateau Experiment
  11. Water Potential/Water Content:  When to Use Dual Measurements
  12. Tensiometers:  Micro-sized - Environmental Biophysics
  13. Green Roofs — Do They Work? (Part II) - Environmental Biophysics
  14. Estimating Relative Humidity in Soil: How to Stop Doing it Wrong - Environmental Biophysics
  15. Using Soil Moisture Sensors on Humans? - Environmental Biophysics
  16. Environmental Biophysics: Top Five Blog Posts in 2015 - Environmental Biophysics
  17. Killing Cheatgrass and Shooting for the Moon
  18. Will Sample Disturbance Lead to Lower Accuracy?
  19. Screening for Drought Tolerance
  20. Measuring Moisture in Concrete
  21. Measuring Frozen Water Potential: How and Why?
  22. Water Content helps Turf Growers find Water/Nutrient Balance
  23. Examining Plant Stress using Hydraulic Conductivity
  24. Sensors Validate California Groundwater Resource Management Techniques
  25. Author Interview: Soil Physics with Python
  26. A History of Thermocouple Psychrometry
  27. This Idea Must Die: Using Filter Paper Method as a Standard
  28. Founders of Environmental Biophysics Series: Sterling Taylor
  29. Remembering John Monteith
  30. Modeling Available Soil Moisture
  31. Do Funding Agencies Favor Collaboration?
  32. Will Complex Scientific Questions Yield Better Science in Desert FMP Project?
  33. Soil Moisture Sensors in Trees - Environmental Biophysics
  34. Data Don't Lie - Environmental Biophysics

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Share to...