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.
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.
Learn more about intensive vs. extensive variables in this chalk talk by soil physicist, Dr. Colin Campbell.
Water potential defines the comfort level of a plant
Water content is not an indicator of plant “comfort” because soil type matters. 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.
Figure 1. Moisture release curves for two different types of soil demonstrate 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 has a much lower matric potential, and the water is significantly less available.
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.
Table 1 illustrates the easy reference scale for some types of crops. Plants will stay out of stress and yield more when kept within this water potential comfort range.
Table 1. An easy water potential reference scale for some crops (source: Taylor, Sterling A. and Gaylen L. Ashcroft. Physical Edaphology. The Physics of Irrigated and Nonirrigated soils. 1072.) Plants stay out of stress and yield more when they are kept within their water potential comfort zone.
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.
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.
Among the thousands of peer-reviewed publications using METER soil sensors, no type emerges as the favorite. Thus sensor choice should be based on your needs and application. Use these considerations to help identify the perfect sensor for your research.
TEROS sensors are more durable, accurate, easier and faster to install, more consistent, and linked to a powerful, intuitive near-real-time data logging and visualization system.
Most people look at soil moisture only in terms of one variable—water content. But two types of variables are required to describe the state of water in the soil.
Webinar: Using the salt balance to irrigate more efficiently
The global pressure for water conservation is being felt in every industry, especially in arid countries. In agriculture, the balance can be hard to find.
In this 30-minute webinar, Dr. Gaylon Campbell, Senior Research Scientist at METER for over 25 years, breaks down how to measure the water and salt balance and how to use these measurements to improve irrigation efficiency. He will discuss:
Climate change impacts on irrigated agriculture
Things that can and cannot increase water use efficiency in agriculture