soil water potential & tension
more information….
Soil water potential is the energy state of water in the soil. It describes and explains how water moves in the soil environment. Soil water potential is also known as soil suction or soil tension, and it is one of the most important parameters in soil physics.
Soil water potential is extensively measured by researchers, engineers, hydrologists, agronomists and students. Consequently, soil water potential is measured with a variety of sensors, loggers and benchtop devices and products. The most accurate and reliable products are manufactured by METER Group and supplied in Australia and New Zealand by Edaphic Scientific.
Our range of soil water potential instrumentation is used across a wide range of applications in Australia and New Zealand from soil columns research at Monash University (Melbourne) to civil engineering at western Sydney’s new airport to mining in Western Australia to plant physiology at Griffith University (Brisbane, Queensland) to agriculture and horticulture in the orchards of northern Queensland and vineyards of South Australia and much more.
Below, soil water potential is explained in detail with a summary video and key research paper citations.
a video on soil water potential
what is soil water potential?
Soil water potential explains the movement of water in the soil environment. Energy must be involved to enable water to move from one location to another location in the soil. The amount of energy available to move water is soil water potential.
Water will always move from high to low soil water potential so there is an equilibrium or balance. For example, water in soil moves from saturated or wet areas to non-saturated or dry areas. Although such a process may seem obvious and intuitive, it is still important that scientists and engineers are able to quantity the potential for water to move from one location to another.
soil water potential versus soil water content?
Soil water potential is closely aligned with soil water content. Often, researchers and engineers in Australia and New Zealand will measure both parameters simultaneously. Yet, it is important to understand the differences between the parameters because they describe separate physical processes.
Soil water potential describes the ability of water to move within the soil environment or vadose zone. That is, soil water potential describes the energy state. On the other hand, soil water content is the amount, or quantity, of water in the soil environment or vadose zone.
Figure 1. The TEROS 12 is an example of a soil water content sensor. The TEROS 12 also measures soil temperature and electrical conductivity (EC).
The dynamics between soil water potential and content depend on the matric, osmotic, gravitational and pressure potential of the soil environment. These components of soil water potential, and their relationship with soil water content, are described in the next section.
There is also a mathematical relationship between soil water potential and content known as soil water characteristic, or soil water retention, curves. For example, a popular approach to describe the relationship is a van Genuchten curve.
Instrumentation supplied by Edaphic Scientific can be used by Australian and New Zealand researchers and engineers to create soil water characteristic curves. These instruments, and soil water retention curves, are described in detail below.
the different types of soil water potential
Soil water potential is the sum of different types of water potential components. That is, soil water potential (ψ) is the sum of matric (ψm), osmotic (ψo), gravitational (ψg), and pressure (ψp) potential:
matric potential
Soil matric potential is the binding of water molecules to soil particles. Via hydrogen bonding, water molecules are attracted to surfaces such as soil particles. This attraction to the surface of soil particles impedes the movement of water in the soil environment.
In porous soils with relatively large air gaps and large particle size, such as sandy soils or artificial substrates used in hydroponics, there is relatively less surface air for the binding of water molecules and water can move relatively freely. In contrast, dense soils, such as loams and clays, have relatively smaller air gaps and soil particles so there is a larger surface area for water molecules to bind. The movement of water in heavier soils is not as free as porous soils. The following video demonstrates matric potential:
Importantly, the matric potential of porous and dense soils can be the same, but water content can vary. The volume, or content, of water is relatively less in porous soils compared with dense soils for the same amount of matric potential. For soil with -10 kPa matric potential, the water content for a porous soil, as an example, may be 15% whereas for a dense soil it may be 35%. Therefore, a much greater volume or quantity of water is required to reach the same level of matric potential in a dense versus porous soil. This point is illustrated in the YouTube video above.
The relationship between soil water potential and water content is the basis of soil water characteristic curves, or soil moisture release curves or soil water retention curves, and further details can be found below.
Sensors such as the TEROS 21 Matric Potential Sensor, and tensiometers such as the TEROS 31 and TEROS 32, only measure the matric potential of the soil.
osmotic potential
Osmotic potential is commonly thought in terms of salts or salinity. That is, the more saline the soil then the lower its osmotic potential. However, osmotic potential is the binding of water molecules to solutes which can include a variety of molecule types. Although salt (i.e., NaCl) is included, other molecules may also be involved.
gravitational potential
Gravity is an important component of soil water potential. Water usually moves down a soil profile because of gravity. But gravitational potential is often overcome by matric or osmotic potential. For example, a strong matric potential gradient can lead to water moving upwards in a soil profile and overcoming the downward force of gravitational potential.
pressure potential
Pressure potential in the context of soil water potential tends to operate at a larger scale. For example, the hydrostatic pressure of groundwater on the vadose environment.
sensor and meter measurement comparison
Edaphic Scientific supports Australian and New Zealand researchers and engineers with advanced soil water potential instrumentation manufactured by METER Group. The table below is a summary of each device. Scroll to the top of this page to find weblinks for each sensor or meter option.
Figure 2. Benchtop laboratory instruments for measuring soil water potential and soil water dynamics. From left to right: KSAT, HYPROP2 and WP4C.
soil water characteristic curves
Soil water characteristic curves describe the relationship between soil water content and soil water potential for a given soil sample or soil type. They are also known as soil water retention curves or soil moisture release curves. The relationship between soil water potential and water content is extremely important for researchers and engineers.
To create the curve, firstly soil water content and potential are concurrently measured on a soil sample from wet to dry. Soil water content is then plotted on the x-axis and soil water potential on the y-axis. A curve is then fitted to the data that best describes the relationship. For example, a popular approach to describe the relationship is a van Genuchten curve.
Figure 3. Soil water characteristic curves describe the relationship between soil water potential (y-axis) and content (x-axis). In this example, three hypothetical soil curves are shown for loamy fine sand, fine sandy loam and silt loam soil types. Image source: METER Group.
Porous soils, such as sands, tend to have a steep slope whereas dense soils, such as clays, have a shallower slope. However, every soil type or sample has a unique curve, and it is highly improbable that two curves for different soils will be exactly the same. Therefore, it is important that a soil water characteristic curve is created for every soil type.
Edaphic Scientific and METER Group provides a range of equipment for soil water retention curves. Commonly, Australian and New Zealand researchers and engineers will create a soil moisture release curve with laboratory instrumentation such as the HYPROP2, WP4C or VSA. However, it is also possible to create curves in the field with the TEROS range of soil water potential and content sensors, as demonstrated in this article:
The following is a brief video on soil moisture release curves:
plant water relations
Soil water potential is important for how plants uptake water and undergo transpiration. Arguably, it may be one of the most important parameters in understanding plant water relations. Therefore, measuring soil water potential is extremely important for Australian and New Zealand plant physiologists, agronomists and horticulturalists.
As defined above, soil water potential describes the movement of water in soils. Water potential gradients also describe the movement of water from the soil and into the plant. At a coarse level, this movement occurs via a water potential gradient from wet soils to relatively drier plant tissue. For the entire process to operate, root tissue water potential needs to be lower than soil water potential and, in turn, leaf water potential is lower than roots and atmospheric water potential is lower than leaf. That is, there is a gradient of water potential from the wet soil to drier roots, leaves and atmosphere. This is known as the soil-plant-atmosphere continuum.
Figure 4. A hypothetical water potential gradient that may occur in a relatively wet tree from wet soils to drier roots, leaves and the atmosphere. The water potential gradient is important for plant water relations and understanding the soil-plant-atmosphere continuum. Image source: METER Group.
Consequently, there are numerous applications where soil water potential is an important parameter in the context of plant water relations. For example, understanding soil water potential is critical in applications such as agriculture and horticulture for optimal irrigation. But, understanding soil water potential is also important for ecology and biodiversity, for example in the germination of seeds or the behaviour of endangered frog species. Soil water potential is also important for understanding plant and ecosystem response to increased atmospheric carbon dioxide. These are only a few examples of the many applications that require an in depth understanding of soil water potential.
To characterise the soil-plant-atmosphere continuum, Edaphic Scientific and METER Group supplies several sensor and meter options. The TEROS 21, TEROS 31 and TEROS 32 are ideal for quantifying soil water potential for plants and the WP4C measures leaf water potential.
case studies
how to measure soil water potential with a soil water content sensor
Soil water content and water potential are typically measured with two separate sensors. However, it is possible to measure both parameters with a single sensor. This article outlines how and the equipment you will need to achieve this.
avocado irrigation management and the TEROS 21 soil water potential sensor
A new, tensiometer-style sensor is improving irrigation management and saving time and costs for Australian and New Zealand avocado growers. The advanced but easy to use technology is also assisting other Australian and New Zealand growers from vineyards to orchards to fields. This article outlines a case study from an avocado orchard in New South Wales, Australia.
Figure 5. Soil water potential monitoring for horticulture irrigation management in an avocado orchard, New South Wales, Australia. A weather station is also installed at the site to provide a complete understanding of water movement in the orchard.
why the old gypsum block is now obsolete
Back in the 1980’s and 1990’s, the gypsum block was a popular sensor for measuring soil matric water potential. Now, in the 2020’s, the gypsum block is largely obsolete. This article outlines how new technology has replaced and improved the old technology.
references
- Hoffmann and Mitchell, 2022. Breeding phenology of a terrestrial-breeding frog is associated with soil water potential: Implications for conservation in a changing climate. Austral Ecology,
- Luo et al., 2022. Soil water potential: A historical perspective and recent breakthroughs. Vadose Zone Journal,
- Parker and Patrignani, 2022. Revisiting Laboratory Methods for Measuring Soil Water Retention Curves. Soil Science Society of America Journal,
- Roy et al., 2018. Development and Comparison of Soil Water Release Curves for Three Soils in the Red River Valley. Soil Science Society of America Journal,
- Schelle et al., 2013. Water retention characteristics of soils over the whole moisture range: a comparison of laboratory methods. European Journal of Soil Science,