Whether you’re a grad student embarking on an environmental measurement campaign, an experienced researcher, or a grower concerned with irrigation management, at some point you’ve probably realized you need to measure soil moisture. Why? Because water availability is one of the main drivers of ecosystem productivity, and soil moisture is the immediate source of water for most plants. What is soil moisture? Below is a comprehensive look at the definition of soil moisture and an exploration of some important scientific terms used in conjunction with soil moisture.
Soil moisture is more than just knowing the amount of water in soil. There are basic principles you need to know before deciding how to measure it. Here are some questions that may help you focus on what you’re actually trying to find out.
Are you interested in the water stored in the soil?
Do you care more about water available for primary productivity to maximize production or understand maximum production on your site?
Are you studying water and solute movement in soils?
Do you aim to optimize water use of crops?
Are you modeling soil hydrology?
Depending on which of these questions you are interested in, soil moisture might mean something very different.
Know which variable you should measure
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: water content, which is the amount of water and water potential, which is the energy state of the water.
Water content is an extensive variable. It changes with size and situation. It’s defined as the amount of water per total unit volume or mass. Basically, it’s how much water is there.
Water potential is an “intensive” variable that describes the intensity or quality of matter or energy. It is often compared to temperature. Just as temperature indicates the comfort level of a human, water potential can indicate the comfort level of a plant. Water potential is the potential energy per mole (unit mass, volume, weight) of water with reference to pure water at zero potential. You can look at water potential as the work required to remove a small quantity of water from the soil and deposit it in a pool of pure, free water.
Water content: It’s just an amount
This article briefly examines two different methods of measuring soil water content: gravimetric water content and volumetric water content.
Gravimetric water content is the mass of water per mass of soil (i.e., grams of water per gram of soil). It is the primary method for measuring water content because the amount of soil water is measured directly by measuring the mass. It’s calculated by weighing the wet soil sampled from the field, drying it in an oven, and then weighing the dry soil.
Thus gravimetric water content equals the wet soil mass minus the dry soil mass divided by the dry soil mass. In other words, the mass of the water divided by the mass of the soil.
Volumetric water content is the volume of water per total volume of soil.
Volumetric water content describes the same thing as gravimetric water content, except it’s being reported on a volume basis.
For example, the components of a known volume of soil are shown in Figure 1. All of the components total 100%. Since volumetric water content (VWC) equals the volume of water divided by the total soil volume, in this case, VWC will be 35%. VWC is sometimes reported as cm3/cm3 or inches per foot.
Gravimetric and VWC: linked by bulk density
Gravimetric water content (w) can be converted to volumetric water content (ϴ) by multiplying by the dry bulk density of the soil (⍴b) (Equation 3).
Because gravimetric water content is the first principles (or direct) method of measuring how much water is in the soil, it is used to develop calibrations and validate readings of almost all the VWC measurements that are sensed either in situ or remotely. If you have a dielectric sensor, you have some relationship that converts what you are reading in your electromagnetic field into a water content. So if you’re unsure that your volumetric water content is correct, sample some soil, measure the gravimetric water content, take a bulk density sample, and check for yourself.
How to measure volumetric water content
Most volumetric water content measurements are made using some kind of sensor. METER water content sensors use capacitance technology. To make this measurement, these sensors take advantage of the “polarity” of water. How does it work?
Figure 2 shows a water molecule. There’s a negative pole at the top with an oxygen atom and a positive pole at the bottom with two hydrogen atoms. If we were to introduce an electromagnetic field (Figure 3) into the soil, this water molecule would jump to attention. If the field were reversed, it would dance the other way. Thus by creating an electromagnetic field with a water content sensor, it is possible to measure the effect of the water on that electromagnetic field. If there is more water in the soil, there will be a larger effect. Learn more about capacitance technology here.
Why measure water content with a sensor?
Using a water content sensor opens up the possibility for a time series (Figure 4), a powerful tool used to understand what’s happening in the soil. Measuring gravimetric water content requires taking a sample or series of samples and bringing them back to the lab. If you need a time series, this is impractical because you would be essentially in the field sampling all the time.
With a water content sensor, you can automatically measure the timing of changes in water content and compare depths in a profile. And the shapes of these curves provide important information about what is happening to the water in your soil.
Table 1 compares different soil sensing methods.
Gravimetric Water Content
Remote Sensing (SMOS)
First principles/direct method
Convenient for time series
Can do time series at limited scale
Enables profile sensing over time
Extremely powerful for spatial sampling
Only 1 snapshot in time
Table 1: A comparison of soil sensing methods
Gravimetric water content is a good first principles measurement but is time consuming, destructive, and only gives a snapshot in time. Water content sensors provide a time series, enable profile sensing over time, and avoid destructive sampling, though a sensor is still inserted into the soil. Remote sensing provides a time series at a limited scale but is extremely powerful for spatial sampling, which is important for measuring water content. METER soil moisture sensors reduce disturbance with a specialized installation tool, designed to minimize site disturbance (watch the video to see how it works).
Saturation: it’s not what you think
In terms of volumetric water content, oven dry soil is 0% VWC by definition. It’s one defined endpoint. Pure water is at the other end of the scale at 100%. Many people think that 100% VWC is fully saturated soil, but it’s not. Each soil type will saturate at different water contents.
One way to look at it is as percent saturation:
% saturation = VWC/porosity * 100
If you know the porosity of any given soil type, it is possible to approximate water content at saturation. But soils seldom reach saturation in the field. Why?
In Figure 6, you can see that as the soil adsorbs water, it creates a water film that clings to the soil particles. There are also pore spaces filled with air. Under field conditions, it’s difficult to eliminate these air spaces. This air entrapment is why the percent saturation will seldom be equal to the theoretical saturation maximum for any given soil type.
What is water potential?
Water potential is the other variable used to describe soil moisture. As previously noted, it’s defined as the energy state of soil or the potential energy per mole of water with reference to pure water at zero potential. What does that mean? To understand this principle, 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 and may be diluted by solutes and even under pressure. As a result, 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 equivalent or greater to the energy with which it’s held. Water potential expresses how much energy you would need to expend to pull that water out of the soil sample.
Water potential is the sum of four different components: gravitational potential + the matric potential + the pressure potential + the osmotic potential (Equation 3).
Matric potential is the most significant component as far as soil is concerned because it relates to the water that is adhering to soil surfaces. In Figure 6, the matric potential is what created the water film clinging to the soil particles. As water drains out of the soil, the air-filled pore spaces get bigger, and the water gets more tightly bound to the soil particles as the matric potential decreases. Watch the video below to see matric potential in action.
A water potential gradient is the driving force for water flow in soil. And soil water potential is the best indicator of plant available water (learn why here). Similar to water content, water potential can be measured with sensors both in the lab and in the field. Here are a few examples of different types of field water potential sensors.
Water potential predicts water movement
Water will move from a higher energy location to a lower energy location until the locations reach equilibrium, as illustrated in Figure 7. For example, if a soil’s water potential were -50 kPa, water would move toward the more negative -100 kPa to become more stable.
This also approximates what happens in the plant soil atmosphere continuum. In Figure 8, the soil is at -0.3 MPa and the roots are slightly more negative at -0.5 MPa. This means the roots will pull water up from the soil. Then the water will move up through the xylem, out through the leaves across this potential gradient. And the atmosphere, at -100 MPa, is what drives this gradient. So the water potential defines which direction water will move in the system.
What is plant available water?
Plant available water is the difference in water content between field capacity and permanent wilting point in soil or growing media (see definitions below). Most crops will experience significant yield loss if soil is allowed to dry even near permanent wilting point. To maximize crop yield, soil water content will typically be maintained well above permanent wilting point, but plant available water is still a useful concept because it communicates the size of the water reservoir in the soil. With some basic knowledge about soil type, field capacity and permanent wilting point can be estimated from measurements made by in situsoil moisture sensors. These sensors provide continuous soil water content data that can guideirrigation management decisions to increase crop yield and water use efficiency.
What is field capacity?
Field water capacity is defined as “the content of water on a mass or volume basis remaining in a soil two or three days after having been wetted with water and after free drainage is negligible.” Glossary of Soil Science Terms. Soil Science Society of America, 1997. It is often assumed to be the water content at -33 kPa water potential for fine-textured soils or -10 kPa in sandy soils, but these are just crude starting points. The actual field capacity depends on the characteristics of the soil profile. It must be determined from water content data monitored in the field. If you’re looking at field capacity data, it’s good to know how that point was arrived at.
Even though we generally specify field capacity in terms of a water potential, it is important to realize that it is really a flow property. Water moves down in the soil profile under the influence of the gravitational potential gradient. It will continue to move down forever, but as the soil dries, the hydraulic conductivity decreases rapidly, finally rendering the downward flow small in comparison with evaporation and transpiration losses. Think of the soil as a leaky bucket. The plants are trying to grab some of the water as it moves down through the root zone.
What is permanent wilting point?
On the opposite end of the scale is permanent wilting point. Permanent wilting point was experimentally determined in sunflowers and defined as -15 bars (-1500 kPa, Briggs and Shantz, 1912, p. 9). It’s the soil potential at which sunflowers wilt and are unable to recover overnight. It’s theoretically the empty tank, where there is a complete loss of turgor pressure, and the plant has wilted. But -1500 kPa is not necessarily the wilting point for all plants. Many plants ‘wilt’ at different points; some plants will start to protect themselves from permanent damage much sooner than -1500 kPa and some well after. So -1500 kPa is a useful reference point in the soil, but be aware that a cactus probably doesn’t care about -1500 kPa, and a ponderosa pine will certainly not shut down at that point. So it can mean different things for different plants or crops (read more: M.B. Kirkham. Principles of Soil and Plant Water Relations, 2005, Elsevier).
You can quickly and easily determine any soil’s permanent wilting point using METER’s WP4C.
Soil type: the lens through which you see
To draw meaningful conclusions about water content you must know something about your soil type.
Figure 9 is a chart of the most common texture classes from sand to clay. Every texture has a different particle size distribution. Table 2 illustrates that at -1500 kPa (permanent wilting point) each texture class has a different water content. And it’s the same for field capacity.
Sandy clay loam
Silty clay loam
Table 2. Representative field capacity and permanent wilting point for different soil textures
Interestingly, a sandy clay loam can have a 32% VWC at field capacity (which is a well-hydrated soil), but for a clay, 32% VWC is at permanent wilting point. This means you should take a soil sample when you’re installing sensors to ensure you know your soil texture and what’s happening in your soil. This is especially important when there are changes in soil type: either changes in the soil profile or spatial variability from site to site. Note that the water potential doesn’t change with the situation. For all these soil types, -33 kPa is -33 kPa whether it’s a clay or a sand. If you look at a silt loam soil as a kind of medium texture soil, its -33 kPa water content is 27% and its -1500 kPa water content is 13%. At a typical bulk density the total pore space is around 50%. If that were filled, the soil would be saturated. So, starting at saturation, (assuming field capacity is -33 kPa) half the water would drain out to reach field capacity. About half of the water that is left is plant available water. Once the plant has extracted all the water it can, an amount of water approximately equal to the plant available water is still in the soil but can’t be removed by the plant.
The PARIO is an instrument that will automatically determine soil type and particle size distribution for any soil.
Soil water retention curves
There is a relationship between water potential and volumetric water content which can be illustrated using a soil water retention curve (sometimes called a moisture release curve or a soil water characteristic curve). Figure 10 shows example curves for three different soils. On the x-axis is water potential on a logarithmic scale and on the Y-axis is volumetric water content. Soil water retention curves are like physical fingerprints, unique to each soil. This is because the relationship between water potential and water content is different for every soil. With this relationship, you can find out how different soils will behave anywhere along the curve. You can answer critical questions such as: will water drain through the soil quickly or be held in the root zone? Soil water retention curves are powerful tools used to predict plant water uptake, deep drainage, runoff, and more. Learn more about how this works here or watch Soil Moisture 201.
The HYPROP is an instrument that automatically generates soil water retention curves in the wet range. You can create retention curves across the entire range of soil moisture by combining the HYPROP and the WP4C.
Soil moisture: Do I need water content or water potential?
Before embarking on any soil moisture measurement campaign, ask yourself these questions:
Do I need to know how much water is stored in the soil?
Do I need to know which way water is going to move?
Do I need to know if my plants can get water?
Do I need to know how much water is in the soil for my plants?
Do I need to know when to water?
If you only need to know how much water is stored in soil, you should focus on water content. If you want to know where water is going to move, then water potential is the right measurement. To understand if your plants can get water, you’ll need to measure water potential. Read more about this in the article: “Why soil moisture can’t tell you everything you need to know”. However, If you want to know when to water, or how much water is stored in the soil for your plants, you probably need both water content and water potential. This is because you need to know how much water is physically in the soil, and you need to know at what point your plants are not going to be able to get it. Find out more about how this works in the article: “When to water: dual measurements solve the mystery”.
More references to answer the question: What is soil moisture?
Kirkham, Mary Beth. Principles of soil and plant water relations. Academic Press, 2014. (Book link)
Taylor, Sterling A., and Gaylen L. Ashcroft. Physical edaphology. The physics of irrigated and nonirrigated soils. 1972. (Book link)