Measurements · Agriculture
Soil Moisture
Also known as soil water content, volumetric water content
Soil moisture / water content.
What it is
What is soil moisture?
Soil moisture is the amount of water held in the soil, most usefully expressed as volumetric water content: the fraction of the soil's total volume that is water, given as a percentage. For example, a reading of 25 percent means a quarter of that volume of soil is water held in the pore spaces between the particles. Volumetric water content is distinct from soil water tension (measured in kilopascals), which describes how tightly the water is held and therefore how hard a plant must work to extract it; the two are related through the soil's texture. Soil moisture varies with depth and changes with rainfall, irrigation, drainage, and the water plants draw up.
Because it measures the reservoir that roots draw on, volumetric water content is the parameter that turns irrigation from guesswork into a measured decision.
Water is usually the limiting factor in plant growth, and knowing the soil moisture lets a grower irrigate the right amount at the right time: enough to keep the crop out of stress, but not so much that water and dissolved nutrients drain below the root zone and are wasted. Too little water closes stomata and stunts growth; too much starves roots of oxygen, encourages disease, and leaches fertilizer into groundwater. Managing to the moisture in the root zone therefore improves yield and quality while saving water and protecting the environment.
For growers, irrigation managers, and researchers, soil moisture is the core input to irrigation scheduling and water-use efficiency, and increasingly to automated irrigation control.
How it is measured
How Soil Moisture Is Measured
The common field sensors are electronic probes that infer water content from the soil's electrical properties. Capacitance and frequency-domain (FDR) probes sense the soil's dielectric constant, which rises steeply with water content; time-domain reflectometry (TDR) measures the travel time of an electrical pulse along buried rods, which likewise depends on moisture. Both are professional buried sensors connected to a Thiamis gateway for continuous logging, and both benefit from soil-specific calibration, since texture, salinity, and temperature affect the raw signal. Probes at several depths on one logger capture how moisture varies through the profile.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing soil moisture sensors
Probes are installed in the root zone at the depths where the crop draws water, in soil that represents the field and in firm contact with undisturbed soil, since air gaps around a probe give falsely low readings. Several depths are usual, so that both the active surface layer and the deeper reserve are tracked, showing how far irrigation or rain has penetrated. The site is chosen away from wheel tracks, drip emitters directly on the sensor, and other atypical spots, because soil moisture is spatially variable and a poorly chosen point will not represent the block being managed.
Every device and option that measures Soil Moisture 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 Davis · Davis Wizard III, Davis Wizard II, Davis Monitor… | Weather Stations | Thiamis |
Where it is monitored
Solutions That Monitor Soil Moisture
Each one links to the solution page showing what that industry measures it for.
Agriculture & Irrigation
Core parameterSoil moisture is the parameter that tells growers when and how much to irrigate, balancing crop water needs against waste and runoff.
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Questions
Soil Moisture FAQ
What is the difference between volumetric water content and soil tension?
Volumetric water content is the fraction of soil volume that is water, in percent, while tension is how tightly that water is held, in kilopascals. Content tells you how much water is present; tension tells you how available it is to the plant. The relationship between them depends on soil texture, so both views are useful.
How do capacitance and TDR probes differ?
Both infer moisture from the soil's electrical response to water, but capacitance and FDR probes measure the dielectric constant while TDR measures the travel time of a pulse along buried rods. TDR is often regarded as less sensitive to soil variation, while capacitance probes are widely used and cost-effective; both need good soil contact and benefit from calibration.
Why measure soil moisture at several depths?
Because water enters from the top and is drawn down through the profile, a single depth cannot show how deep irrigation or rain has reached or where the roots are still finding water. Probes at several depths reveal the wetting front and the deeper reserve, which is what allows precise scheduling.
How do I monitor soil moisture?
Pair a Thiamis gateway with capacitance, FDR, or TDR probes installed in the root zone at several depths for continuous logging and irrigation scheduling. Use the configurator to combine soil moisture with soil temperature and weather parameters for a complete picture of the crop's water environment.
Monitor Soil Moisture on the Record
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