HydraProbe is a rugged soil sensor with patented technology that provides continual, consistent accuracy measuring the three most significant soil parameters simultaneously—moisture, salinity and temperature.

As the most scientifically researched soil sensor available, it has been depended on by the USDA, NOAA, NASA, leading irrigation companies, and many universities for over 20 years. It’s been engineered to be exceptionally rugged and will provide data that you can trust year after year.

Patented Sensor Technology

HydraProbe uses unique “Coaxial Impedance Dielectric Reflectometry” to provide consistent long-term accuracy of moisture, salinity and temperature in any soil type. This also provides low inter-sensor variability, so every sensor measures the same without the need to calibrate.


Strong, non-bending, non-corrosive stainless steel tines


Fully sealed electronics—fully immersible in water.


5 year warranty


Durable 18 gauge, UV-resistant high-density polyethylene cable can remain buried or be exposed.


Maintains accuracy for years with no calibration.


Continual, long-term data without calibration.


Durable stainless steel tines, fully potted components and a 5-year warranty.


Forget calibrating, ignore the soil type. Just set it and forget it.


Consistent research-grade accuracy every season, every location.

  • Stable—no sensor drift, ensuring continual accuracy.
  • Patented technology that accurately measures moisture and electrical conductivity permits more accurate optimization of watering and fertilization than with just moisture.
  • Depended on by the USDA, NOAA, leading irrigation companies, and many universities for over 20 years. Used by NASA for ground truthing of satellite-based soil imaging.
  • Soil moisture calibration has been rigorously peer-reviewed, making it one of the most trusted soil sensors available.
  • Can remain in-situ indefinitely, or relocated and redeployed without worry.
  • Ideal for remote locations, harsh environments and applications where data is critical.
  • Enables measurement of native (undisturbed) soil, even hard-packed clay.
  • Industry-leading 5-year warranty.


  • Repeatable accuracy and stability without the need for calibration in most soils.
  • Digital sensor using the SDI-12 protocol—no setup, just connect to data logger. Compatible with any SDI-12 capable data logger.
  • Zero maintenance required.
  • Unparalleled spatial and temporal measurement consistency. No sensor-to-sensor variations across locations, seasons, soil types or moisture range.
  • Instant measurement of the 3 most significant soil parameters simultaneously—moisture, salinity and temperature.
  • Unlike most TDR or capacitance-based sensors, HydraProbe is less sensitive to changes in temperature, salinity, and soil mineralogy.

Used in more water supply forecast and climatological networks than any other soil sensor

Prevalence of Soil Measurement Technologies in Soil Moisture Networks

HydraProbe Installations Worldwide - Dec. 2016

1-20 21-100 101-300 301-1,000 1,001-5,000 5,001-20,000 >20,000

The Science Behind HydraProbe

HydraProbe is a “dielectric impedance”-based sensor developed by the physics department at Dartmouth College. Unlike capacitance or time domain based soil sensors, HydraProbe fully characterizes the dielectric spectrum using a radio frequency at 50 MHz. Complex mathematical computations performed by an onboard microprocessor process the reflected signal measurements to accurately determine the soil’s dielectric permittivities—the key parameters behind the soil moisture and bulk EC measurement. Low inner-sensor variability means there is no need for sensor-specific calibrations. This method has passed the most rigorous scientific peer review from dozens of journals such as the Vadose Zone Journal, American Geophysical Union, and The Journal of Soil Science Society of America. Read more about the different soil sensor technologies at soilsensor.com.

About EC (Electrical Conductivity / Salinity)

  • The bulk EC (electrical conductivity) of the soil is correlated to the soil’s salinity because when salts are mixed with water they will conduct electricity. The bulk EC parameter is sometimes called “salinity”.
  • Many nutrients are salts—a source of salinity. Nutrient accumulation, poor drainage and saline irrigation water can lead to the unwanted buildup of salinity in soil.
  • High bulk EC can affect moisture readings and create errors with capacitance based moisture sensors.
  • HydraProbe’s soil moisture measurement is less sensitive to salinity than other capacitance based probes.
  • The soil bulk EC can change dramatically with water content and can be affected by the quality of the irrigation water, fertilization, drainage, and other natural processes.
  • Compaction, clay content and organic matter, can influence moisture holding trends over time, also affecting bulk EC capacities in soil.
  • The effect of bulk EC on the moisture availability to a plant’s roots is great. As salinity changes the water needs also change.
  • A temperature corrected bulk EC parameter is available so the user can make comparison independent of soil temperature.
  • Because HydraProbe also measures the dielectric permittivities, algorithms can be applied to approximate the EC of the soil pore water allowing for better soil salinity characterizations.

The HydraGO lets you take HydraProbe to go.

Take soil measurements anywhere, without the effort or expense of setting up a permanent soil monitoring system. Your smartphone communicates wirelessly with the HydraGO using Bluetooth.

Simply insert the probe into the soil, and tap on the “Sample” button in the app. The location of each measurement is recorded along with the soil measurement data. All data can be saved and emailed as a .CSV for analysis in Excel.

Technical Specifications





Real dielectric permittivity (isolated) 1 to 80 where 1 = air, 80 = distilled water 0.001
Soil moisture for inorganic & mineral soil ± 0.01 WFV for most soils

± 0.03 max for fine textured soils*
From completely dry to fully saturated (from 0% to 100% of saturation) 0.001
Bulk electrical conductivity ± 2.0% or 0.02 S/m whichever is typically greater 0 to 1.5 S/m 0.001
Temperature** ± 0.3°C -10°C to +60°C 0.1°C
Inter-sensor variability 3 m-3) n/a




Power supply 9-20 VDC 9-20 VDC
Power consumption
Cable 3-wire: power, ground, data 4-wire: power, ground, com+, com-
Max. cable length 60 m (197 ft.) 1,219 m (4,000 ft.) Non-spliced: 304.8 m (1,000 ft.)
Baud Rate 1200 9600
Communication protocol SDI-12 Standard v. 1.2 Custom or open spec
Addressing Serial; allows multiple sensors to be connected to any RS485 or SDI-12 data logger via a single cable.


Operating temperature range
  • Standard temperature probe range: -10°C to +60°C
  • Standard extended temperature probe range: -30°C to 60°C**
  • Extra extended temperature probe range: -40°C to 65°C**
Storage temperature range -40°C to +65°C
Water resistance Tolerates continuous full immersion
Cable 18 gauge (22 gauge for RS-485 version), UV resistant, direct burial
Vibration and shock resistance Excellent; potted components in PVC housing and 304 grade stainless steel tines


Length 4.9” (124 mm)
Diameter 1.6” (42 mm)
Optional slim housing version available: 1.4" (35.8 mm)
Weight 7 oz. (200 g)
Optional slim housing version available: 6.5 oz. (184 g)
Cable weight 0.86 oz/ft (80g/m)
Sensing volume (cylindrical region) Length: 2.2” (5.7 cm)
Diameter: 1.2” (3.0 cm)

Measurement parameters

1 Voltage 1
2 Voltage 2
3 Voltage 3
4 Voltage 4
5 Voltage 5
6 Soil Temperature in Celsius
7 Soil Temperature in Fahrenheit
8 Water fraction by volume
9 Loss Tangent
10 Soil Conductivity (temperature corrected) in Siemens / meter
11 Real dialectric permittivity
12 Real dielectric permittivity (temperature corrected)
13 Imaginary dialectric permittivity
13 Imaginary dialectric permittivity (temperature corrected)
15 Soil conductivity in Seimens / meter
16 Diode Temperature in Celsius
17 Saved for future development
18 ADC Reading 1
19 ADC Reading 2
20 ADC Reading 3
21 ADC Reading 4
22 ADC Reading 5

* Accuracy may vary with some soil textures.

** For standard extended temperature range and extra extended temperature range probes, contact us to inquire.


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Written by the soil experts at Stevens, our soil resource guide contains a wealth of information and will benefit anyone involved with soil. Whether you’re a soil scientist, a farmer or a soil researcher, this 52 page book is a fantastic reference and source of up-to-date theories, practices and advice. 


Soil Geomorphology

Soil Properties

Salinity / Electrical Conductivity (EC)

Dielectric Permittivity

Soil Monitoring Applications

Soil Moisture and Irrigation

Soil Sensor Technologies

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Sensor Accuracy

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Soil Physics Workshop – Syllabus

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One of the world’s largest meteorological networks, SNOTEL, is employing soil moisture sensors to monitor the impact of soil water content on stream flow forecasts By Keith Bellingham, Soil Scientist & Geochemist / Certified Professional Hydrologist Download a PDF of this article  Many complex political, social, environmental and scientific challenges impact water resources. Snow telemetry…
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Scientific Studies

Title Main Author Publication Date Journal Reference Application
Dielectric Loss and Calibration of the HydraProbe Soil Water Sensor Seyfried, M. S. 2005 Seyfried, M. S., L. E. Grant, E. Du, and K. Humes, Dielectric Loss and Calibration of the Hydra Probe Soil Water Sensor, Vadose Zone Journal 4:1070–1079 (2005). Derivation of the HydraProbe’s general soil moisture calibration
Estimating root zone soil moisture at distant sites using MODIS NDVI and EVI in a semi-arid region of southwestern USA Schnur, M. T. 2010 Ecological Informatics. doi:10.1016 / j.ecoinf.2010.05.001 Using HydraProbe soil sensor to assess regional effects on vegetation and root zone soil moisture in arid lands.
The NOAA Hydrometeorology Testbed Soil Moisture Observing Networks: Design, Instrumentation, and Preliminary Results Zamora, R. J. 2011 Journal of Atmospheric and Oceanic Technology, 28, 1129-1140. doi:10.1175/2010JTECHA1465.1 Using HydraProbe to forecast floods and assess flood risk.
Evaluation of Lichtenecker's Mixing Model for Predicting Permittivitty of Soil at 50 MHz Leao, T. P., E. P. 2015 American Society of Agricultural and Biological Engineers, 58(1), 83-91. doi:10.13031/trans.58.10720 Dielectric Mixing and dielectric permittivity of heterogeneous materials.
Soil Moisture for Hydrological Applications: Open Questions and New Opportunities Brocca, L. C. 2017 Advances in Hydro-Meteorological Monitoring, Special Issue of Water, 9(140). doi:10.3390/w9020140 Soil Moisture and its effect on climate, drought and regional weather.
Climate Models Predict Increasing Temperature Variability in Poor Countries Bathiany, S. V. 2018 Science Advances, 4(5). doi:10.1126/sciadv.aar5809 Using soil moisture measurements to make improved climate models.