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Soil moisture measurement methods—compared

Soil moisture measurement methods—compared

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Which method is right for your application?

Understanding the difference between soil moisture sensors can be confusing. The gravimetric technique is the only direct way to measure soil moisture, but it is labor-intensive and practically impossible if you need a time-series. Most researchers choose soil sensors that use indirect measurement methods—taking a property you can measure and inferring a property that’s more difficult to measure.

The two charts below compare the most common soil moisture sensing methods, the pros and cons of each, and in what type of situation each method might be useful. All METER soil moisture sensors use a high-frequency capacitance sensing technique and an installation tool for easy installation and to ensure the highest possible accuracy. For more in-depth information about each measurement method, watch our Soil moisture 102 webinar.

Types of soil moisture sensors

SensorProsConsWhen to Use
Electrical Conductivity
Probes
1. Continuous measurements can be collected with data logger
2. Lowest price
3. Low power use
1. Poor accuracy: calibration changes with soil type and soil salt content
2. Sensors degrade over time
1. When you only want to know if water content changed and don’t care about accuracy
TDR Probes
(Time Domain)
1. Continuous measurements can be collected with data logger
2. Accurate with soil-specific calibration (2-3%)
3. Insensitive to salinity until the signal disappears
4. Respected by reviewers
1. More complicated to use than capacitance*
2. Takes time to install because you must dig a trench rather than a hole
3. Stops working in high salinity
4. Uses a lot of power (large rechargeable batteries)
1. If your lab already owns the system. They are more expensive and complex than capacitance, and studies show both TDR and capacitance to be equally accurate with calibration
Capacitance Sensors1. Continuous measurements can be collected with data logger
2. Some types are easy to install
3. Accurate with soil-specific calibration (2-3%)
4. Uses little power (small batteries with little or no solar panel)
5. Inexpensive, you can obtain many more measurements for the money you spend
1. Becomes inaccurate in high salinity (above 8 dS/m saturation extract)**
2. Some low quality brands produce poor accuracy, performance.
1. You need a lot of measurement locations
2. You need a system that’s simple to deploy and maintain
3. You need low power
4. You need more measurements per dollar spent
Neutron Probe1. Large measurement volume
2. Insensitive to salinity
3. Respected by reviewers, since method has been around the longest
4. Not affected by soil-sensor contact problems
1. Expensive
2. Need a radiation certificate to operate
3. Extremely time-intensive
4. No continuous measurement
1. You have a large budget, a lot of time, and dedicated personnel to make the measurements
2. You are measuring highly saline or swell-shrink clay soils where maintaining contact is a problem
COSMOS1. Extremely large volume of influence (800 m)
2. Automated
3. Effective for ground truthing satellite data as it smooths variability over a large area
4. Not affected by soil-sensor contact problems
1. Most expensive
2. Measurement volume poorly defined and changes with soil water content
3. Accuracy may be limited by confounding factors such as vegetation
1. When you need to get a water content average over a wide area
2. You are ground truthing satellite data
*Acclima and Campbell Scientific make TDR sensors/profile probes that have on board measurement circuitry, which overcomes the challenge of complexity most TDR systems face.
**This depends on measurement frequency, the higher the frequency, the lower the sensitivity.

A comparison of sensor benefits

 Electrical ConductivityTDRCapacitanceNeutron ProbeCOSMOS
PriceLowestModerate to highLow to moderateHighHighest
AccuracyLowHigh*
(with soil-specific calibration)
High*
(with soil-specific calibration)
Low (Improves with field calibration)Unknown
ComplexityEasyEasy to intermediateEasyDifficultDifficult
Power useLowModerate to highLowN/AHigh
Salinity SensitivityExtreme1. None in low to medium salinity
2. Yes in high salinity
Yes in high salinityNoNo
DurabilityLowHighHighHighHigh
Volume of InfluenceSmall area between probe A and probe B0.25 liter to 2 liters depending on probe length and shape of the electromagnetic field0.25 liter to 2 liters depending on probe length and shape of the electromagnetic field20 cm diameter sphere when soil is wet, 40 cm diameter sphere when soil is dry800 meter diameter
*Some low quality brands exhibit low accuracy and poor performance. The largest threats to accuracy for both TDR and capacitance sensors are air gaps caused by poor installation, followed by clay activity in the soil (i.e. the smectite clays), followed by salinity.

Our newest soil moisture sensors

We created the new TEROS sensor line to eliminate barriers to good accuracy such as installation inconsistency, sensor-to-sensor variability, and sensor verification. TEROS soil moisture sensors combine consistent, flawless installation with an installation tool, extremely robust construction, minimal sensor-to-sensor variability, a large volume of influence, and advanced data logging to deliver the best performance, accuracy, ease-of-use, and reliability at a price you can afford.

Want more details? In the video below, soil moisture expert Leo Rivera explains why we’ve spent 20 years creating the new TEROS sensor line.

 

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Everything you need to know about measuring soil moisture—all in one place.

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