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By Emily Bedwell, Extension Irrigation Technology Specialist, University of Idaho
Crops have very specific water needs, and those needs change dynamically throughout the growing season as plant growth and development shift from vegetative to reproductive stages. Keeping soil moisture within an optimal range is critical for maintaining yield, quality and overall crop health. Both overwatering and underwatering can create costly challenges; excess water can increase disease pressure and nutrient leaching, while inadequate moisture can limit yield and quality.
Soil moisture sensors are powerful tools for helping ensure that these changing water demands are met. By providing continuous, real-time data, soil moisture sensors offer a clear picture of moisture conditions throughout the root zone. This information can help fine-tune irrigation, avoid unnecessary applications and maintain soil moisture at levels that support crop water needs at each growth stage.
New technologies can be overwhelming, especially when there are many options that each have their own pros and cons. Here is a breakdown of some common types of soil moisture sensors, how they work and what to consider before purchasing.
Soil Water Tension Sensors
Measures: Soil water tension
Units: kilopascals (kPa) or centibars (cbars) (1 cbar ≈ 1 kPa)
Tension-based sensors measure how strongly the soil holds onto water, which simulates how hard roots must work to extract moisture from the soil. There are two common types of tension-based sensors: true tensiometers and granular matrix sensors.
Tensiometers –These are true tensiometers consisting of a water-filled tube and a porous ceramic cup. As the soil dries, water is drawn out of the instrument, creating suction measured on a gauge.
Pros:
- Easy to interpret
- Direct measurement of plant water stress
- Perform well across soil textures
Cons:
- Require refilling, adjusting for air bubbles
- Limited reading range in very dry soils
- Freeze sensitive
Granular Matrix Sensors – While granular matrix sensors technically are not tensiometers, they measure the same unit: soil water tension. Instead of water inside a tube, these sensors contain a granular matrix that absorbs moisture from the soil. An electronic reader measures the electrical resistance in the matrix, which correlates with soil tension.
Pros:
- Less affected by soil texture
- No maintenance (no refilling needed)
- Can remain in soil year-round
- Long life (5+ years)
- More affordable option
Cons:
- Require good soil contact
- Slower to respond after irrigation
- Slightly less intuitive than a gauge
Capacitance Sensors
Measures: volumetric water content (VWC) or available soil moisture (ASM)
Units: percent
Capacitance sensors send an electrical signal into the soil. Because water conducts electricity differently than soil or air, the sensor calculates the volume of water per volume of soil. For example, a reading of 22% VWC means that 22% of the soil’s volume is water.
Capacitance sensors can either be a single-depth sensor or a probe that includes several measurement points along a single shaft, providing data throughout the root zone.
Pros:
- Low maintenance
- Multi-depth data (probes)
- Show water movement after irrigation
Cons:
- Require calibration for accuracy
- Readings vary by soil texture
- Higher cost per sensor
- Installation can be difficult
- Proper installation is critical for data accuracy
Time-Domain Reflectometry (TDR) Sensors
Measures: volumetric water content (VWC) or available soil moisture (ASM)
Units: percent
TDR sensors send a fast electromagnetic pulse down metal rods. The “travel time” of that pulse changes depending on how much water is in the soil. This provides a highly accurate measurement of VWC.
Pros:
- Very accurate when calibrated
- Low maintenance
Cons:
- Require careful installation
- Most expensive type of sensor
Understanding VWC% vs. ASM%
Some sensors report soil moisture in VWC, while others report it in ASM. Understanding the difference between these two units is one of the most confusing parts of soil moisture data.
Volumetric Water Content (VWC%)
- Physical measurement of water volume per soil volume
- Does not indicate ease of plant uptake
- Soils with identical VWC% but varying sand/silt/clay composition can have different irrigation needs
Available Soil Moisture (ASM%)
- Calculated value based on field capacity and wilting point of a specific soil texture
- Represents how much water is usable by the crop
- A 60% ASM reading does not equal 60% VWC
Some sensors estimate ASM based on a soil type-specific conversion, but without calibration, these values are only approximations.
Where to Install Soil Moisture Sensors?
So, you have decided which sensors are the best fit for you, and now you are ready to install them in your fields. The main question I hear related to soil moisture sensors is “Where do I install them?” Consider these dos and don’ts for installing sensors in your field.
Do:
- Install in a representative area of field
- Install beyond the second tower of center pivot
- Ensure proper installation (no air gaps around sensor)
- Install in predominant soil type of field
- Install more than one sensor if possible
- Install deep enough to represent root zone
Don’t:
- Install on field edge
- Install in a low or high elevation area in field
- Forget to calibrate your capacitance sensors
- Install your sensor(s) and never validate readings with field checks
How to Get Started
No matter which sensor you choose, the goal is the same: to gain better insight into what’s happening below the soil surface. Soil moisture sensors provide data to help conserve water, improve yield and quality, and reduce pest and disease risk. For assistance with selecting, purchasing or installing soil moisture sensors, contact your local irrigation dealer or crop consultant. Also, you can contact your county’s Extension educator or your state’s irrigation specialist for help evaluating the benefits of integrating soil moisture sensors into your operations.
Author’s note: Emily Bedwell is an assistant professor and the Extension irrigation technology specialist at the University of Idaho in Kimberly, Idaho. She can be reached at ebedwell@uidaho.edu.