Sensors
Soil moisture sensors: types, how to choose and how to install them
Not every sensor measures the same thing. Some measure how much water is in the soil; others, how hard the roots must work to extract it. The types available, which one suits your crop and how to install it so the data is useful.
In short
- There are two families: sensors that measure volumetric water content (capacitive, TDR) and sensors that measure soil water tension (tensiometers, granular matrix).
- For field irrigation, capacitive sensors are the most common choice thanks to fast response, low maintenance and remote reading.
- Install them in pairs at one third and two thirds of the root zone depth, in at least two locations per uniform zone.
- The absolute value matters less than the trend: how it rises with each irrigation and falls as the crop uses water.
What does a soil moisture sensor measure?#
A soil moisture sensor reports the state of the water around the roots — the reading that most directly answers do I need to irrigate? There are two ways to measure it, and the difference matters before you buy:
- Volumetric water content (VWC): what share of the soil volume is water. Measured by capacitive and TDR sensors, indirectly, from the soil's electrical properties.
- Soil water tension: how much force the roots need to extract water, in centibars or kilopascals. Measured by tensiometers and granular matrix sensors.
The first tells you how much water there is; the second, how available it is to the plant. The same moisture percentage can be plenty in sandy soil and scarce in clay; tension already accounts for that difference.
Types of soil moisture sensors#
Capacitive (FDR)#
They measure the soil's dielectric constant, which changes sharply with water. They respond fast, have no consumable parts and integrate easily with LoRaWAN transmitters. Their sensing volume is small and readings are affected by salinity and soil contact, so installation matters.
TDR (time domain reflectometry)#
They measure how long an electrical pulse takes to travel along the rods. They are accurate across a wide range of textures and widely used in research; they usually cost more than a capacitive sensor.
Tensiometers#
A water-filled tube with a porous ceramic tip: as the soil dries, water moves out and the gauge reads the tension. They are inexpensive and unaffected by salinity, but have a limited range, respond slowly and need maintenance (refilling).
Granular matrix#
They measure the electrical resistance of a porous block in equilibrium with the soil. They cover a wide tension range (0 to about 200 centibars) and can be read remotely, but they are sensitive to temperature and salinity and should be calibrated per soil type.
The two-prong resistive probes sold for hobby projects are good for learning, not for irrigation decisions: the exposed electrodes degrade within weeks and the reading drifts.
Soil moisture sensors compared#
| Type | Measures | Strengths | Limits |
|---|---|---|---|
| Capacitive (FDR) | Volumetric content | Fast response, no maintenance, remote reading | Small volume; sensitive to salinity and poor contact |
| TDR | Volumetric content | Accurate across many textures | Higher cost |
| Tensiometer | Tension | Inexpensive, unaffected by salinity | Limited range, slow response, maintenance |
| Granular matrix | Tension | Wide range, remote reading | Sensitive to temperature and salinity; calibrate per soil |
How to choose a soil moisture sensor#
Define what it's for. To time irrigation and get alerts, a well-installed capacitive sensor is enough; for research or highly variable soils, consider TDR.
Look at your soil and water. With saline water or soil, tensiometers or a temperature- and salinity-compensated capacitive sensor give steadier readings.
Choose the connectivity. In open fields, a sensor with a LoRaWAN transmitter reaches several kilometers on batteries that last years. Avoid devices that only log to local memory.
Favor open protocols. A sensor that speaks standard LoRaWAN or MQTT lets you change platforms without changing hardware.
How and how deep to install it#
- Depth: in pairs, at one third and two thirds of the crop's root zone depth.
- Location: between plants in the same row, inside the dripper's wetted bulb, away from wheel tracks and paths.
- Number: at least two locations per uniform zone. If the field has light and heavy soils, measure them separately.
- Contact: no air gaps. Backfill with the same soil and tamp gently; poor contact is the most common cause of bad readings.
How to read the data#
A single number says little; the curve says a lot. A day or two after a heavy irrigation the value levels off: that is your field capacity reference. From there, moisture falls as the crop uses water, and irrigation has to arrive before the refill point.
Always compare both sensors in the pair: if the upper one rises with each irrigation and the lower one doesn't, irrigation is too short; if both stay saturated for long, there is too much water. To see how this fits into a complete system, read smart irrigation: what it is and how it works; to decide where to install, satellite helps: NDMI vs NDVI.
Bloomiot Workspace takes the data from your LoRaWAN or MQTT sensors, even third-party ones, and tells you when to irrigate and when something fails. Check the supported formats in the documentation.
Frequently asked questions
What is the best soil moisture sensor?
It depends on the use. To time irrigation in the field, a well-installed capacitive sensor with remote reading is the most practical choice. Tensiometers work well with saline soils or water, and TDR sensors are preferred when more accuracy is needed in highly variable soils.
How deep should a soil moisture sensor be installed?
The usual recommendation is to install them in pairs, at one third and two thirds of the crop's root zone depth. That way you see each irrigation in the upper layer and confirm that water reaches the deeper roots.
How many soil moisture sensors do I need?
At least two measurement locations per uniform zone — the same soil, crop and irrigation block. If the field has light and heavy soils, each soil type needs its own measurement.
Do soil moisture sensors need calibration?
Sensors ship with a generic calibration that is fine for tracking trends. For more accurate absolute values they are calibrated per soil type. In practice, to time irrigation it is enough to set field capacity and the refill point by watching how the sensor responds on your own field.
Sources
- Soil moisture
- Sensors
- Tensiometer
- TDR
- Irrigation
About the author
Elio Fagúndez
Founder of Bloomiot. Mechatronics engineer specializing in hardware development, embedded systems and IoT; he designed the platform that turns satellite and field data into irrigation decisions.