In short
There are two families of probes: those measuring how much water is in the soil (capacitive, as a volume percentage) and those measuring how hard it is for roots to extract it (tensiometers and granular matrix blocks, in kilopascals). To automate irrigation, several go in each zone in the root area and watering starts when most cross the threshold, with safety limits.
Probe types
| Type | What it measures | Advantages | Drawbacks |
|---|---|---|---|
| Capacitive | Volumetric water content (%) | Maintenance-free, continuous reading, easy to link by radio | Best calibrated for each soil; salinity can affect the reading |
| Tensiometer | Tension or matric potential (kPa) | Shows root effort directly; widely used in intensive horticulture | Must be refilled with water; limited range in very dry soil |
| Granular matrix block | Soil tension (kPa) | Robust, low-cost, covers drier soils | Slower response; less accurate in very wet soil |
| Multi-parameter probe | Moisture, temperature and electrical conductivity | One probe watches both water and salinity | More expensive; soil EC is not the same as solution EC |
Where and how many
- In the active root zone, near the dripper but not right under it.
- For deep-rooted crops, at two depths: one to decide when to water and one to check water is not lost below.
- At least two or three per zone, at representative points, not at the edge or in waterlogged spots.
- In nursery pots and trays, in representative containers of each bench or batch.
Rules that prevent surprises
- Water when most probes in the zone cross the threshold, for example two out of three, not on a single one.
- Set a minimum pause between cycles and a daily maximum.
- If a probe stops reporting or gives impossible values, fall back to scheduled watering and send an alert.
- Review calibration when changing substrate or season.