A soil moisture sensor tells you how wet the soil is at the depth where it sits. Used well, it takes the guesswork out of irrigation. Used badly — wrong depth, wrong place, uncalibrated — it gives confident numbers that mean little.
How capacitive probes work
Water changes the electrical properties of soil. A capacitive probe measures that change and converts it into a moisture reading. Because different soils respond differently, each probe is calibrated between the soil’s own dry and wet points — which is why the same reading means different things in sand and in clay until it is calibrated.
Dry and wet thresholds
Every probe carries two thresholds: below the dry threshold the crop needs water; above the wet threshold the root zone is saturated and at risk of root disease. Our defaults are dry below 30 % and wet above 70 % on the calibrated scale, then adjusted for crop and soil.
| Reading | Meaning | Action |
|---|---|---|
| Below dry threshold | Root zone drying out | Irrigate — in short pulses |
| Between thresholds | Comfortable band | Leave it |
| Above wet threshold | Saturated | Hold irrigation; check drainage |
Placement matters more than the sensor
- Place the probe at the depth of the active roots, not at the surface.
- Put it in the wetted zone of a dripper — not between drippers, not right under one.
- Choose a spot that behaves like the rest of the zone, not the low corner or the sandy ridge.
- For deep-rooted trees such as avocado, two depths show whether water is draining or sitting.
How often to read
Our probes report every 30 minutes between 06:00 and 18:00 and every two hours overnight, and answer a manual request from the dashboard within about two minutes. That is frequent enough to see each irrigation pulse soak in and drain, and sparing enough to run indefinitely on a small solar panel.
One reading is a number. The curve over a day — how fast it rises after irrigation and how fast it falls after — tells you about drainage, root activity and whether your run times are right.