How do soil moisture sensors work in agriculture?

Field sensors read water in the root zone and compare it to a set threshold, then trip an irrigation valve when the soil crosses that line. Capacitive probes report a percentage tied to the water around the sensor. Tension probes report how hard roots must pull to draw water, measured in centibars. Both measure one point at one depth.

What soil moisture level should trigger irrigation?

Siltwell's default fires a dry alert when the two most recent readings are both below 25% on a capacitive probe, or both above 60 cb on a Watermark tension probe. Those defaults apply to every bed. In our own garden no bed reached 25% from mid-July to mid-September; the lowest capacitive reading we logged was 32%.

Which soil moisture sensor is most accurate?

Tension probes report centibars, a physical pull that transfers across soil types, so a cb number means something close to the same thing in sand or clay. Capacitive percentages depend on calibration and soil, so they read best as a trend within one bed. Neither sensor sees past the few inches around the probe.

Can you use an agricultural soil sensor in a home garden?

Yes. The same capacitive or tension probe mapped to one bed gives you the same reading a field uses, scaled to a few square feet. The one change is that at home the threshold triggers an alert to you instead of a valve, and you choose whether to water.

October 8, 2026

Soil moisture sensors for agriculture in a home garden bed

Agricultural soil moisture sensors read water in the root zone and trip an irrigation valve when the reading crosses a set threshold. The same probe and the same threshold logic work in a single raised bed. At home the reading triggers an alert to you, and you decide whether to water that bed.

How field sensors trigger irrigation

A field sensor sits in the soil and measures one of two things. A capacitive probe reads the water around it as a percentage. A tension probe reads how hard roots must pull to draw water, in centibars, so a higher number means drier soil. Both feed a controller that opens a valve once the number passes a line you set.

The percentage and the centibar reading describe the same ground in different units. Capacitive output shifts with calibration and soil texture, so the same water content can read differently in sand and clay. Tension transfers better across soils, because a root pulls against the same physics everywhere.

What the same sensor does in a home bed

Siltwell reads an Ecowitt WH51 as a capacitive percentage or a Davis Watermark probe as tension in centibars, each mapped to one bed. It fires a dry alert and a push when the two most recent readings are both past the threshold. It tells you which bed is dry. It does not open a valve or run any pump.

That per-bed dry alert is the home version of a field controller's trigger. You map the sensor to a bed, and the bed tells you when it needs water. Connecting the station and turning on per-bed soil alerts is part of Pro. For the wiring and mapping steps, see how to connect a soil sensor per bed, and for the controller comparison see what a soil sensor for irrigation does.

What level should trigger watering

The default dry line is 25% on a capacitive probe or 60 cb on a Watermark, and the alert fires only when the last two readings both cross it. One low reading won't trip it, which filters out a single odd sample. In our own garden the beds stayed well clear of that line through the dry weeks: one raised bed swung from 32% to 62%, the ground bed from 32% to 62%, and another raised bed held a narrower 39% to 54%.

No screen sets a custom dry line per bed today, so every bed uses the 25% default (or 60 cb for a tension probe). If your soil holds water differently, read the percentage as a trend within that one bed rather than a fixed wet-or-dry mark.

What a probe can't see

A probe reads one point at one depth. A dry pocket on the far side of the bed, or a crust that sheds water before it reaches the root zone, won't show in the number. Place the probe in the root zone of the planting you care about, and treat a surprising reading as a cue to dig a finger in and check.

If you want the alert that comes with these readings, connect the station and map the probe to a bed. See how to connect a station, then set the sensor against the bed you water most.

Common questions.

How do soil moisture sensors work in agriculture?: Field sensors read water in the root zone and compare it to a set threshold, then trip an irrigation valve when the soil crosses that line. Capacitive probes report a percentage tied to the water around the sensor. Tension probes report how hard roots must pull to draw water, measured in centibars. Both measure one point at one depth.
What soil moisture level should trigger irrigation?: Siltwell's default fires a dry alert when the two most recent readings are both below 25% on a capacitive probe, or both above 60 cb on a Watermark tension probe. Those defaults apply to every bed. In our own garden no bed reached 25% from mid-July to mid-September; the lowest capacitive reading we logged was 32%.
Which soil moisture sensor is most accurate?: Tension probes report centibars, a physical pull that transfers across soil types, so a cb number means something close to the same thing in sand or clay. Capacitive percentages depend on calibration and soil, so they read best as a trend within one bed. Neither sensor sees past the few inches around the probe.
Can you use an agricultural soil sensor in a home garden?: Yes. The same capacitive or tension probe mapped to one bed gives you the same reading a field uses, scaled to a few square feet. The one change is that at home the threshold triggers an alert to you instead of a valve, and you choose whether to water.

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