August 25, 2026 · Updated September 13, 2026
What a soil moisture sensor for irrigation system tells you
A soil moisture sensor reads the water held around its prongs and reports it as a percentage. That number decides whether a bed counts as dry. The sensor tells you a bed needs water. Opening a valve or running a pump stays with you.
How the sensor reads the soil
A capacitive probe sits in the root zone and measures how much water surrounds it, then reports a percentage. A higher number reads wetter. A reading of 45% holds more water than one of 30%. The probe reads one spot at one depth, so where you place it sets what it can see.
Some probes report tension in centibars instead, which measures how hard roots have to pull to get water. A high centibar number reads dry. Both signals answer the same question in different units. Both feed one decision about whether the bed needs water today.
What triggers a watering decision
Siltwell fires a dry-bed warning when the two most recent readings from a mapped probe both fall under that bed's threshold. Every bed opens at 25% capacitive moisture, and you can override it. For a tension probe the trigger is both readings above 60 centibars. Requiring two readings stops a single glitch from waking you at 2 a.m.
The threshold is the whole decision, so it earns careful setting. A number pulled from a chart rarely matches your soil. We cover how to read a season and land on a per-bed figure in setting a per-bed threshold.
What our own beds read through summer
In our own garden four probes ran from mid-July to early September. The first raised bed held 39% to 54% all summer. Raised bed 2 swung from 36% to 57%. The third raised bed and the ground bed both moved across a 32% to 62% band. None of them touched the 25% default.
That tells you two things. Beds dry at different rates from the same rain, so one threshold across the garden would mislead you. A default set too low can stay silent all season. So the number that counts is the one you tune to the bed in front of you. The raised beds with fast-draining mix dipped lowest in early September, when our rain dropped to 1.35 inches for the month after 6.56 inches in August.
What the sensor can't decide
A probe reads its own patch. A deeper layer may still hold water while the top reads dry, and a corner of the bed can drain faster than the spot you chose. The probe misses both. Running your irrigation is also outside what it does. Siltwell reports which bed is dry and leaves the valve to you or to a controller you set up. To send water to the driest zone rather than the whole yard, see which zone needs water.
Rain-skip applies to the whole garden rather than a single bed. Siltwell holds the watering prompt once the next day or two forecasts 5 mm or more of rain. When 5 mm or more fell over the last three days, it marks the garden covered. The per-bed probe still shows which bed dried out first once the wet spell passes.
To act on this, map a probe to each bed, watch what each one reads for a few weeks, and set the threshold from that record. Connecting a supported station and its soil sensors is covered on the station setup page, and from there each bed reports its own dry days instead of one number for the whole garden.
Common questions.
- Can a soil moisture sensor control irrigation?
- On its own it reports whether a bed is dry; it doesn't open a valve or run a pump. Siltwell fires a dry-bed warning when a mapped probe reads low, and you decide to water. If you want a valve to act on that reading, you wire the probe to a controller yourself.
- How does a soil moisture sensor decide when to water?
- It compares the reading against a threshold you set for that bed. Siltwell warns when the two most recent readings both sit below the threshold, which stops a single odd reading from firing an alert. Above the threshold, the bed is holding enough water for now.
- What moisture threshold should trigger watering?
- The default is 25% capacitive moisture, and each bed can carry its own number. Set it from what the bed reads over a season rather than a round figure, because a sandy raised bed and heavy ground soil drain at different rates. Our own beds never dropped near 25% through summer, so a lower default would have stayed silent all season.
- Does a sensor stop watering when it rains?
- A soil probe measures the ground, so a real soak shows up as a higher reading and the bed stops looking dry. Siltwell's rain-skip guidance is separate and works at the garden level, holding when 5 mm or more is forecast and marking the garden covered when 5 mm or more fell in the last three days.
Related guides
How to set up a garden soil moisture sensor per bed
Pick a capacitive probe, map it to a bed, and set a dry threshold so Siltwell tells you which bed needs water.
What a plant moisture sensor tells you outdoors
Learn what a plant moisture sensor reads in an outdoor bed, why one dip can mislead you, and how a per-bed alert decides when to water.
Set a baseline soil moisture sensor reading per bed
Learn how to set a per-bed baseline for a soil moisture sensor so a live reading tells you when a bed is dry, without false alarms.
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