September 1, 2026 · Updated September 13, 2026

Set a baseline soil moisture sensor reading per bed

A soil moisture sensor reports a percentage of the water it senses in the ground, and that percentage means nothing until you know the number your bed reads once it has dried down. That dry-line number is the baseline, and you set one per bed. With it in hand, a live reading tells you whether to reach for the hose.

How a capacitive probe turns water into a percentage

A capacitive probe sends a small electric field into the soil around it and measures how much water sits in that field. Wet soil holds more charge, so the probe reports a higher percentage. As the soil dries, the charge falls and the number drops. The reading tracks the water in one small volume of soil, at one depth, next to one probe.

That volume matters. Two beds holding the same water can read different percentages because their soil, their compaction, and the depth of the probe differ. A percentage is a reading of one spot in one bed, comparable to itself over time and to little else. This is what a sensor cannot tell you. It will not say your bed is drier than the neighbor's in absolute terms, only how this bed compares to how it read last week.

What a bed's own range looks like

In our own garden the four probed beds each settled into a distinct range across late summer. Raised bed 3 fell to 32% on July 13 and recovered to 62% by August 21. The ground bed touched those same extremes yet reached its driest on August 6. Raised bed 1 held a tighter band between 39% and 54%, while raised bed 2 moved across 36% to 57%.

One season and one weather pattern produced four different ranges. The spreads differ because each bed grows different crops, drains at its own rate, and dries from the top down at its own speed. A single threshold of, say, 40% would read as fine for raised bed 1 and as a standing alarm for raised beds 2 and 3. Each bed needs its own line.

Setting the threshold that fires a dry alert

Siltwell fires a dry warning for a bed when its soil probe reads below a threshold you set. Its default sits at 25% capacitive moisture, and each bed can take a number you choose instead. The alert waits for the two most recent readings to both sit below the line, so one noisy reading won't trip it. On a Watermark tension sensor the same logic uses 60 cb.

To find your number, water a bed deeply and watch it dry down over the following days. Note the reading where the plants start to look thirsty in the afternoon, or the point where you would reach for the hose on your own judgment. Set the threshold a few points above that mark, so the warning reaches you before the bed is stressed rather than after.

Confirming dry without false alarms

A reading confirms itself against the soil you can feel. Push a finger or a screwdriver a few inches down near the probe. If it comes out dry and the percentage sits at your threshold, the number and the soil agree, and your baseline is sound. If the soil is still damp while the number reads low, move the probe deeper or to a spot with better contact and set the line again.

With the baseline set, a low reading points you to the one dry bed, and the rest can wait. It reports the number and sends the warning. It does not run your hoses, so the watering itself stays a step you take.

Start with one bed. Set its threshold at the default 25%, watch it for two weeks against how the plants look and how the soil feels, then nudge the number to match what you learn. Repeat for each bed as its range shows itself. Our guide on reading a probe per bed covers where to place it, and when to water from the reading covers what to do the moment the alert lands.