August 31, 2026 · Updated September 13, 2026

How a soil water potential sensor tells you a bed is dry

A soil water potential sensor measures how hard your plant's roots must pull to draw water, reported in centibars of tension. A percent moisture sensor measures how much water the soil holds by volume. Both point you to a dry bed, and they read the same soil differently because the meaning of a percent shifts with soil type.

For most home beds either sensor works once you set the right threshold. Tension carries better between soils, because 60 centibars means roughly the same struggle whether the bed is sand or clay. Percent probes are cheaper and pair with more consoles, and they log a clean trend once you learn each bed's wet and dry ends.

What tension in centibars measures

Tension rises as soil dries. Near 0 cb the soil is saturated and roots pull freely. As water leaves, the remaining film clings harder to soil particles, so the number climbs. Above 60 cb many vegetables start to slow their uptake, and Siltwell treats two recent readings both above 60 cb as a dry bed.

Tension's strength is that it tracks what the plant feels. It reports how hard the roots must pull for water. The weakness is hardware. Tension blocks need firm soil contact, respond slowly after a rain, and can drift as they age. They also read a narrow band around the sensor, so placement decides what you learn.

What volumetric percent measures

Percent moisture is the fraction of soil volume filled with water. Capacitive probes read it by sensing how the soil changes an electric field, and the Ecowitt WH51 works this way. Siltwell's default dry mark is 25%, with a per-bed override, because the same percent means different things in different soils.

That soil dependence is why one threshold cannot cover a whole garden. Across our own garden, from mid-July into early September, one raised bed held between 39% and 54%, and a ground bed moved between 32% and 62%. Same summer, same rain, different ranges, so each bed carries its own number. We set thresholds from a baseline reading rather than a book value, and the method for that lives in our per-bed probe reading guide.

How a per-bed alert avoids a false alarm

Whichever sensor you use, a single reading can mislead you. A probe can log one low spike when a dry pocket sits against the sensor, or one high spike right after you water past it. Siltwell holds back a dry-bed warning until the two most recent readings both fall under the threshold. That rule covers the percent path and the tension path together. A cron checks every ten minutes, so a lone bad reading passes without a push.

Neither number tells you the whole story. Each probe samples a single point at a single depth. When the surface reads dry, the probe still misses whatever water deeper roots reach. Set the probe at the depth your crop roots, and read the trend across days rather than one moment.

Which one for you

• Pick tension if you run several soil types and want one threshold that means the same thing everywhere.
• Pick percent if you want a lower-cost probe that logs cleanly and pairs with a console you may already own.
• Run either through Siltwell and the per-bed dry alert fires only after two low readings agree.

Start by taking a baseline reading in each bed a day after a soak and again when a plant first wilts. Those two points set the threshold that matters for your soil. If you are still deciding between a handheld meter and a fixed probe, our meter versus bed sensor comparison walks through the trade-off.