August 31, 2026

Soil Water Potential vs. Percent Moisture: The Two Numbers That Tell You a Bed Is Actually Dry

Search "soil water potential sensor" and you land on pages that either sell you a probe or lecture you on the physics of tension. Almost none of them answer the question you actually have standing over a bed: what reading means "water now"? There are two different numbers in play — tension in centibars and volumetric percent — and they measure soil water in genuinely different ways. Here's what each one tells you, and the honest threshold that should trigger a watering.

Two numbers, two different questions

Capacitive probes — the kind in most home weather-station ecosystems — report volumetric water content, a percentage of how much of the soil is water. It answers "how much water is in the ground?" Tensiometers and Watermark-style sensors report soil water potential in centibars (cb) of tension — how hard a root has to pull to get that water free. That answers a slightly different, more plant-centric question: "how much effort does a root have to spend?"

The distinction matters because the same soil can read "20% moisture" and mean very different things in sand versus clay. Tension normalizes for that — a plant in clay and a plant in loam both start to struggle at similar centibar values even though their percentages differ. Neither number is "right." Percent is cheaper to measure and intuitive; tension maps more directly to plant stress. What both share is that a single reading is a poor trigger.

The threshold nobody prints on the box

The SERP is full of tension charts — "0–10 cb saturated, 10–30 cb field capacity, 30–60 cb time to water" — and vendor moisture tables that imply a magic percentage. Treat those as orientation, not gospel; your soil sets the real line. Here's the number Siltwell actually acts on, and it's deliberately conservative:

• Capacitive percent: a bed is flagged dry when its two most recent readings are both below the threshold — default 25%, which you tune per bed.
• Watermark tension: a bed is flagged dry when the two most recent readings are both above 60 cb.

The 25% and 60 cb figures are starting points, not laws of nature. A fast-draining sandy bed and a moisture-holding clay bed will cross those lines at different real-world dampness, which is exactly why the per-bed threshold is adjustable. You watch how a bed behaves for a couple of weeks, then nudge the number so "alert" lines up with "the soil my plants actually want water at."

Why "two readings, not one" is the whole trick

Soil sensors spike. A probe can catch a splash from the hose, a pocket of air after cultivation, or just electrical noise, and throw a single reading that looks alarmingly dry (or wet) for one cycle. If you trigger on any one reading, you get nagged constantly and stop trusting the alerts — the fastest way to end up ignoring a sensor entirely.

Siltwell's soil job runs every 10 minutes but only fires a "this bed is low" warning and push when the two most recent readings are both past the line. One odd blip won't do it; a genuine drying trend will. That's the piece the physics-heavy articles skip — they'll explain what 60 cb means but never tell you how to keep a noisy sensor from crying wolf.

One honest boundary worth stating plainly: this alerts you which bed is dry — it does not run your irrigation. Siltwell tells you "water this bed, skip that one"; the hose is still yours to turn on. If you want the per-bed reading walkthrough, the per-bed probe guide covers mapping a sensor to a bed, and meter vs. bed sensor compares the jab-in dial to a logging probe.

Which sensor to actually buy

For most gardeners, a logging capacitive probe is the practical choice: cheaper than a tensiometer, no reservoir to refill, and it feeds a continuous trend rather than a one-off dial reading. Siltwell works with a specific set:

• Ecowitt WH51 — capacitive percent, best value, pairs to any Ecowitt gateway or Wittboy console.
• Davis 6345 — for WeatherLink setups.
• Ambient WH31SM — needs a WS-2000 or WS-5000 console.

If you're on Tempest, note it has no soil sensor of its own — you'd add an Ecowitt WH51 alongside it. AcuRite and La Crosse aren't supported here, since they lack a soil probe and an open API.