August 28, 2026

What Soil Moisture Percentage Should You Water At? The Number a Meter Reading Won't Give You

Search "soil moisture measurement" and you'll learn how to take a reading — push a probe in, note the percentage — but almost no guide commits to the part you actually need: what number do I water at? A reading with no threshold is just a fact with nowhere to go. The useful answer is a line you decide to act on, and it isn't the same for every bed.

The reading is the easy half

Taking a moisture reading is genuinely simple. A capacitive probe buried a few inches into the root zone gives you a percentage; the finger test gives you a cruder version of the same signal. What almost nobody tells you is that the percentage on its own is meaningless until you pair it with a decision: below this, water; above it, leave the bed alone.

The reason guides dodge the number is that it's genuinely relative. A capacitive reading isn't an absolute soil-science value — it shifts with the sensor, the soil texture, and how you've packed the bed. A sandy bed and a compost-heavy one can read the same percentage and be at very different real dryness. So a single universal "water at X%" would be a lie, which is why honest sources go quiet exactly where you need them.

A starting number, then tune it per bed

You still have to start somewhere, so here's a real one: for a capacitive probe, around 25% is a sensible dry trigger. Siltwell uses 25% as the default threshold for a bed with a mapped soil sensor — the point below which it considers the bed low. It's a starting point, not a commandment.

The important move is tuning it per bed once you've watched how each one behaves. Siltwell lets you override the default with a per-bed threshold, because your fast-draining raised bed and your heavy clay border shouldn't share one number. After a week or two of watching a bed dry down and how the plants respond, you'll know whether it wants a slightly higher or lower line. That's the difference between a reading and a rule you can trust.

• Fast-draining or sandy beds: tend to hit the dry mark sooner and may want a slightly higher threshold so you're not caught out.
• Compost-rich or clay beds: hold water longer and can ride a lower number without stress.
• Containers: dry fastest of all — treat each pot as its own bed with its own line.

Why two readings, not one

Here's the trap with spot-checks: you take one reading, it's low, you march outside — and it was a blip, or you'd caught the surface after a hot afternoon while the root zone was still fine. One number is noisy.

Siltwell only calls a bed low when the two most recent readings are both below its threshold. A cron checks mapped soil sensors every ten minutes, and it takes two consecutive readings under the line before it fires a "this bed is dry" warning and a push. That's a deliberate guard against the single odd reading sending you out with a hose for nothing. It's the plain-language version of "water this bed, skip that one" — per bed, confirmed, not on a hunch.

One thing worth being blunt about: that alert tells you a bed is dry. It doesn't open a valve or run your irrigation — Siltwell warns; you (or your own timer) do the watering. If you're weighing a probe against a plain meter for exactly this reason, the meter-vs-bed-sensor guide lays out the trade-off, and whether cheap moisture meters actually work is worth reading before you buy the two-prong kind.

From a one-off measurement to continuous monitoring

A handheld reading tells you about one bed at one moment. The upgrade is a buried probe that reads that bed all day, so the threshold you tuned actually gets watched for you instead of only when you remember to check. Map a supported soil probe to a bed and the "is it below the line, twice in a row" logic runs on its own.

Supported probes: the Ecowitt WH51 (best-value capacitive, pairs to any Ecowitt gateway or Wittboy console), the Davis 6345, and the Ambient WH31SM (needs a WS-2000/5000 console). A Tempest station has no soil sensor, so you'd add an Ecowitt WH51 alongside it.